Sustainable Access to Mars: Interplanetary Transportation Architecture

MatthewRRobinson · 2012-04-11 17:48:25

Much of the space community, and perhaps much of the world, is looking forward to the first manned mission to Mars. But, in all the proposals and designs I've seen, MRDM is the biggest perpetrator, the program is terribly unsustainable, even less sustainable than Apollo. Apollo launched with ~200 tons of hardware, and returned only 6. Unlike the ISS, which was constructed by, although it was more expensive than other LEO vehicles, a reusable vehicle, access to the moon required a vehicle that was 50% heavier, and entirely non-reusable.

If Elon Musk's goal of creating a colony on Mars that costs half a million to go to is to ever become a reality, a mission architecture that throws away billions of dollars of hardware with every flight is absolutely unacceptable. A popular thought is, "we'll get there eventually, but it's not time now", that's the exact same thought that's stagnated our space program for the last half-century, and will doubtlessly ground our species until our death if it continues. The imaginary future where prices are lower and spaceflight is easy will never come unless we build it - and that is exactly what this mission architecture is for. Even if reusability doesn't bring costs down to $500,000 per ticket round-trip, which it very well may, lack of reusability will ensure each mission costs tens of billions, and hundreds of millions if not billions per ticket, at least.

Sustainable Access to Mars is key for any long-term endeavor on Mars, be it a single station, or multiple outposts across the martian surface, or even a colony, Sustainable Access to Mars (SAM) is a must. SAM isn't the entire Mars Mission, but is the architecture by which cargo and personalle are delivered to and from Mars, so discussion on a single site, multiple sites, crew size, whether to have a ground vehicle or not, etc. are not directly relevant to SAM. I.e, SAM is about getting to and back from Mars, not what happens on the surface.

Here's what I propose:

First, a fleet of 3 unique vehicles is used (not including the surface hab, or possible ground vehicles, which are not unique to SAM, and the specific details of which are not relevant to SAM) ;

1- The Earth Return Vehicle (ERV)
The Earth Return Vehicle consists of two modules mounted together, similar to Apollo's Command Module and Service Module. They do not separate under any ordinary circumstances, but can separate for emergency aborts.
The vehicle's side is coated with sufficient PICA-X heatshield to allow for tens or hundreds of interplanetary-velocity (~10.5 km/s) Martian aerocaptures and re-entries. (PICA-X can be used to create a heat shield capable of hundreds of Earth re-entries *source, 2)

The "Command Module" portion is a modified Red Dragon, modified to include an opening Pica-X coated nosecone (fitted over the docking port) and a ballute. The nosecone opens for docking, but otherwise remains closed. The Dragon retains enough of it's own heatshield (apart from the ERV's heat shield) for a single interplanetary Earth re-entry (~12 km/s) without the "Service Module" in case of failure of Earth Orbit Capture. It also retains enough landing thrust to land on Mars without the Service Module portion in case of ERV Main Propulsion System failure during vertical powered descent over Mars.

The "Service Module" portion uses CH4/LOX engines and propellant tanks sufficient to give the ERV vehicle enough Delta-Vee for Mars ascent and Trans-Earth Injection (combined, roughly 6.4 km/s minimum).

2- The Supplementary Transit Habitat Module (Suphab)
The Suphab is an extremely small, lightweight module. It, along with the ERV, provide the habitat space and supplies for Earth-Mars and Mars-Earth transit.

3- The Trans-Mars Bus (Bus)
The Trans-Mars Bus is the "tractor" that delivers the surface habitat and/or cargo to Mars, and delivers the ERV's and Suphab to and from Mars.
The Bus consists of five propellant tanks, a communications array, an avionics bus, a low-power powerplant, a Timberwind NTR Main Propulsion System, and a robotic arm (which can be fitted with a robonaut for EVA, esp. for EVA on the NTR MPS if that should become a necessity).

Four propellant tanks, from a head-on view, are arranged in an "X" configuration, at each point of the X. The fifth propellant tank is, from a head-on view, at the center of the "X", but offset behind the other four propellant tanks, leaving the actual center of the "X" empty. The engine assembly is mounted on the rear of the fifth propellant tank. The first four propellant tanks act as a radiation shield from interplanetary radiation, and the fifth acts as both a radiation shield from interplanetary radiation and a radiation shield from the NTR engines.

(The center of the "X" will have enough room for the nose of the ERV (The dragon "CM") docked with the Suphab which docks to the Bus, but does not have enough room to encompass a surface habitat.)

First:
The Bus is launched into LEO, along with an unmanned ERV and surface habitat. The entire assembly docks in orbit, then the Bus uses it's NTR to perform TMI for a low-energy transfer to Mars.

On arrival to Mars, the Bus, Surface Hab and ERV all undock, and the Surface Hab and ERV each aerocapture separately, then enter Mars' atmosphere for reentry.

After they have undocked, the Bus, now free of payload mass, performs a final course correction burn for a free-return around Mars back to Earth.

The habitat uses a heat shield, parachutes and a very small amount of thrust to land. The ERV, on the other hand, uses it's re-usable (although ablative PICA-X) heat shield, then a non-detaching ballute to descend. Once at low altitude and near terminal velocity, an engine ignites and uses a very modest amount of Delta-Vee in a powered vertical descent. Once landed, the ballute (which has a hose connected to the vehicle) is vented by opening a valve on the ERV and is retracted and stored, making it re-usable.

The ERV then begins ISRU.

The Bus, on the other hand, now free from any cargo mass, performs a free-return back to Earth, and captures back into Earth orbit using a very small amount of remaining propellant.

Second:
This time, the bus is docked with the Suphab and a manned ERV. The Suphab being significantly lighter than the Mars surface hab, the assembly is able to perform a much higher-energy TMI, meaning a shorter transit time for this manned flight.

On arrival to Mars, the ERV undocks and re-enters; the Suphab remains docked to the Bus for the duration of the mission, making it, too, reusable.

One of the key arguments against total ISRU is the lack of abort ability during re-entry and landing, however, the manned ERV is composed of a service module and a dragon command module that can separate and use it's own heat shield, ballute (the same that is normally used), and thrust to land, making abort possible during reentry. Although the service module would be lost, the ERV that landed on Mars earlier would be used to return to Earth, anyways.

Should the ERV fail to aerocapture completely, let us assume the velocity we failed to lose in aerocapture is less than the delta-vee carried for the powered descent phase: The ERV can then use it's powered descent propellant to perform a propulsive MOI, then perform a very small Delta-Vee burn at apoapsis to drop the periapsis into Mars' atmosphere. In this emergency landing, the dragon CM can separate and land under it's own power.

Once again, with the free return complete, the Bus+Suphab perform EOI.

Return To Earth:
For the Return, the Bus performs a low-energy TMI transfer from Earth with Suphab docked. The ERV launches from Mars and performs it's own TEI. The two rendezvous and dock, allowing the crew to access the Suphab, and the Bus to use it's NTR to capture into Earth orbit with the ERV and Suphab docked, meaning no hardware is lost whatsoever.

However, should the rendezvous with the Bus fail, the ERV, having been restocked with supplies from the surface hab (possibly produced on Mars), has sufficient supplies for the crew to survive the duration of the TEI transit on small rations. Upon arriving to Earth, the dragon portion of the vehicle can separate and survive Earth re-entry, enabling the crew, but not the entire ERV, to return intact.

Additional Surface Habs
By launching only a surface hab without an ERV, and docking with the Bus to make a surface hab+Bus assembly, it is possible for the Bus to conduct a higher-energy TMI transfer, decreasing transit time significantly, perhaps making it possible to launch a manned hab to Mars without an ERV attached.

An additional surface Hab does not necessitate an additional ERV: if part of the "surface hab" package is a surface vehicle, then an additional ERV does not need to be launched if the new surface hab is within driving distance of an already existing ERV. The effect, however, is that the average on-Mars stay time for the crew would increase, but only if the additional surface hab is manned, adding crew but not an additional ERV.

~

The mission is very low mass, and most importantly, no hardware needs to be replaced with each mission, meaning only 1-2 super heavy launches per additional mission for NTR LH-2 propellant and minor mass such as perishables for the Earth-Mars flight (on-surface and return provisions could be produced on Mars), which is 1-2 super heavy launches depending on the precise mission mass. At the cost of an additional launch, an additional HAB+cargo can be added to the surface station to expand an existing one, or for a new one to be created, but that is the only instance where new hardware is specifically needed; when you want new hardware to stay on Mars.

Using SpaceX low-cost re-usable launch vehicles, the cost for continuing service to Mars could be comparable to that of the Space Shuttle's continuing service to the ISS, and possibly even cheaper (more sustainable and/or more open to expansion), when considering each return mission happens less often than once a year during launch windows.

This is, opposed to MRDM v5, which calls for 7 super heavy (Ares V/SLS) launches per mission! *source, p.30

~

The key is the lower costs, making it possible to sell tickets to Mars for lower prices than would otherwise be possible. With this setup, and the possibility of a profit, human sustained presence on Mars is assured. However, with a throw-away hardware architecture, that even Mars Direct is guilty of, you're limited by the price of the hardware.

Let's say it's somehow possible to make an interplanetary Mars-transfer spacecraft that can crew 100 passengers, and costs less than a space shuttle, 2 billion dollars. Ignoring the costs of launching the crew and passengers into LEO, making a profit, costs of launching propellant into orbit, launching the spacecraft itself into orbit, and all other costs, that is still $20/million per person, even using those insane numbers like 2/billion for a 100-passenger to Mars spaceship. Now, that is why reusable hardware is a must for sustained access to Mars.

~EDIT:

Given the political sensitivities of NTR, it would be interesting to look at this proposal with the "Bus" using chemical propulsion instead. Two alternatives I can imagine are;

1. LOX/LH2 propulsion.
or
2. LOX/CH4 propulsion.

In the LOX/CH4 propulsion method, the mission is altered somewhat. Instead of going on a free return, and performing a ~4,500 m/s EOI, the bus would perform a propulsive MOI after separating from it's cargo. The ERV would carry significantly greater tankage, as well, and there would be a much higher rate of CH4/LOX production by ISRU. Over the course of the mission, the ERV would launch, dock, and refuel the Bus from it's own propellant tank, one launch at a time. This would involve new technology; on-orbit refueling, and involve the ERV launching into Mars orbit and reentering a number of times, but would save a tremendous amount of mission mass. The new technology isn't particularly difficult, but the multiple ERV launches and returns might make the mission somewhat risky.

But neither of those things should be exaggerated. The new technology (in-flight refueling) is something jet aircraft do all the time. Doing it in space just means forming a seal; something we've done countless times with on-orbit docking. When you consider on-orbit refueling as a combination of hard-docking and mid-air refueling, the technology really isn't new at all.

As for the multiple launches and reentries, not only did the Space Shuttle fly 134 reentries with only 1 re-entry failure (due to foam breaking off from the ET), but soon we'll have real-world data from SpaceX on the exact same thing as the ERV with the reusable Falcon 9. (Note, this year, SpaceX will begin scale testing of this with the "Grasshopper", source, source2)

As for the Timberwind NTR, I don't think it's quiet as far-fetched as a lot of people may think. We've been using RTG's for a long time, and recently, with Curiosity, it's been publicized as a "nuclear-powered Mars rover" a number of times. It may be an RTG, but in the public's eye, it's a nuclear powerplant, and seeing as it draws electrical power from radioactive decay of radioactive material, it's not really an incorrect understanding. It seems like a huge step towards nuclear power in space. (Which, btw, actually has flown before, source) it's not quiet as far-fetched as many think it is.

Last edited by MatthewRRobinson (2012-04-11 21:12:00)

RobS · 2012-04-11 21:29:38

Are people having a timeout problem? I just wrote a post and lost it!

Anyway, Musk seems to want to create sustainable access to Mars, just as you propose. His system starts with reusable Falcons and Falcon Heavies that return to the launch site. He said somewhere a reusable system puts 40% less in orbit, but that's still about 36 tonnes for a Falcon Heavy. If you put that much in orbit for hundreds of dollars per pound (i.e., less than $36 million for 36 tonnes) you don't need Timberwinds or ballutes; you go to Mars with RP1 and LOx, because it would be so cheap. RP1/LOx gives an exhaust velocity of 3.4 km/sec; you need a mass ratio of 3.6 to achieve 4.3 km/sec (the velocity to go to Mars on a free return trajectory, 6 months outbound). So you need to launch a 36-tonne payload and 3 36-tonne propulsion stages. Each would come with a docking collar attached to it with explosive bolts. They'd simply dock together, and when a particular stage burned up its fuel you'd fire the explosive bolts and separate. The first two propulsion stages would gradually aerobrake to low Earth orbit and return to the launch site; the third would go to Mars and possibly land there, where eventually you could refuel it with methane/LOX (which is very similar to RP1). Refueled, with 33 tonnes of methane and LOX (with a 3-tonne structure and heat shield) it could launch 13 tonnes into Mars orbit, or maybe 10 with enough fuel to return to the surface. If you built it into the lander/ERV the ERV would already be on top.

MatthewRRobinson · 2012-04-13 22:03:24

RobS wrote:

Are people having a timeout problem? I just wrote a post and lost it!
Anyway, Musk seems to want to create sustainable access to Mars, just as you propose. His system starts with reusable Falcons and Falcon Heavies that return to the launch site. He said somewhere a reusable system puts 40% less in orbit, but that's still about 36 tonnes for a Falcon Heavy. If you put that much in orbit for hundreds of dollars per pound (i.e., less than $36 million for 36 tonnes) you don't need Timberwinds or ballutes; you go to Mars with RP1 and LOx, because it would be so cheap. RP1/LOx gives an exhaust velocity of 3.4 km/sec; you need a mass ratio of 3.6 to achieve 4.3 km/sec (the velocity to go to Mars on a free return trajectory, 6 months outbound). So you need to launch a 36-tonne payload and 3 36-tonne propulsion stages. Each would come with a docking collar attached to it with explosive bolts. They'd simply dock together, and when a particular stage burned up its fuel you'd fire the explosive bolts and separate. The first two propulsion stages would gradually aerobrake to low Earth orbit and return to the launch site; the third would go to Mars and possibly land there, where eventually you could refuel it with methane/LOX (which is very similar to RP1). Refueled, with 33 tonnes of methane and LOX (with a 3-tonne structure and heat shield) it could launch 13 tonnes into Mars orbit, or maybe 10 with enough fuel to return to the surface. If you built it into the lander/ERV the ERV would already be on top.

Wow, that is really dumbfoundingly straight-forward. Entirely reusable, and very direct. For here and now, that is probably the best architecture there is.

As for the third stage, since it's on a free return trajectory, if it's not needed on Mars, it can simply loop around and come back to Earth and aerocapture, granted a little extra propellant for more course corrections and orbital adjustments (which could probably be reduced to very little: simply make it aerocapture into a highly elliptical orbit and perform the orbital adjustments at apogee.)

But still, that involves 4 heavy launches for every 36 tons launched at Mars. It would surely require some extra analysis, but it would be interesting to see how the costs of a Timberwind engine would compare with the reduced launch costs. You only pay for a development program once, so then the question is; how many times can you reuse the Timberwind-bus MTV (can you even turn a profit?), and how long would it take for it to pay itself off? All assuming, of course, the NTR program is allowed in the first place (I wasn't around then, but I haven't heard anything about protestors at the NERVA program.)

It would be possible, using NTR, to cut those launches in half, and using a design like the Falcon 9 RLV upper stage, it could aerocapture back at Earth instead of even needing any propellant (Though, with a mass ratio of 1.55 giving the needed propulsion, it's also very easy to carry enough propellant for propulsive EOI).

Furthermore, you could either use the extra velocity (5.76 km/s at mass ratio of 1.8) for a shorter transit time, or add in more payload with the second launch for more tonnage to Mars.
(Booster: 30t total, 4t dry, 42 tons of payload to TMI, 100 m/s spare (4.4 km/s). In total, that's 2 heavy launches for 42 tons of payload TMI.)

~

Timberwind aside, using LOX/LH2, you could still get 4.75 km/s even if each booster had 6.5 tons dry mass. But at 4 tons (I think 3 is a bit optimistic, including landing gear, RCS and heat shield), 5.26 km/s, for a shorter transit time, or, launch 2 regular boosters and a smaller booster (8t lighter than the others) packaged with 8 tons of extra payload, with 200 m/s to spare for course correction (for a total of 44 tons TMI, enough for a 20t ERV, 16t HAB, and 8t rover/ISRU/supply package.).

If I assume the 3-ton figure for dry mass on each propulsion stage, it can be done with only 2 boosters at 451 ISP. With the 4-ton figure, 2 boosters can launch a payload of 31 tons to TMI (with 200 m/s to spare).

~

The LOX/LH2 looks most promising, I don't think Timberwind would be worth 2 launches instead of 3, but in truth only numbers will tell accurately enough to make a decision on (but only experience will tell for sure), examining the costs of Timberwind development, political struggle, and engine cost/reusability of MTV v.s. savings from less launches.

Btw; I used a spreadsheet to get this, checking the 340 ISP RP-1/LOX, 36 ton wet mass, 3 ton dry mass, I had to drop the payload to 33 tons to get 4.3 km/s out of it, and that's with no extra Dv for contingency or course corrections.

Last edited by MatthewRRobinson (2012-04-13 22:22:16)

clark · 2012-04-13 23:08:28

We all realize how far away from being possible this all really is, right?

Right?

Of course, I have always been the crazy one.

SpaceNut · 2012-04-14 07:14:04

Large payloads of 20 plus landing on mars still has not been solved...but still 4 launches to get the 20 plus to mars, is still alot of launches to get in a 2 month window....for each unit for leg of use

GW Johnson · 2012-04-14 10:49:17

If for your transfer vehicles you are considering propellant modules in the 30 ton class assembled by simple docking in LEO, I think that is the right approach. Unmanned supplies and equipment can go "slowboat" this way on min energy trajectories for capture into Mars orbit, and the basic transfer vehicle could be reused, but only if you do not jettison the empty modules.

Myself, I would use the NERVA design that almost flew for this, not Timberwind, which never got that far through test. Later, as better nuke engines become available, you could replace the NERVA's. But why throw away hardware if you don't have too?

That sort of transfer begs the question of propellant supply at both ends, so that the vehicle may return for reuse. For nuke rockets, hydrogen is usually considered, which is fairly easily made from water using solar PV, if the rate of production required is not too high. Both Mars and Earth have lots of water, as ice on Mars, both liquid and ice here.

Why not shoot ice payloads almost naked into LEO and LMO with light gas gun technology? That technology is powerful enough now to work on Mars, and almost ready to do that job with small payloads to LEO. So the payloads are small? So what? Use solar thermal to melt, and solar PV to electrolyze, in both LEO and LMO. As long as the individual transfer flights are many months apart, you don't care that the batch process is small scale and slow. Your propellant reserves build up over that time interval. It can be automated. Product is both hydrogen and oxygen, items very valuable.

Single stage one-way nuke transfer by min energy trajectories, with fully reusable vehicles, means you build them and launch the fueled modules from Earth only once. After that, resupply is very cheap, once the light gas guns and solar propellant satellites are in place. This is not "battlestar galactica" stuff, either. The biggest one-way payload to ship to establish this capability is the light gas gun equipment to be landed piecemeal on Mars.

Now, this does also beg the question of equipment transfer from LMO to the surface at Mars. If piecemeal components can fit inside one-way Dragons rigged as landers, that's the start. But, if you have NERVA engine technology resurrected, that also supports one-stage fully reusable lander technology, using nothing more sophisticated than direct rocket-braking descent. These things could carry a big load fueled for a two-way trip from LMO. Check it out yourself at Isp near 1000 sec, engine T/W around 4, vehicle structural fraction 20% to be tough as an old boot, and 70% propellant fraction to be fueled for up to 30 degrees out of plane, full rocket delta-vee two-ways. I got a 10% payload fraction.

If you make hydrogen both in LMO and on the surface, you could carry far heavier payloads fueled only one-way like that.

The more desirable follow-on to NERVA would be one of the gas core concepts, but that's future stuff. We could do the NERVA thing right now, but we need to pick the brains of those who did it for their engineering art. There are very few of those guys left. That was 4 decades ago they did that.

If it turns out there's significant ice inside Phobos, so much the better. No light gas on Mars is necessary.

It takes a while to bootstrap into a setup like that, but if you plan for it from the outset, the right choices can be made along the path. There's a real sustainable transport system to Mars. Simple, direct, and launch things only once (keep on using hardware once launched).

GW

Last edited by GW Johnson (2012-04-14 11:00:36)

louis · 2012-04-15 09:13:23

clark wrote:

We all realize how far away from being possible this all really is, right?
Right?
Of course, I have always been the crazy one.

Stop being so gnomic Clark - can't you state what you mean clearly?

AS far as I am concerned with Musk we could get there in 10 years with the right investment.

louis · 2012-04-15 09:14:42

SpaceNut wrote:

Large payloads of 20 plus landing on mars still has not been solved...but still 4 launches to get the 20 plus to mars, is still alot of launches to get in a 2 month window....for each unit for leg of use

Surely we could manage 2 x 2 launches (i.e. using two separate sites).

louis · 2012-04-15 09:23:28

The technology is obviously of supreme importance, but I think we have to see it in context:

1. There are many opportunities to offset the costs with a range of commercial schemes including meteorite/regolith collection and sale; art installations; commercial sponsorship; mining of precious metals; conducting science experiments; sale of TV and film rights; and manufacture of lightweight luxury goods such as jewelry and watches (not necessarily the whole process - perhaps just finishing on Mars with some Mars materials).

2. Once the Mars economy becomes self-sustaining ,the people of Mars can afford to subsidise the costs of transit, in particular through their own labour and material input in terms of building rocket hardware on Mars. A colony of 10,000 could probably build its own rockets and transit habs subject to importation of some specialist computer and material parts.

Russel · 2012-04-15 15:55:21

I think if this "getting to mars and back safely" problem can be solved in a way that everyone can see is optimal, then we've got to start putting on the table exactly what principles we are working with, use every physical trick we have at our disposal, and exhaustively (and methodically) work through the space of all options.

And indeed a lot of the tricks (or tools) being used are about saving fuel. Which in and of itself is a reasonable goal.

It has to be said though, that as launch costs go down, it gets harder to justify doing some things that may save some fuel. Or put another way it gets harder not to use fuel where it would either add to safety or save money in avoiding complexity or development costs.

Presently we're talking about potential near term costs in the order of $2,500 per Kg into LEO. That figure might go down to $1,000 if Musk has anything to do with it.

This crew are promising $500, specifically for fuel

http://quicklaunchinc.com/

Even if you work with the more conservative figure then 100 tonnes of fuel costs $100M. Yes, ok, I'd love that for pocket money, but it starts to become a second order problem relative to all the development costs and hardware involved. Yes in the decades to come we're going to have to get better at it still (and options like fusion propulsion come to mind) but if we're talking about exploration and about the near term (next 15 years) then I personally wouldn't worry too much about fuel. I'd simply not want to waste it.

Now about the architecture. Obviously, every bit of mass you don't have to re-accelerate is a bonus. More importantly though is this. Every bit of mass you don't have to get back off Mars is even better. Hence my allergy to huge earth return vehicles with engines sized to get all that fuel off the planet all in one go.

As regards to the starting post on this thread, we save on energy by having the "bus" not capture into Mars orbit and thus enjoying a free return. Fair enough. But remember the energy saved is only on the mass of the (nearly) empty bus itself - which I take has been built fairly lightly to start with.

We then lose in terms of the safety issues from "missing the bus". Yes, it can be done if you have to. But you are adding risk and then begging the question "can it be done better?"

Another issue is that yes, you can save fuel by having a low mass space habitat. But there is a lower limit - because mass equals shielding. Now, I've also thought about arranging tankage to provide extra shielding - and that in itself is a good idea. But the problem there is you can only rely upon the shielding of the relatively thin walls of the tankage. The fuel itself would provide good shielding, but the problem is, you use the fuel up as you go. So again, I think that sets a fundamental minimum to the mass of the space hab.

That isn't to say though that you can't engineer a lighter space hab with the same level of radiation protection simply by making it smaller. Its just that it gets rather cosy as you do..

We still have unresolved issues to do with EDL. And I'd be worried about reclaiming parachutes. You'd have to retract it in a relatively short time whilst still in the air, and maintain your vertical descent enough to avoid it folding on top of you. Not a big problem. But again, if you apply the principle of total reusability you can get yourself tied in knots - reminds me of the folks trying to design the Plastiki.

Ok, now for a concrete proposal of my own. Feel free to be be mean.

My architecture revolves around two basic vehicles. Each has two slightly different forms. The first of these is the key to the whole system.

Its a vehicle designed for ascending from the surface of Mars into orbit, taking with it a cargo. There are two versions of this vehicle. One manned. One purely carries fuel. But there is a large degree of commonality to design and interoperability. The manned version is your Mars landing/ascent capsule. Its also your Earth landing capsule The unmanned version is a fuel ferry. You'll need at least two of these for the sake of redundancy.

The main feature of the fuel ferry is that its able to transport processed methane/lox to Mars orbit a few tonnes at a time. So refueling the following vehicles typically requires multiple trips. One thing we do have on our side, is time.

The second vehicle again has two basic forms but its basically a methane/lox rocket. The common elements are the engine, tankage and heat shield. One version carries the space hab. This version also docks with the manned landing/ascent capsule. The other version is simply a tractor for ferrying unmanned items. Again we have commonality of design and interoperability. We also have only that mass of tankage and engine that is sufficient to the task.

As the manned version of this vehicle approaches Mars, the crew enter the landing/ascent capsule, undock and land. The vehicle/space hab pair aerocapture into orbit. Then the vehicle refuels. On approach to Earth the same procedure applies. So this vehicle is fully reused.

The unmanned vehicle basically follows the same process, cycling between the two planets and picking up fuel in Mars orbit. This vehicle also has the job of transporting the hydrogen tank (for fuel processing) and when this happens, the hydrogen tank remains attached to the vehicle and shares the aerocapture process. This means you can recycle the hydrogen tank - if it makes sense to do so.

Depending on how you design the Mars habitat and optimise, you may need 2 or 3 tractors to establish your presence on Mars. When you're finished you have all the expected gear on the surface, including habitat, power supplies, rovers, and finally the 2 fuel ferries and possibly a spare landing/ascent capsule (yes I love redundancy).

One of those tractors carries the initial fully fueled hydrogen tank. There is also an unmanned flight of the space hab. All of these vehicles are refueled in orbit. So at most you need 5, but possible 4 of these vehicles. And they're all essentially the same platform.

Before the first manned mission everything is checked out, fueled and functional. There is a spare, fully fueled space hab/return vehicle in Mars orbit. There are two fully functional fuel ferries. There is even a spare landing/ascent capsule waiting on Mars. Yes, all of this probably means having to spend 4 years from the word go to the first manned flight - due to the rate at which you can produce fuel.

The first manned mission proceeds as you would expect, on a fast trajectory, transferring to a capsule and then landing. The space hab they traveled in now becomes a spare in orbit.

When the crew ascend into Mars orbit they claim the original space hab. That then makes its journey to Earth. The second space hab is now the spare and it gets refueled.

And so the cycle repeats..

As you can see, what I have proposed is nearly fully recyclable.. to the limits of endurance of the hardware involved.

It is possible to leave a large enough reserve of Hydrogen in orbit around Mars that it will serve ongoing missions. A larger tank is also more efficient in terms of volume to area ratio. So you also have a buffer against unexpected losses and less leakage/evaporation. By making this tank large you also make it possible for the tank to serve several missions - potentially half a dozen. If it fails, then you have the reassurance that at any given time there is always another fully fueled space hab vehicle also in Mars orbit.

The humans enjoy the fact that the only thing that flies with the humans is the space hab, the capsule, the minimum mass of tankage and engine, and only enough fuel to do TMI.

That's about as optimal as it gets.

Indeed, you can further optimise, by giving the space hab a fairly high orbit around Mars. This means you've got a lower energy trans Earth injection. So you can actually engineer a faster flight home if you wish.

It leaves you with abort options at every stage. The possibilities for repair and improvisation are many.

Now if you're a fan of electric propulsion you've got the option of using that for the unmanned transport. Likewise for nuclear. The point is that by optimising the manned part of the flight (which is what the original poster does) you're taking away a lot of the motivation for high Isp engines. Its down to the cost of fuel and I don't think 100 tonnes or so is too much to pay for safety and simplicity in that part of the mission. I personally don't mind nuclear, but I'd sooner trust my life to a bog standard chemical rocket than to a fairly new nuclear design (and its not radiation - its reliability)

I leave others to do the math, but I think that's fairly optimal - and elegant - and safe. And I'm sure it can be improved. Have fun!

And yes, I've skated over some EDL issues. Bite me

MatthewRRobinson · 2012-04-15 18:40:40

Russel wrote:

I think if this "getting to mars and back safely" problem can be solved in a way that everyone can see is optimal, then we've got to start putting on the table exactly what principles we are working with, use every physical trick we have at our disposal, and exhaustively (and methodically) work through the space of all options.
And indeed a lot of the tricks (or tools) being used are about saving fuel. Which in and of itself is a reasonable goal.
It has to be said though, that as launch costs go down, it gets harder to justify doing some things that may save some fuel. Or put another way it gets harder not to use fuel where it would either add to safety or save money in avoiding complexity or development costs.
Presently we're talking about potential near term costs in the order of $2,500 per Kg into LEO. That figure might go down to $1,000 if Musk has anything to do with it.
This crew are promising $500, specifically for fuel
http://quicklaunchinc.com/
Even if you work with the more conservative figure then 100 tonnes of fuel costs $100M. Yes, ok, I'd love that for pocket money, but it starts to become a second order problem relative to all the development costs and hardware involved. Yes in the decades to come we're going to have to get better at it still (and options like fusion propulsion come to mind) but if we're talking about exploration and about the near term (next 15 years) then I personally wouldn't worry too much about fuel. I'd simply not want to waste it.

A very interesting concept. It'll be interesting to see what becomes of it. But, I'm somewhat cynical. Wiki says some guns have gotten up to 11 km/s, and that's pretty surprising. But aerodynamic drag goes by the velocity squared, and currently rockets that leave the ground with zero speed and accelerate from 2-4 g's in the atmosphere, suffer something on the range of 2 km/s losses from aerodynamic drag. That's while traveling below 1 km/s.

IIRC, some early rockets (The Mercury-Atlas?) experienced a max Q around 10,000 pounds per square foot, at 6 G's liftoff.

So, the pressures they'll be working with for max Q, with the velocity they'll need to overcome aerodynamic drag, will both be extremely extravagant. Granted, an ice cone with a SRB on the back might be able to take a lot.

It'll be interesting to see how this technology goes... While I think there's a fair chance it'll work, I wouldn't be depending on it.

Russel wrote:

Now about the architecture. Obviously, every bit of mass you don't have to re-accelerate is a bonus. More importantly though is this. Every bit of mass you don't have to get back off Mars is even better. Hence my allergy to huge earth return vehicles with engines sized to get all that fuel off the planet all in one go.
As regards to the starting post on this thread, we save on energy by having the "bus" not capture into Mars orbit and thus enjoying a free return. Fair enough. But remember the energy saved is only on the mass of the (nearly) empty bus itself - which I take has been built fairly lightly to start with.

The point, actually, isn't to save the mass of the dry bus, but to save the expensive hardware that makes up the bus.

Russel wrote:

We then lose in terms of the safety issues from "missing the bus". Yes, it can be done if you have to. But you are adding risk and then begging the question "can it be done better?"

Like I said, the ERV would have enough supplies to return on low rations, having been refilled from the surface habitat. Upon arriving to Earth, the dragon capsule portion could separate and re-enter. Loss of ERV, but crew, capsule, and Bus all still survive.

Russel wrote:

Another issue is that yes, you can save fuel by having a low mass space habitat. But there is a lower limit - because mass equals shielding. Now, I've also thought about arranging tankage to provide extra shielding - and that in itself is a good idea. But the problem there is you can only rely upon the shielding of the relatively thin walls of the tankage. The fuel itself would provide good shielding, but the problem is, you use the fuel up as you go. So again, I think that sets a fundamental minimum to the mass of the space hab.
That isn't to say though that you can't engineer a lighter space hab with the same level of radiation protection simply by making it smaller. Its just that it gets rather cosy as you do..

I think the need for radiation protection is rather over-rated. IIRC, the exposure for even a 6-month journey would be something that would increase your chance of cancer by a small percentage in the later years of life. In fact, I was even thinking of doing away with the whole Bus' design and just using a single propellant tank. Because the real threat is CME's, and other such solar storms. Those can release enough radiation to kill. So, use a single propellant tank slightly wider than the habitat, and point it, and the engine assembly, at the sun. Free enormous radiation shield. Of course, inside the habitat you also use the supplies (food, water, etc) to make a sort of radiation shelter to further increase protection.

Russel wrote:

We still have unresolved issues to do with EDL. And I'd be worried about reclaiming parachutes. You'd have to retract it in a relatively short time whilst still in the air, and maintain your vertical descent enough to avoid it folding on top of you. Not a big problem. But again, if you apply the principle of total reusability you can get yourself tied in knots - reminds me of the folks trying to design the Plastiki.

I think I'll drop that idea altogether...

Russel wrote:

Ok, now for a concrete proposal of my own. Feel free to be be mean.
My architecture revolves around two basic vehicles. Each has two slightly different forms. The first of these is the key to the whole system.
Its a vehicle designed for ascending from the surface of Mars into orbit, taking with it a cargo. There are two versions of this vehicle. One manned. One purely carries fuel. But there is a large degree of commonality to design and interoperability. The manned version is your Mars landing/ascent capsule. Its also your Earth landing capsule The unmanned version is a fuel ferry. You'll need at least two of these for the sake of redundancy.
The main feature of the fuel ferry is that its able to transport processed methane/lox to Mars orbit a few tonnes at a time. So refueling the following vehicles typically requires multiple trips. One thing we do have on our side, is time.
The second vehicle again has two basic forms but its basically a methane/lox rocket. The common elements are the engine, tankage and heat shield. One version carries the space hab. This version also docks with the manned landing/ascent capsule. The other version is simply a tractor for ferrying unmanned items. Again we have commonality of design and interoperability. We also have only that mass of tankage and engine that is sufficient to the task.
As the manned version of this vehicle approaches Mars, the crew enter the landing/ascent capsule, undock and land. The vehicle/space hab pair aerocapture into orbit. Then the vehicle refuels. On approach to Earth the same procedure applies. So this vehicle is fully reused.
The unmanned vehicle basically follows the same process, cycling between the two planets and picking up fuel in Mars orbit. This vehicle also has the job of transporting the hydrogen tank (for fuel processing) and when this happens, the hydrogen tank remains attached to the vehicle and shares the aerocapture process. This means you can recycle the hydrogen tank - if it makes sense to do so.
Depending on how you design the Mars habitat and optimise, you may need 2 or 3 tractors to establish your presence on Mars. When you're finished you have all the expected gear on the surface, including habitat, power supplies, rovers, and finally the 2 fuel ferries and possibly a spare landing/ascent capsule (yes I love redundancy).
One of those tractors carries the initial fully fueled hydrogen tank. There is also an unmanned flight of the space hab. All of these vehicles are refueled in orbit. So at most you need 5, but possible 4 of these vehicles. And they're all essentially the same platform.
Before the first manned mission everything is checked out, fueled and functional. There is a spare, fully fueled space hab/return vehicle in Mars orbit. There are two fully functional fuel ferries. There is even a spare landing/ascent capsule waiting on Mars. Yes, all of this probably means having to spend 4 years from the word go to the first manned flight - due to the rate at which you can produce fuel.
The first manned mission proceeds as you would expect, on a fast trajectory, transferring to a capsule and then landing. The space hab they traveled in now becomes a spare in orbit.
When the crew ascend into Mars orbit they claim the original space hab. That then makes its journey to Earth. The second space hab is now the spare and it gets refueled.
And so the cycle repeats..
As you can see, what I have proposed is nearly fully recyclable.. to the limits of endurance of the hardware involved.
It is possible to leave a large enough reserve of Hydrogen in orbit around Mars that it will serve ongoing missions. A larger tank is also more efficient in terms of volume to area ratio. So you also have a buffer against unexpected losses and less leakage/evaporation. By making this tank large you also make it possible for the tank to serve several missions - potentially half a dozen. If it fails, then you have the reassurance that at any given time there is always another fully fueled space hab vehicle also in Mars orbit.
The humans enjoy the fact that the only thing that flies with the humans is the space hab, the capsule, the minimum mass of tankage and engine, and only enough fuel to do TMI.
That's about as optimal as it gets.
Indeed, you can further optimise, by giving the space hab a fairly high orbit around Mars. This means you've got a lower energy trans Earth injection. So you can actually engineer a faster flight home if you wish.
It leaves you with abort options at every stage. The possibilities for repair and improvisation are many.
Now if you're a fan of electric propulsion you've got the option of using that for the unmanned transport. Likewise for nuclear. The point is that by optimising the manned part of the flight (which is what the original poster does) you're taking away a lot of the motivation for high Isp engines. Its down to the cost of fuel and I don't think 100 tonnes or so is too much to pay for safety and simplicity in that part of the mission. I personally don't mind nuclear, but I'd sooner trust my life to a bog standard chemical rocket than to a fairly new nuclear design (and its not radiation - its reliability)
I leave others to do the math, but I think that's fairly optimal - and elegant - and safe. And I'm sure it can be improved. Have fun!
And yes, I've skated over some EDL issues. Bite me

Interesting... Kind of like Mars Direct and the OP architecture's ERV's. Except, instead of refueling on the surface, you're using spacecraft in orbit, that take the role of the ERV's.

I don't see where the mass saving is in doing that, though. The propellant for TEI is still gained via ISRU on Mars, in both cases. Merely the architecture means you don't need ISRU propellant to lift the mass of the Mars Transit Vehicle from Mars surface to MO. But the propellant for doing that in any case is provided via ISRU, and it would probably cost a lot more total mass, actually, because you have to lift the ferry from the surface to MO, land it, and repeat, probably quiet a number of times.

My own OP, I think, needs some heavy revision. For one thing, the idea of using boosters akin to a F9 RLV upper stage - with a Falcon Heavy payload of 36 tons - is just genius. Only upon completion of the the final booster's TMI burn is the assembly on an escape trajectory from Earth, so all earlier separated boosters are in a highly elliptical orbit. They aerocapture and return, just like the F9 RLV upper stage. The final booster essentially becomes my original "bus", except instead of propulsive EOI after free return, it can do an aerocapture. And instead of Timberwind NTR's, it can use LOX/LH2.

Want a higher energy trajectory? Just launch an extra booster. It may require that the next-to-last booster not separate, though (so essentially the final booster is just a propellant tank).

What I love is the sheer simplicity of it. It's almost as simple as Mars Direct, except all the hardware is reusable. It's the best of both worlds.

Last edited by MatthewRRobinson (2012-04-15 18:42:45)

Russel · 2012-04-15 21:26:59

Ok, I'll probably have to break this up into bits..

Regarding the space-gun concept, I think they've got a pretty good handle on the physics. Whether they've got a good handle on the costs is another issue. But I'm also confident that you can bring the cost of fuel down to under $1000/Kg even with conventional rockets - if you can get the reusability working.

About saving on expensive hardware. Its kind of inevitable that space hardware is going to be expensive so you might as well design it to last and then keep reusing it. I think you've captured that idea well. I'm trying to do the same. This of course raises the issue of how do you reliably predict wearout - and how do you design for serviceability. If you want the "space hab" part of the enterprise to work as a reusable item you have to design it to be easily stripped down including the plumbing for the toilet.

I toyed with the idea of making elements free-return and every time I thought about it I didn't like it when it came to human safety.

Come to think of it, a free return approach could be used for some of the unmanned deliveries - that would save on Mars-produced fuel.

I actually hit upon the idea of making the fuel ferry as small as reasonably possible just yesterday. And when I came to write it up it hit me that the ideal size is such that it shares components with the landing/ascent vehicle. There's actually a thought process behind this.

Originally I was simply going like this. Ok, so you want to get to Mars. Well, you need a space hab. Then you need propulsion. Why not use the same propulsion to make a one stop trip to the surface to ferry hydrogen down and then return with fuel. That's where I started from. Then I realise you end up getting tied in knots figuring out which bit takes what path and what connects to what. But the thing that kept showing up in the basic calculations is that anything you design to take off from Mars with a fuel load sufficient to get anyone home, direct or via rendezvous in orbit, has to have more thrust (and thus more engine and tankage mass) than you'd need anywhere else in the mission. My initial solution was to make this beast a prisoner to the Mars surface/orbit and then I ended up with a separate engine etc for the actual TMI/TEI parts. Problem is, I hated the sheer bulk of the thing. So that's the thought process behind this.

I don't know enough about the radiation issue. I'm an engineer, not a physicist.

I asked this question in the landing thread and RobS replied that solar storm radiation is anisotropic. I had a bit of a look around on the net and its not exactly clear cut. A lot of research, but nothing definitive. My gut feeling is this. You'd get some benefit from focusing some mass in the direction of the sun. Probably stuff like the heat shield. However what I'd feel confident about is a space hab with a conventional aluminium shell but sandwiched with a couple of cm of polyethylene composite. Then I'd build the sleeping quarters as cosy as possible and then wrap that in the water storage tank. And layer things from that. I doubt it could ever be a 10 tonne structure, not with all the supplies, life support etc. But you could easily keep it down to (say) 25 tonne. Yes, I know, every Kg counts.

I'm still not sure what I'd do regarding EDL on the fuel ferry but I have my wild untested ideas. Obviously, its got a conventional heat shield. Obviously the heat shield is scaled to suit the human version of the same craft. Meaning that its over-sized for the fuel ferry in its landing phase. That helps a bit. I don't mind a small throw away supersonic chute if that helps to get you into the realms of a propulsive landing. Of course then you need robotically reattach a new one with every trip.

My hope is that the theory is correct that the propulsive thrust under conditions more like Mach 2.5 can be solved. Its easily tested. Wait for the Dragon to acquire its launch abort system and put it into several test re-entries, and then fire the super-dracos at successively higher and faster conditions. Chances are this experiment will have to be run by them in any case.

If you want to go hairier, one thought is basically an extension of the fixed heat shield concept, but above the craft - basically a circle of mesh. Or yet another air brake if you wish. Again, enough to put you safely into the realms of propulsive landing. You'd have to design it to present less drag going up, of course. But this isn't as big an issue in the martian atmosphere.

.. pause for lunch...

Russel · 2012-04-15 21:44:29

"I don't see where the mass saving is in doing that, though. The propellant for TEI is still gained via ISRU on Mars, in both cases. Merely the architecture means you don't need ISRU propellant to lift the mass of the Mars Transit Vehicle from Mars surface to MO. But the propellant for doing that in any case is provided via ISRU, and it would probably cost a lot more total mass, actually, because you have to lift the ferry from the surface to MO, land it, and repeat, probably quiet a number of times."

Its not really intended to save Mars-derived propellant. In fact it probably requires somewhat more. What it does save is on development costs and time and emphasises safety. You'v got numerous abort options and one thing I really like is designing for the situation where what you're trying to do isn't in the manual.

As for the comments about boosters and the final line.. Can you explain more?

I'm happy to answer more specific questions too.

Russel · 2012-04-15 21:49:26

I should add that part of the mass saving is in keeping the vehicle that takes fuel to orbit (or beyond) low mass. And that mass is delivered essentially only once - in the preparatory flights. What you gain is more headroom and the freedom to have more redundancy. As I said initially I'm not a fan of saving fuel at all costs - simply not wasting it.

RobS · 2012-04-15 22:06:18

I'm not convinced that nuclear engines are necessary, considering the utter irrationality of the public and the cheapness of Falcon Heavies, if they indeed are reusable and work as Musk hopes. He was talking about returning boosters, refueling them, and launching them again on the same day! I find that kind of turnaround time hard to believe. But even if you have to check them out and do minor repairs over a few weeks, that's still pretty good turn around. And if the turn around is slower than that, you stick to RP1 and LOX, which has no significant boiloff issues over 6 months or a year. Surely you could launch 3 boosters and a payload over that time frame.

Furthermore, solar rockets have already been tested in the lab (using either electric heaters or intense lights on a small scale) and we know the concept will work. We know we can scale it up to 50 or 100 pounds of thrust for 10-15 minutes at perigee. We know the engines and mirrors are extremely light in weight. We know the Isp is 900 seconds or more, like solid core nuclear. You don't have hydrogen boiloff issues because you are tapping your hydrogen tank every few hours at perigee for propulsion. You can push your payload to almost earth escape velocity with maybe half a tonne of propellant for every tonne of payload pushed, and then you jettison the solar rocket and its big, empty tanks and use a small hydrogen/oxygen rocket for the remaining trans-Mars injection (0.5 km/sec Hohmann, 1 km/sec for a 6-month trajectory). That'll work just fine for cargo and would be cheap. For people, you push as much of the mass to near-escape this way as possible, then fly the crew up quickly via chemical rocket and execute TMI.

So, a solar thermal rocket can be developed for pennies, compared to solid core nuclear, can do 80% of the work, and involves no political fallout. Why pursue something difficult and expensive?

As for what to send to Mars, Dragon is being discussed because it is being developed, but I think it is too small, and Falcon Heavy has a 5.2 meter fairing anyway (Dragon's 3.6? meters; smaller). So I'd scale up the Dragon for interplanetary use. That's what I called a Griffin or Gryphon (cooler spelling; an equally mythical creature to a dragon, but one that can fly). A conical capsule 5.2 meters in diameter and 6-7 meters high has 50 to 60 cubic meters of volume, much better than the 10 of the Dragon, easier to accommodate people and cargo. The space shuttle had 68 cubic meters and could accommodate 7 for two weeks. You'd still need an inflatable, but you'd have room to put it somewhere when you deflated it, and a three-level Gryphon capsule would not have to be completely depressurized; you could just depressurinze the nose cone or the middeck for retrieving the inflatable, performing space walks, etc., while your non-EVA crew, along with the vegetable garden or the rabbits (assuming you were bringing some plants and animals to Mars) would remain in a pressurized environment. The Gryphon is about half the volume of Mars Direct's ERV, but bigger than a tiny capsule. I'd create three versions:

1. Bloc-1: Full 3-level capsule for interplanetary cruise, with a small methane/oxygen kick stage for trans-Earth-injection attached to the bottom and a light-weight inflatable hab for private cabins, lounge, exercise area; leave the heavy stuff, life support, kitchen, waste disposal, in the capsule lower level.

2. Bloc-2: A Mars descent and ascent version, where the lower level accommodates extra tankage for hydrogen feedstock down and methane/oxygen fuel back up; the middeck is mostly cargo storage; and the nosecone is the small crewed space for landing and ascent to the Bloc-1 earth return Gryphon.

3. Bloc-3: Later, once fuel is being made on Mars and on Phobos or Deimos: the three-level version would be used with tanks under it large enough to accommodate a launch from the Martian surface to orbit, which would also be roughly the right size for TMI. You'd launch it from Earth, fuel it in orbit, empty the tanks during TMI, aerobrake into an elliptical orbit, refuel with enough propellant to land, refuel there and return to an elliptical orbit, refuel for TEI, return to earth, aerobrake into orbit, and repeat. For a 20-tonne vehicle you'd need 50 tonnes of methane/oxygen storage for TMI and MOI, maybe 8 tonnes for Mars landing, and maybe 15 tonnes for TEI.

There might be a Bloc-4 version as well where the interior is completely stripped so that it serves as a straight cargo lander.

Fuel in Mars orbit: I'd harvest Phobosian and Deimosian chondrite. Chondrite is a certain percentage water and carbon (I haven't looked it up lately, but Phobos and Deimos may be lower in volatiles anyway). Alternately, a Gryphon stripped of everything in its capsule but with 50 tonnes of fuel in its tanks could probably launch into Mars orbit enough fuel to refuel a second Gryphon with crew. The other possibility is to make the Gryphon's fuel tanks big enough so the vehicle can launch straight to Earth. The delta-v required is about the same as is required of the second stage of many rockets (i.e., after the shuttle's solids dropped off, or the Saturn V's first stage dropped off).

The other thing I'd do, once we have a Mars transportation system developed, is to set up a destination in Mars orbit. Arriving vehicles would stop there to refuel, as would departing vehicles. It would store emergency supplies, have an inflatable ready and available. Consumables for the return trip to Earth would arrive there via Hohmann transfer and await arrival of the returning crew. The main module there would have 1 or 2 Canadarms for grabbing cargo. It'd have docking ports, too. The orbit could be a 24.6-hour elliptical orbit, periapsis 400 km, apoapsis something like 30,000 km. It could also be a Phobos transfer orbit or a Deimos transfer orbit. The equivalent arrival/departure facilty at the Earth/Moon lagrange point was called "Gateway" by Michael Duke. In a novel the Mars equivalent I called "Embarcadero" and the Mercury one at the sun/Mercury L2 polint I called "Portal." You get the idea.

Last edited by RobS (2012-04-15 22:12:10)

Russel · 2012-04-16 02:55:13

Just for fun, try putting "aneutronic fusion propulsion" into google.

Russel · 2012-04-16 03:09:09

Incidentally, I ran the numbers and a conventional dragon capsule would be unsuitable as a fuel ferry. Essentially you need to build something either a lot lighter, or larger, or both.

I'm still convinced of the need for inter operable components though.

Russel · 2012-04-16 03:21:40

I know it sounds a bit off-the-planet (well, it is actually).. but that gas gun concept (for launching fuel).. I wonder if you could build that on Mars, and if so would that buy you a bunch of optimisations. Less air resistance. Less orbital velocity. Less gravity acting its structure..

louis · 2012-04-16 14:20:11

Russel wrote:

I know it sounds a bit off-the-planet (well, it is actually).. but that gas gun concept (for launching fuel).. I wonder if you could build that on Mars, and if so would that buy you a bunch of optimisations. Less air resistance. Less orbital velocity. Less gravity acting its structure..

Sounds much more likely to succeed on Mars. Should definitely be looked into.

I think the Armadillo style simple "gas cylinder" craft might also work on Mars.

MatthewRRobinson · 2012-04-16 21:16:32

Russel wrote:

"I don't see where the mass saving is in doing that, though. The propellant for TEI is still gained via ISRU on Mars, in both cases. Merely the architecture means you don't need ISRU propellant to lift the mass of the Mars Transit Vehicle from Mars surface to MO. But the propellant for doing that in any case is provided via ISRU, and it would probably cost a lot more total mass, actually, because you have to lift the ferry from the surface to MO, land it, and repeat, probably quiet a number of times."
Its not really intended to save Mars-derived propellant. In fact it probably requires somewhat more. What it does save is on development costs and time and emphasises safety. You'v got numerous abort options and one thing I really like is designing for the situation where what you're trying to do isn't in the manual.
As for the comments about boosters and the final line.. Can you explain more?
I'm happy to answer more specific questions too.

I, too, like the idea of using components common to multiple vehicles. But one thing, is, though, that the only major difference in-between a larger craft and a smaller craft would be tankage and structural components. You could use the same type of engines, for example, just use more of them. Avionics, ECLSS, powerplants, etc. could all still be common to both. The vehicles have enough differences already so that they can't be the same type of vehicle, like Endeavor and Discovery, don't they? What differences do they have?

As for the boosters, it's really the simple architecture RobS mentioned in the reply to the OP, and I built on it. A Falcon 9 RLV lifts 60% the payload of a non-RLV Falcon 9. Applying a very similar number to the Falcon 9 Heavy (F9H), and we assume the F9H RLV can lift 32 tons to LEO.

Make multiple boosters, all identical. They're very similar in design to the upper stage of a Falcon 9 RLV, Pica-X heatshield, can return and land on Earth, and they have a dry weight ~4 tons or less. They don't need significant thrust, and they have docking rings that can connect them to eachother and can quickly separate.

You launch 2-3 boosters, depending on how much umph you want (low energy, high energy, small payload, larger payload) using F9H RLV's, then you launch the payload to be launched TMI, and the whole thing docks together as an assembly.

The boosters perform TMI in a staged manner, separating upon burnout. All of the boosters, except the final one, will be in a highly elliptical orbit around Earth. With a small delta-vee at apogee, either immediately or after a few orbits, they can re-enter Earth's atmosphere and land just like the F9 RLV's second stage.

For the final booster on TMI; you have two options. If Mars needs a booster, then you have it land on Mars with the payload. If not, then it performs a Free-Return and re-enters and lands back at Earth.

That's how to launch stuff to Mars.

Now, for what "stuff" you launch at Mars, I need to do a little more research on that.

When doing ISRU, what fraction of the final propellant produced on Mars do you need to carry in LH2 from Earth?
(Ex, 6 tons from Earth: 30 tons on Mars. What's the real ratio?)

Russel wrote:

I toyed with the idea of making elements free-return and every time I thought about it I didn't like it when it came to human safety.

What's wrong with human safety on free-returns?

Russel · 2012-04-17 08:07:04

"What's wrong with human safety on free-returns?"

Well, it goes like this. If you have a place of safety both on Mars and in orbit, you're in a very good position when things go wrong. That would be my ideal.

Connecting up with something that is already on a free return trajectory means accelerating to Mars escape velocity. And it probably also means having little remaining fuel. So you've committed yourself to deep space.

Now, I've got very little problem with the task of actually docking. Or put it this way, provided something else doesn't go wrong, I trust the navigation system more than anything else.

But what can go wrong? Well, propulsion failure of one sort or another. Mostly this will put you into a high orbit. And maybe you can recover from that. But there is a small window into which you can put yourself into Mars escape but then fail to dock. Then you're on a free return trajectory that most likely will come somewhere near Earth but someone will have to go send a rocket and some duct tape for you to manage to recover.

There are of course worse failures.

I don't mean to argue too much here.. I'd like to see a free return idea work but for me to feel comfortable with it, you'd have to show me a rocket that's simple, has been tested on Mars itself, and is obviously reliable. Probably meaning you've taken some compromises in performance too.

Oh, and the other thing about free returns is the kind of failure where nothing goes wrong - its just that you've failed to launch into a tight window. That to me is more likely.

With a target that's orbiting, you get a second chance.

Oh btw.. Perhaps if you're looking for a suitable compromise its putting a vehicle into high orbit - even a week long one if you wish. Deimos transfer orbit might be worth looking at - that saves you half your DeltaV over low Mars orbit. I actually had that in the back of my head earlier.

Also, you can have hybrids where at some stage you capture into low orbit for convenience of refueling, but then use electric thrust to get you to a high orbit. Like I said, one thing we do have is time.

Russel · 2012-04-17 08:18:21

Speaking of commonality of components, I'm wondering how much commonality there can be between those engines that have to climb out of Mars's surface, and those that only need less acceleration that start from orbits.

As for my fuel ferry idea.. I'm still working on that. The more I think about it the more I realise that the problem is dry vehicle mass relative to total payload (fuel used to launch plus fuel delivered). Ergo, its got to be a pretty lean, simple vehicle - unlike the manned ascent capsule. But still I hope for some commonality.

MatthewRRobinson · 2012-04-17 20:53:11

Russel wrote:

Well, it goes like this. If you have a place of safety both on Mars and in orbit, you're in a very good position when things go wrong. That would be my ideal.

That thought crossed my mind, but then again, it's not like you can take it home unless it's during a return-launch window, so it's still not that much different from a vehicle that's only there during the window. Though there is an argument to having an extra life-support system nearby, but even to that, can an orbiting vehicle support the entire crew for additional months than the months already needed for Mars transit? At some point you're going to have to accept some risk, and I think having so many backup life-support systems is past that point. We already have a rover, a HAB, and at least a MAV on the surface, assuming there's not an extra MAV filling it's tanks with ISRU for next mission. That's 4x the options Apollo had, granted it's a much longer stay, and I don't think 5 life support systems has so much advantage over 4 as to warrant two additional very costly delta-vee maneuvers (MOI, TEI), 5/4 is long past the point of diminishing returns. The safety standard has been 3 systems, so I'd think 3 life support systems is optimal.

Russel wrote:

Connecting up with something that is already on a free return trajectory means accelerating to Mars escape velocity. And it probably also means having little remaining fuel. So you've committed yourself to deep space.
Now, I've got very little problem with the task of actually docking. Or put it this way, provided something else doesn't go wrong, I trust the navigation system more than anything else.
But what can go wrong? Well, propulsion failure of one sort or another. Mostly this will put you into a high orbit. And maybe you can recover from that. But there is a small window into which you can put yourself into Mars escape but then fail to dock. Then you're on a free return trajectory that most likely will come somewhere near Earth but someone will have to go send a rocket and some duct tape for you to manage to recover.

Unfortunately, I don't think it's really possible or practical to have a backup for a failure in-between escape velocity and TEI, or when the mission first launches, escape velocity and TMI. If you're on a course that will keep you from docking, then you're probably on a course that won't take you near Earth - that problem is just as present with or without free return, though. And really there's no possible "safety net" for a total MPS failure in that velocity window. Instead, we just make the MPS more trustworthy by doing things like using more than one engine (MRDM uses 3, a bit overkill IMO). One idea I like might be to have your RCS use the same propellant as your MPS - if the MPS fails, you can use RCS thrusters to finish the burn, granted it'll take much longer.

Or, like Dragon, just use your RCS in the first place. Using dozens of thrusters built with great endurance should be very safe.

Russel wrote:

There are of course worse failures.
I don't mean to argue too much here.. I'd like to see a free return idea work but for me to feel comfortable with it, you'd have to show me a rocket that's simple, has been tested on Mars itself, and is obviously reliable. Probably meaning you've taken some compromises in performance too.
Oh, and the other thing about free returns is the kind of failure where nothing goes wrong - its just that you've failed to launch into a tight window. That to me is more likely.
With a target that's orbiting, you get a second chance.

That, I think, is by far the best argument against free return, and the only one that has me really doubting it. I wonder what it takes to design the rocket to launch in a dust storm? How fine is the dust? Will the vehicle need extra shielding for it? When it comes to starting the engines, I'm sure you'd want to purge them first. But one very serious threat might be dust gathering in the combustion chambers of the RCS. Maybe you could cover them after the initial landing, then run a check and make sure they're clear before launch? Or maybe even make an RCS purge system...

That eliminates a dust storms keeping you from launching, but since those can last months, you may want that ability even if you're not doing a free return docking. The real issue, is a system on the MAV/ERV failing and the vehicle being unusable until the window closes. Most architectures I've seen have two such vehicles, though, so you could simply use the backup. And on top of that, any failure will probably mean that vehicle is out of commission, anyways. It's a small window where the failure is major enough that you can't launch with it, but minor enough so that it can be repaired.

And, worst to worst, you could just make the MAV/ERV capable of sustaining life for the entire trip back, just on low rations and small space.
To be fair, though, at that point there's really not much of a reason to make it dock with anything.

And also, to be fair, I admit it is possible a system could fail that could warrant a shut-down and restart which would make you miss the launch window, that would be very possible.

Russel wrote:

Oh btw.. Perhaps if you're looking for a suitable compromise its putting a vehicle into high orbit - even a week long one if you wish. Deimos transfer orbit might be worth looking at - that saves you half your DeltaV over low Mars orbit. I actually had that in the back of my head earlier.
Also, you can have hybrids where at some stage you capture into low orbit for convenience of refueling, but then use electric thrust to get you to a high orbit. Like I said, one thing we do have is time.

Rather than a hybrid (which would add a huge amount of complexity and cost to the vehicle), If I was going for MOI, then I'd just refuel it in orbit. As for your MAV ferry, I'd consider making it like the ERV - give it a much higher mass ratio than necessary, and use propellant from the main tanks to fill the MTV. Granted, though, that that re-fuelling in orbit incorporates a lot more risk into the mission.

~

On the topic of MAV+MTV v. ERV in general, one of the worst decisions I've seen was MRDM v5. Keeping the MTV in orbit saves mass on having to take the Mars Transit life support and cabin and having to land it and take off again. So I can see the point in that. But then, they decided that refuelling it in orbit would be unsafe, so they brought along the propellant from Earth, so overall that drastically increased the mission mass. That's just insane, to me.

You did have a fair point on capturing a free-return MTV; if a vehicle system fails before/during launch, then you miss the capture.

But if you keep the MTV in orbit, then you either
1) accept the fact the risk is greater, because of the many launches to refuel it in orbit
(The launches may not have to make a tight window, but we're talking about a spacecraft that can launch multiple times from the ground (not a pad), with absolutely no ground support; so it's impossible to inspect, refurbish, or service the vehicle to any significant extant, and there's no ground infrastructure to handle it, and if it fails once, then the entire mission is doomed without any possible abort/contingency.)
2) accept a drastically higher mission mass because it brought the propellant from Earth.

In either case, it seems facing this situation, the very best solution is just a big ERV that sits on the surface.

-You don't have to make any tight launch window, like with a free-return MTV
-You don't have to bring the propellant from Earth
-You don't have to take many risky launches to refuel it in orbit

So it either saves a lot of mass, or it makes the mission drastically safer.

I realize I started this post advocating a free-return MTV, but by now, upon looking at it in detail, I think the above points very strongly make an all-out surface ERV the very best case.

*An orbiting MTV with propellant from Earth drastically increases mission mass.
*An orbiting MTV refueled in orbit drastically increases mission risk.
-You could make the MAV capable of TEI, but then there's not much reason to have the MTV aside from showers and comfort; at that point, your MAV is almost as massive as a full-up ERV, anyways. Overall mission mass could probably be reduced at this point by dropping the MTV and making the MAV an ERV.
*A free-return MTV is also a lot more risky.

Overall, I just don't see anything better than an all-out ERV. It's either a little heavier but much safer, or much lighter and just as safe.

Russel · 2012-04-18 04:31:26

Ok, speaking of launch windows, its like this..

An Earth return has a launch window in the order of 2-3 weeks (depending on your reserves of fuel)

However, to meet with a free return vehicle, the window is decided tight. Perhaps less than an hour.

When you have an orbiting vehicle, the launch window is even shorter, but it repeats. If your orbiting vehicle is in an orbit of no more than a couple of days, that gives you a number of chances to make that window before the earth return window closes.

In answer to the next question, can an orbiting vehicle support the entire crew for months? Yes - if its big enough - which by definition it has to be. At least I think that's the question. If you mean what happens if something goes wrong and you miss your earth return? Well, then you can use up the supplies on the orbiting vehicle, then return to surface if you need to.

This is a part of the reason why I suggest leaving an orbiting spare. There's actually the orbiting space hab from the previous mission, plus the one you brought with you.

Finally you've got the option of return to surface - refuel - wait for another earth return - repeat. Again, having a big hydrogen tank up there is part of that.

I don't think that having a backup life support system (2 in orbit, one on the ground) is all that costly. The ground system is essentially a one-trip with spares. And the orbiting ones simply get juggled.

I guess you could count the life support in the capsule, but that's only going to get you a couple of days.

Hope that deals with that first paragraph. I have to admit I might have missed something.

As to reliability of propulsion in critical situations - yes you can make it pretty darn reliable. But as I said, I'd want to see it tested unmanned first.

As for risk taking. Sure. But see my comments above. If you choose to take a risk, you've got to prove its an essential risk. Nothing you can do about it. I think when I get more time we might adopt some common assumptions and then do the math.

I am sympathetic to just landing one big vehicle on Mars, and then later lifting off. My fear with that is the complexities of EDL. Meaning every time I think of it I end up wanting to land separately in a capsule. Also, I think one of the issues you run into on Mars is simply packaging. What do you stack on top of what and do you end up with cranes..

As far as refueling in orbit goes. Its a risk I wouldn't want to take with humans around. Rather, as I suggest, you do it robotically. Again this is part of the thinking with having a spare at all times, including the fuel ferry itself.

I'm not sure if we're at cross-purposes when we speak of hybrids. The hybrid I meant was a hybrid in terms of Delta-V budgets. In practice this means aerocapture of your return vehicle/space habitat into an orbit that suits refueling. But its easy to contrive an electric boost into a high orbit. Then when your manned ascent craft goes to dock in orbit, it has to use more Delta-V, but the much heavier vehicle then needs much less Delta-V to get back to Earth return.

Yes, I'm aware that having a fuel ferry that has to make multiple trips is asking a lot of its EDL gear and system reliability. So I'm kinda backing away from that and wondering how much propellant you can stack onto a relatively light (when landing) vehicle.

Also, I can't entirely rule out the idea of just using ISRU for surface purposes and achieving a vastly simpler system, simply at the expense of (maybe) another 150 tonnes or so of propellant in earth orbit. If so, I'd just build a vehicle that can get the crew to Mars and back, and strap on enough tanks. If so you can adopt a high orbit straight off.

Again, I think I need to do the math to get a better feel for it.

SpaceNut · 2012-04-18 21:08:32

Good run down MatthewRRobinson and Russel on the options for ERV (Earth Return Vehicle only parked in Mars Orbit), MAV (Mars Accent Vehicle not capable or Earth return) and MTV (Mars Transit Vehicle simular to the ERV but is used both way but still needs the MAV) ....

Liquid Hydrogen parked in mars orbit would need cryrogenic cooler system to reduce boiloff since depending on the configuration of vehicle used is on the order of 2 years to 5 depending on mission style.

As for making use of the MAV as a mars return to surface vehicle it does not have unless repack and reinstalled spare parachutes or heatshield for such a purpose. The Mav lands once is refuel insitu and is intended a once use vehice back to orbit to mate up with a waiting ERV or MTV....

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