Sustainable Access to Mars: Interplanetary Transportation Architecture

Mark Friedenbach · 2012-05-02 02:33:19

John Finn at NASA-Ames developed a solid-state compressor able to separate Martian atmosphere into its constituent gasses and compress it to 17psi. It was built and tested under martian-analogue conditions, and is capable of running indefinitely without wear and tear. Pioneer Astronautics has demonstrated production of rocket propellant from such compressed gas sources.

There is certainly work to be done in scaling up and integrating both processes, but there are no potential show stoppers here. Atmospheric ISRU is a solved problem.

GW Johnson · 2012-05-02 08:26:40

I dunno what a "solid state compressor" is, but I'm glad to hear there is one and that it works. Sounds like that "do it a different way" I suggested has "already been done". That's great news.

What kind of propellants did they make at Pioneer?

GW

Mark Friedenbach · 2012-05-02 11:43:17

I think in this case solid-state simply means no moving parts. I'd have to find the technical report to see the mechanism that was actually used, although I believe it was some sort of absorption-copression. EDIT: here's a description of it link

Pioneer's work was methane/LOX, I believe, although I know they've also worked on ethylene and methanol.

GW Johnson · 2012-05-02 13:37:43

Mark:

I looked at the link. That's the absorption-degas analog of the dry ice confined-heating self compression I had thought about. Very clever. I never knew adsorption had reached storage volumes that large. That's a good technique. Once it's near 17 psia, then ordinary compressors could take it to 2000 psi if need be.

The fuel-maker sounds intriguing, too. I've recently become a fan of methane-LOX here as a cleaner alternative to kerosene-LOX.

These items are the sort of gear that needs to get scaled up and taken along on the first mission or two, to get thoroughly checked-out and wrung-out of any hidden "gotchas". After that, it's ready for prime time, and the very thing we need.

You must be younger than I am, with more recent industry experiences and connections, to know of such things.

GW

SpaceNut · 2012-05-02 21:35:50

Thanks for the reminder of Pioneer Astronautics Mark Friedenbach had forgotten about them but then again thats from the period of time that is missing.....

SpaceNut · 2012-05-02 21:43:28

http://www.pioneerastro.com/Projects/Mm … Report.doc

SpaceNut · 2012-05-07 20:01:37

Something that we have touched on is the leveraging of what we build for not only a transportation system but also of the other pieces that we will need. When Nasa was going for the moon I recommended that we make use of the lunar chariot and rovers to allow for a leveraged point to traveling on Mars.

The Triway into Space Declaration

I could not have said it better....

Rather than fighting over a lunar base (above) versus a Mars base or asteroid expeditions, space advocates should seek to support technologies than mutually enable all three. Pursuing each of these space expansion goals separately can blind the activist to the synergies and savings that can come from pursuing one goal with the other two in mind. Similar transportation, habitat module, and life-support technologies will be needed for all three destinations. These technologies might include modular bio-regenerative life-support structures and technologies; multi-use spacesuit and vehicle development; mining, processing, and construction technologies; modular and robotic factory systems; and small “pocket” hospitals.

This step also sets up free trade from each destination to the others....

Leveraging the way to mars was part of the constellation thoughts with the crew going to the ISS , the moon and to mars...before it was cancelled....
http://www.nasa.gov/pdf/203075main_ECLS … iefing.pdf

Last edited by SpaceNut (2012-05-11 06:45:12)

SpaceNut · 2012-05-11 05:10:29

Back to methane creation again with the disappearance of Poop Power I will need to find the links for how we can use different types of waste created on the way to mars to augment rocket fuel production.

5 Ways to Turn Poop Into Power
Anaerobic Digesters Turn Waste Into Energy so why not use this as a supplement and deliver the waste to the surface in a dedicated lander...

SpaceNut · 2012-05-11 06:26:31

Any journey to Mars for man will include some form of life support closed or open and with it we have the need to remove mans waste products.
http://en.wikipedia.org/wiki/ISS_ECLSS
The highest priority for the ECLSS is the ISS atmosphere, but the system also collects, processes, and stores waste and water produced and used by the crew—a process that recycles fluid from the sink, shower, toilet, and condensation from the air.

That said the carbon is already at higher pressure than that of mars to help in boot strapping methane creation along the way
http://www.nasa.gov/centers/ames/resear … pport.html

The waste solids can then be gasified to create a process to recover the carbon from it as well as other elements once we are on Mars.

MatthewRRobinson · 2012-05-21 11:00:21

GW Johnson wrote:

On the compression thing: I cannot claim to be familiar with everything that has been proposed. I am familiar with recip and turbine. And basic thermodynamics. Yes, we have compressors on submarines that charge air banks from 1 atm to 500 atm. They are gigantic, heavy things, carried by a sub weighing 4-7000 tons. That kind of thing does not miniaturize well. And, we're talking about a compression ratio final/input of 500.
Most of the aircraft compression machinery we have is under-30 for compression ratio, most of them under 15 or so. What goes on in a shop air compressor and what goes on inside a piston engine are comparable at compression ratios in the 6-11 range. All of these devices are light enough to fly, and all of them miniaturize well. It is the lower compression ratio that allows them to be miniaturizable and be flightweight.
Mars's atmosphere is 7 mbar, except up on the highlands and mountains where it's nearer 2 mbar. But let's go with 7 mbar. Picking a challenging miniaturizable/flightweight compression ratio like 30, that's a bottled gas pressure of 210 mbar, or about 20% of 1 atm. Most of the chemistry processes I know of take place at 1-30 atm. Well, it seems like that might be a serious problem. Although, exploring low-pressure chemistry is something we can do, right here. But dollars to doughnuts, I'll bet you it ain't ready yet to go to Mars.
On the other hand, lets look at the submarine high-pressure air bank compression ratio of 500, and use that at 7 mbar. 3500 mbar, about 3.5 atm, that's usable with chemistry processes we already use industrially. The only problem is the huge tonnage of machinery for a throughput that scales down directly with inlet density (about 0.6% of that here). Heavy, inefficient. I see not much promise down that path.
Take some dry ice frost, pack it tightly into a steel can with a valve and an outlet pipe, and seal it up. Heat it with low-grade energy (solar thermal would work, albeit slowly). The CO2 vaporizes, but cannot expand, so the pressure rises by the specific volume ratio at a constant process temperature. That's a factor in the hundreds to around a thousand (don't have a thermodynamic properties table for CO2 handy).
There's a compression ratio comparable to the sub's high-pressure air bank, done with steel cans, high-pressure tubing, ordinary gas bottle plumbing, and a solar-thermal panel. Admittedly, it's a batch process, not so amenable to robotic operation. But it could be done. All you need is a source of dry ice.
That's the numbers for the kind of problem we are talking about here. If you want to make methane and oxygen out of CO2 and H20, on Mars, the poles are the place to do it. Unfortunately, that's not where the first landing(s) will take place.
That's also a part of my point: every site on Mars is different. There is no one "average Mars" we can put in a simulation chamber here on Earth.
GW

Okay, as someone far less knowledgable, I have a lot of respect, but something about this greatly bothers me.

Pressure is a force exerted over an area. Material strengths retain a set amount of force. So I don't think you can use compression ratios to determine the type of mass requirement that a system will have. Not to mention a submarine's air charge system needs to work in minutes, not years like an ISRU unit, and they have no need to be lightweight, and as such are built with very heavy materials. Isn't that all a bit like saying we can't build a Moon rover because jeeps are so heavy?

Back to my earlier point of pressure, I really don't actually know, but I think material strength means that a material can take a certain amount of force loading before breaking, right?
If so, then you can't use compression ratios at all.

Let me compare a compression ratio of 500 from a submarine's air charge systems to a Mars ISRU unit, working with one square meter of material, and how much material strength it needs (directly proportional to the net force on the material).
First, the submarine. Pressure from outside (because we're working with one square meter, I'm replacing pascals with newtons) is 1 atm, which on one square meter is 101,325 newtons.
Force on the inside, 500 atm, is 50,662,500 newtons per square meter.
So, simple vectors say to subtract, not to divide. So the force that the material will need to withstand is 50,662,500 N - 101,325 N = 50,561,175 N

Now, the Mars ISRU.
0.7 mbar = 709.275 N/m^2
3.5 bar = 354,637.5 N/m^2
At one square meter;
354,637.5 N - 709.275 N = ~353,928 N
Which is compared to
50,561,175 N

Which means a submarine's air compression systems must be built to withstand 143 times greater force per exposed surface area than an ISRU unit.

I'm not arguing compressor design, I know next to nothing about that aside from pistons, what I'm saying is that all the parts exposed to the charged air in a submarine must withstand a much greater force of pressure than the parts in an ISRU compressor.

And as for ISRU as a whole, the thread is really about mission architecture, so if you don't want to bet human lives on it with the first landing, then the three leading architectures (at least as I see it) and the only ones NASA's seriously considered, MRDM 5, Mars Semi-Direct and Mars Direct, all are designed so that we don't bet human lives on ISRU working. As RobS said, there'll be a fully fueled Earth Return System (be it an ERV or MAV+MTV) waiting on Mars when the crew arrives.

The point I'm interested in returning to is ERV vs. MAV+MTV. I haven't seen a reply to that yet, so I'm guessing we're agreed on ERV?

The the mission architecture is starting to look a lot like Mars Direct, albiet using reusable vehicles instead. Differences: A single-stage ERV that can perform aerocapture for EOI, dock with a booster or a few in LEO, and get launched on TMI by boosters that will either free return around Mars back to Earth and EDL, or just EDL from a highly elliptical orbit. Thereby making every vehicle reusable, except of course for the ones that are staying on the Martian surface.

Aerocapture EOI is perhaps the weakest point. On the first pass, we only need to capture into a highly elliptical orbit, though, no need to burn 2 km/s lowering the apoapsis if we can come back and do that later, and thus make the burn a lot less intense. All that's needed is to drop below escape velocity, into an elliptical orbit to perform another aerobraking maneuver at periapsis to drop into LEO.

So, then, what kind of Delta-Vee is needed? Since we're leaving Earth with 4.2 km/s, I think it's safe to assume our return velocity will be in a similar range, about 4.2 km/s above LEO velocity, for a total of about 12 km/s. A velocity of 10 km/s at about 70 km altitude gives an aposapsis of only 27,000 km, far below geostationary orbit. Or, alternatively, you could save .8 km/s of atomspheric burn and initially enter an orbit with an aposapsis halfway to the moon (120,000 km). So the arrival aerocapture scrape only needs to burn off 1.2 km/s, starting at 12 km/s. Surely the small delta-vee makes this doable, despite the high initial velocity?

GW Johnson · 2012-05-21 11:48:22

The compression ratio is important because of product density (largely its pressure). Material strength need be only what is necessary to contain the product you require. Most of this chemistry stuff takes place at 1 to several atm. If 1 atm is good enough, the compression ratio from 7 mbar to 1013 mbar is about 140, which is doable in any of several ways.

Since I last corresponded on this thread, I found out about an absortion (adsorption?) compressor rig. You absorb CO2 at 7 mbar, confine the "sponge", heat it to drive the CO2 out, which is near 1 atm. It's an analog to the confined dry ice vaporization I suggested, except it works straight off atmospheric CO2. Works anywhere on Mars. The product comes out near 1 atm pressure, which is apparently chemically useful for synthesizing methane from CO2 and water. No super-high pressures there.

That'll work, we just gotta try it enough to work all the bugs out, before risking depending on it. Some of that can be done here, the final check being maybe an unmanned trial on Mars.

Purity is an issue to be evaluated. I'm not sure, but I think this absorption compression thing is the same basic technology that's in the oxygen system in the F-22. It seems to be having troubles, and I'd hazard a guess that's a purity-of-product issue. Mars's atmosphere is not just pure CO2, so the same risk applies.

GW

SpaceNut · 2012-05-21 20:03:00

MatthewRRobinson wrote:

The point I'm interested in returning to is ERV vs. MAV+MTV. I haven't seen a reply to that yet, so I'm guessing we're agreed on ERV?

Most have trouble following the acronyms from one mission to the next as the function while being simular are way different in duration and or size....

Key elements of the mission
http://nssdc.gsfc.nasa.gov/planetary/mars/marsretu.html

both delivered MAVs will have been long since fully fueled by the ISRU plant(s)
The MAV will rendezvous the orbiting fully fueled ERV soon after launch and, after successful docking, the crew will transfer themselves and their scientific samples into the ERV.
The ERV will certainly need to be extremely robust as it will have been untended in Mars orbit for nearly 4 years before being used.

So fear of insitu not working is less but still a 4 year wait for the use of the ERV is a question....

Now for the Mars Transit Vehicle or Mars Semi-Direct
http://nssdc.gsfc.nasa.gov/planetary/mars/marslaun.html

The propulsion system that will most likely be used by the Mars transit vehicles once in LEO will be Nuclear Thermal Propulsion.

This mission launches 2 sets of 3 vehicles with specific purposes for each ....

Key elements are as follows
http://www.marssociety.org/home/about/mars-direct

The general outline of Mars Direct is simple. In the first year of implementation, an Earth Return Vehicle (ERV) is launched to Mars, arriving six months later. Upon landing on the surface, a rover is deployed that contains the nuclear reactors necessary to generate rocket fuel for the return trip.

The mission MRDM 5 of Nasa was based on the Ares V constellation and we all know that its not happening....

GW Johnson · 2012-05-22 16:24:10

There's a lot of boxes to think outside of, in addition to the ones implied by the mission designs in the previous post. Two of my favorites are inappropriate holdovers from Apollo: one launch/one mission, and one mission/one landing. Those two are still part of many of the concepts in the previous posting, to one extent or another.

What you want to do is collect a list of tried-and-proven concepts, and then rearrange them outside the constraints of these boxes. You don't want to use anything not yet well-proven by the time you actually go. Significant ISRU ought to be tested thoroughly on the first manned mission to Mars (after very extensive unmanned testing both here and on Mars).

I'd be afraid to bet lives on it until after that final test, though. Just my humble opinion, being a suspenders-and-belt-and-armored-codpiece sort of guy, yet still unafraid to think outside some largely-unrecognized boxes, at the same time.

GW

MatthewRRobinson · 2012-05-23 20:26:43

SpaceNut wrote:

MatthewRRobinson wrote:
The point I'm interested in returning to is ERV vs. MAV+MTV. I haven't seen a reply to that yet, so I'm guessing we're agreed on ERV?
Most have trouble following the acronyms from one mission to the next as the function while being simular are way different in duration and or size....
[....]

ERV: Earth Return Vehicle,
An unmanned ERV is sent to Mars. At the next launch window, a manned ERV is sent to Mars. The ERV lands, and the crew ride either one back home at the next return window.
It's essentially Zubrin's ERV from Mars Direct, except I would personally prefer something reusable (and thus single-staged) and that can perform an aerobrake in Earth's upper atmosphere so it can be reused.

MAV+MTV refers specifically to the architecture used in MRDM 5.
MTV: Mars Transfer Vehicle
Large, trans-habbed equipped spacecraft that cannot enter an atmosphere, to serve as a habitat for transit in-between Earth and Mars orbit.
MAV: Mars Ascent Vehicle
Small spacecraft that only goes from martian surface to Mars' orbit, and possibly back again.

GW Johnson wrote:

There's a lot of boxes to think outside of, in addition to the ones implied by the mission designs in the previous post. Two of my favorites are inappropriate holdovers from Apollo: one launch/one mission, and one mission/one landing. Those two are still part of many of the concepts in the previous posting, to one extent or another.
What you want to do is collect a list of tried-and-proven concepts, and then rearrange them outside the constraints of these boxes. You don't want to use anything not yet well-proven by the time you actually go. Significant ISRU ought to be tested thoroughly on the first manned mission to Mars (after very extensive unmanned testing both here and on Mars).
I'd be afraid to bet lives on it until after that final test, though. Just my humble opinion, being a suspenders-and-belt-and-armored-codpiece sort of guy, yet still unafraid to think outside some largely-unrecognized boxes, at the same time.
GW

As has been stated a few times, most mission architectures, those discussed here and those that NASA's seriously considered, involve sending any ISRU assets to Mars a full launch window before sending any astronauts. By the time the astronauts arrive, it'll all be fully fueled up. If it fails to fuel up, then mission designers and engineers will have to scramble to assemble a different launch scheme to send the assets to Mars fully fueled, and fix the ISRU technology.

GW Johnson wrote:

The compression ratio is important because of product density (largely its pressure). Material strength need be only what is necessary to contain the product you require. Most of this chemistry stuff takes place at 1 to several atm. If 1 atm is good enough, the compression ratio from 7 mbar to 1013 mbar is about 140, which is doable in any of several ways.
Since I last corresponded on this thread, I found out about an absortion (adsorption?) compressor rig. You absorb CO2 at 7 mbar, confine the "sponge", heat it to drive the CO2 out, which is near 1 atm. It's an analog to the confined dry ice vaporization I suggested, except it works straight off atmospheric CO2. Works anywhere on Mars. The product comes out near 1 atm pressure, which is apparently chemically useful for synthesizing methane from CO2 and water. No super-high pressures there.
That'll work, we just gotta try it enough to work all the bugs out, before risking depending on it. Some of that can be done here, the final check being maybe an unmanned trial on Mars.
Purity is an issue to be evaluated. I'm not sure, but I think this absorption compression thing is the same basic technology that's in the oxygen system in the F-22. It seems to be having troubles, and I'd hazard a guess that's a purity-of-product issue. Mars's atmosphere is not just pure CO2, so the same risk applies.
GW

Well, once we've got it at 1 atm or more, then shouldn't purifying it be a somewhat simple process? Given that we have something on the order of more than a year to do it, it shouldn't require a very massive system to purify it by whatever method is deemed best.

GW Johnson · 2012-05-24 10:22:50

I'd use a piece of well-proven ISRU equipment as part of my mission design, as long as there was a back-up or "way out" designed into the mission. You have to do that with everything, anyway. Once out of Earth orbit, there is no practical possibility of rescue. And I think we can all agree that there is nothing in this world as expensive as a dead crew. Especially in government work.

That being said, what I see being discussed and referenced in the forums is beginning to convince me that effective and reliable ISRU for propellants might be something we actually can get "well-proven" in time to make the first trip. Purity issues can be addressed, yes, but you have to find them first.

Since the adsorption compressor principle we are talking about for Mars ISRU seems to be the same one as is in an oxygen system on the F-22 that is having problems, well, I think we just might have found one of the "gotchas". If we get that fixed, that's one less "gotcha" with the different hardware/same principle we'd use on Mars.

It all gets down to doing the development phase correctly, that much larger effort that goes in between the laboratory demonstrations and the actual application. Development work is much more art than science. None of the "gotchas" you are trying to find were ever seen before, much less written about. That's detailed, frustrating, dirty-fingernails work. And there's a whopping lot of it to do, before you ever consider your new product "well-proven".

That's what the ISRU equipment needs to go through before we bet lives on it. But it seems to me this could be done. In time to go, maybe even within the decade, if need be. That's just my experience-based hunch. I do like what I'm reading about this stuff.

GW

Void · 2012-12-28 08:07:20

The material I am going to post seems like it would fit here:

Dual-Mode Water Rocket:
http://www.sbir.gov/sbirsearch/detail/259659
http://www.orbitec.com/frames/advancedPropulsion.html

Vortec Engine:
http://www.sae.org/mags/AEM/11560
http://www.orbitec.com/propulsion.html
http://www.google.com/search?q=Vortex+r … 93&bih=423

orbitec-vortex-liquid-fuel-rocket-engine-usaf-upper-stage-5.jpg

Orbitec Web Site:
http://www.orbitec.com/

You may have already discussed the above, under other descriptions, but I missed it if you did. At any rate, the diagram is good to look at I think.

I am curious about this propulsion system, the Dual-Mode Water Rocket, and wonder how it might be used in a Mars mission.

One issue I saw on other threads, was that a vlasmir engine might force a mission to linger in the Van Allen belt too long.
With the above, I would imagine that the chemical engine could be used to get it through quickly.

It is also obvious that this thing could be refilled where ever water is convenient. Maybe from the Moon and Mars.

I also think that it should be possible to use a robotic ion rocket to place refueling stations at intermediate points for a trip to and from Mars, allowing the mass carried in the actual Dual-Mode Water Rocket to be lower.

It also seems to be stated that the vortex engine is considered to be lighter and more reliable. However, I do not know that it would be suitable for landing and launching from the surface of Mars. I do not know if it would make any sense to try to do an aerocapture of it at Mars. I am going to speculate that the answer would be no. In that case a separate lander is needed.
Or maybe it would be in two parts, the Vortec Engine part as a lander.

But as I have said, by having refueling at various points in the mission, perhaps an advantage is gained.

I can see also that for this method, it would also serve well to visit the Moons of Mars.

I wonder what thoughts you might have about this.

Last edited by Void (2012-12-28 08:21:52)

JoshNH4H · 2012-12-28 15:01:22

I would think that an interplanetary transportation infrastructure would simply involve chemical rocket engines fueled by lunar H2/LOX and either phobosian H2/LOX, or Hydrogen payload brought to phobos combined with local Carbon/Oxygen to make Methlox for the return. Existing engines can be used to fire both propellant combinations. A Nuclear Thermal rocket would also be a good choice.

Given low interplanetary delta-Vs, I don't see why any further technology advances are needed.

The first technology looks like an improvement on an arcjet; The second is just an innovation in the cooling of chemical engines. Both are worthwhile technologies but neither is very revolutionary.

Void · 2012-12-29 06:38:59

If this works, maybe it also changes things for other forms of propulsion.

http://sciencenordic.com/sailing-solar-winds

http://en.wikipedia.org/wiki/Electric_sail

http://www.electric-sailing.fi/

Quote:

The electric sail is a new space propulsion concept which uses the solar wind momentum for producing thrust (Janhunen, P., Electric sail for spacecraft propulsion, AIAA Journal of Propulsion and Power, 20, 4, 763-764, 2004, Janhunen, P. and A. Sandroos, Simulation study of solar wind push on a charged wire: solar wind electric sail propulsion, Ann. Geophys., 25, 755-767, 2007). The electric sail is somewhat similar to the more well-known solar radiation pressure sail which is often called simply the solar sail.
A full-scale electric sail consists of a number (50-100) of long (e.g., 20 km), thin (e.g., 25 microns) conducting tethers (wires). The spacecraft contains a solar-powered electron gun (typical power a few hundred watts) which is used to keep the spacecraft and the wires in a high (typically 20 kV) positive potential. The electric field of the wires extends a few tens of metres into the surrounding solar wind plasma. Therefore the solar wind ions "see" the wires as rather thick, about 100 m wide obstacles. A technical concept exists for deploying (opening) the wires in a relatively simple way and guiding or "flying" the resulting spacecraft electrically.
The solar wind dynamic pressure varies but is on average about 2 nPa at Earth distance from the Sun. This is about 5000 times weaker than the solar radiation pressure. Due to the very large effective area and very low weight per unit length of a thin metal wire, the electric sail is still efficient, however. A 20-km long electric sail wire weighs only a few hundred grams and fits in a small reel, but when opened in space and connected to the spacecraft's electron gun, it can produce several square kilometre effective solar wind sail area which is capable of extracting about 10 millinewton force from the solar wind. For example, by equipping a 1000 kg spacecraft with 100 such wires, one may produce acceleration of about 1 mm/s^2. After acting for one year, this acceleration would produce a significant final speed of 30 km/s. Smaller payloads could be moved quite fast in space using the electric sail, a Pluto flyby could occur in less than five years, for example. Alternatively, one might choose to move medium size payloads at ordinary 5-10 km/s speed, but with lowered propulsion costs because the mass that has to launched from Earth is small in the electric sail.
The main limitation of the electric sail is that since it uses the solar wind, it cannot produce much thrust inside a magnetosphere where there is no solar wind. Although the direction of the thrust is basically away from the Sun, the direction can be varied within some limits by inclining the sail. Tacking towards the Sun is therefore also possible.
The electric sail won the 2010 Finnish Quality Innovation Prize among Potential innovations. The prize was handed out by the President of Finland Tarja Halonen on November 11, 2010.

I guess what I am after, is fuel depots, supplied by robotic verisions of a propulsion system having such an ability.
It would be OK to have them be combustable chemicals, but that is harder to maintain at a distance.
A block of ice in a sealed wrapper which was highly reflective should keep in orbit around Mars, or in intermediate orbits in locations between Earth and Mars. If the vehicle, which can propulse with water or chemicals could retank at intermediate locations, then the typical mass of the vehicle could be smaller, which may allow more payload to be carried, or a quicker trip or both.

In addition those fueling stations could have some emergency repair/survival capabilities, no specifics given for that.

Last edited by Void (2012-12-29 06:57:23)

GW Johnson · 2012-12-31 11:42:15

The following is about long-term sustainable interplanetary travel, not what we need for the initial mission to Mars or anywhere else beyond the moon. (Although, it would be nice to have for the initial mission to Mars.)

For long-term sustainable interplanetary travel, I'd look first for the most widely-available, highly-abundant volatile to use for propellant. What we're seeing from all the planetary science probes is: that answer is "water". It may not be pure, but ice is everywhere, and solid contaminants are really easy to separate with gravity, even if it is artificial (centrifugal force).

So, if we use the water, we also find it is really easy to ship anywhere, because we can ship it frozen with minimal vapor containment to prevent sublimation. An iceberg is not lost if you take a meteor hit; a remarkable safety feature. Just patch the hole in your vapor containment, which is holding at most about 6 mbar pressure. That containment might just be a big plastic bag. We do have to learn how to patch holes in plastic bags in-vacuo and in zero-gee. But it can be done. One way is to use an aluminized thermoplastic, and just heat bond it with a hand-held iron.

OK, so water makes the most practical propellant material we have. So, now how exactly do we use it? Solar or nuclear electrolysis is a known way to produce hydrogen and oxygen, which can be both rocket propellants and fuel cell reactants. We already know we're going to do this at some level, and how to do it. Known tinkertoys in-hand. But the energy cost to make the hydrogen and oxygen, store them successfully, etc, will never be trivial, no matter how the space commerce economy finally takes shape. You'd really like to use the water directly, as water, and just use your waste heat to melt the stored ice as you go.

Guys, that's a nuclear thermal rocket. A water-variant of the old NERVA. Everybody thinks the Isp will be far lower due to the molecular weight effect, and at the same core temperatures, it is. But water as steam will transfer a whole lot more heat than hydrogen. You can run the reactor core at a higher temperature and a whopping lot more generated power. I'd bet the finished Isp isn't as low as everybody fears, once the development is done. How would you like Isp 600+ at engine T/W > 5? I'd bet real money this could be done, within about 5 years of having resurrected the original LH2 NERVA, an item that itself should take at most about 5 years, if done by the right team.

The follow-on is a gas-core nuclear thermal rocket, which takes away the core temperature limitation, and eliminates the safety concerns associated with a "live" core inside an engine shell, between burns. Operated up to a modest power level, regenerative cooling is possible. How would you like 2000+ Isp at engine T/W > 30? Beyond that, you need a place to put the waste heat. A heavy radiator is probably needed. Would you like Isp 5000+ at engine T/W around 0.1? Still high enough for an impulsive burn for interplanetary travel. I'd bet we could have one of these working within about a decade of having the water-NERVA working.

High-Isp / high thrust nuclear propulsion with plain water as the propellant. Water which you can get nearly everywhere you go. Water which is easy to store and ship as minimally-enclosed icebergs. Sounds like a series of very smart technology development programs we should already be undertaking. Too bad no one is.

Sounds also like the government monopolies on nuclear power need to be broken, so that somebody actually motivated can go do this. Political anathema to some, I know. But it must be done.

Just how sustainable and low-cost do you want to be? Especially if you use a thorium reactor. Thorium is very likely "everywhere", too. It's very plentiful here, and on the moon. The probes should be looking for it "out there", shouldn't they? Too bad they're not. Not yet, anyway.

GW

Void · 2012-12-31 18:08:39

I have no special notion to push, I am just throwing spitballs at the wall to see if anyting can stick.

Nuclear with thorium would be OK with me, because it is quite possible to make sure that you have very little dangerous material in the reactor, or at least that is what I thought I read.

I am just thinking however even for that vehicle it might still be prefered to have efficient robots place ice blocks along the routes. I think it is implied that you also do not object to that.

Time will tell, I have always felt that the space program has an evil nanny, that puts the pillow over the babies head when it appears that it might be about to start up.

But little by little, human abilities are advancing. Maybe some day they will reach a "Critical Heat Level" and suddenly it will make all the sense "out of this world" to get going with the various things, you have mentioned.

Bye GW

Last edited by Void (2012-12-31 18:10:05)

RobS · 2012-12-31 22:00:38

The problem I see, void, is that ice or propellant depots won't do you much good if you are shooting past them at a dozen kilometers per second. There is no reason to have propellant depots between Earth and Mars, for example; you want to fire your engine deep in Earth's gravity well and coast the rest of the way, because that uses the least fuel. I don't think fuel depots are useful in deep space, unless they are at your starting point or end point.

GW Johnson · 2013-01-01 10:51:04

I wasn't really thinking about fuel depots "along the way" in the sense of mid-trajectory. I was thinking more about "topping off your tanks" from the local ice when you actually make your stop. Although, if it's a stopping place that is often frequented, a fuel depot there makes sense.

GW

Russel · 2013-01-02 08:14:59

A couple of thoughts regarding the Mars return trip.

I see the problem being in 3 parts.

Part 1 is going from low orbit to high orbit. There is a delta v of about 3.2Km/s from LEO to Earth C3. Similarly there is roughly 1.4Kms from LMO to Mars C3.

Part 2 is the delta v required for going from the C3 of one planet to another. So that's 1.5+ Km/s.

Part 3 is aerobraking. Which I specifically define as braking from high orbit to low orbit and not capture from above a C3 orbit.

Part 1 can be answered with low thrust applied selectively. Meaning you generate an increasingly elliptical orbit. And within that orbit you apply thrust where the vehicle is travelling fastest. In other words closest to the planet in question. Now, intermittent thrust does require more patience, but it also provides other opportunities.

Part 3 can be done without heavy thermal protection, since you've started within C3 you're guaranteed to remain within orbit. So your task is to very gradually brake with a number of passes through the upper atmosphere. One thing I'll note here is that the cold of deep space is to your advantage. And carrying a large chunk of ice is to your advantage too.

Part 2 is actually the tricky part. Because with low thrust you can double (or worse) the effective delta-v, at least between C3 points. There's basically two approaches you can take here. One is to cop it sweet and wear a higher effective delta-v but trade that off against a much higher Isp. The other approach is to use a higher thrust system for one final kick on your final orbit. And you don't need to use a higher thrust mode for the full C3 to C3 delta-v. You just need enough to have enough velocity when you're a few million Km out so you're not wasting too much propellant in low thrust mode. Its a complex trade off.

Here is one concrete solution. Lets suppose we solve two basic problems. One is building a large solar array - on the order of hundreds of KW - and light. The other is a light weight cryogenic system of modest power level. Think about a re-usable transit vehicle whose sole purpose is to convey people back and forth between Earth and Mars, engaging in slow aerobraking at either end of the trip.

Propellant is water. On top of that is an auxiliary store of LH2/LOX. In low Earth orbit the vehicle would be replenished directly from Earth with already liquefied LH2 and LOX. Also provided from Earth is enough water to make the round trip.

Phase 1: is unmanned. The vehicle uses an array of MET thrusters generating in the order of 100 Newtons in order to generate a high and elliptical orbit. Imagine a GTO orbit.

Phase 2: the crew is launched into a matched orbit, docks and joins the transit vehicle.

Phase 3: 2 to 3 orbits and a check out of the vehicle, the orbit is now a few hundred m/s away from Earth C3.

Phase 4: At the lowest point in the final orbit, the LH2/LOX is used to apply a delta-v of roughly 1Km/s. This is on top of MET thrusting. So by the time you're past the orbit of the moon you've gained most of the energy you need for Mars transfer.

Phase 5: Continued MET thrusting for some period (days) to achieve the final Mars injection. We'll assume a Isp of 800 here.

Phase 6: Some months later, a long thrust brings you into orbital capture.

Phase 7: Aerobraking into low Mars orbit.

Phase 8: A long period in Mars orbit. Towards the end of this period the available solar power is used to electrolyze and this time liquefy more LH2/LOX. This time a smaller quantity is needed.

Phase 9: The crew re-joins the transit vehicle. This time in low Mars orbit.

Phase 10: As with Phase 1, but this time manned (over some weeks) the orbit is raised as before.

Phase 11: As you'd guess, on the final orbit, the LH2/LOX is used for a final kick.

And, you can guess the rest. Finally after we are captured into Earth orbit the crew transfers into the Earth lander. Whether the vehicle actually needs to go back to a lower orbit is an open question.

Ok, what does this gain us?

Working backwards. We arrive in Earth orbit with (say) 25 tonnes of vehicle - for the sake of the argument. Effective delta-v on MET post Phase 11 we'll take as 1.3Km/s. So a mass ratio of 1.18. And thus the mass on exit from Mars is 30 tonnes.

Before that we assume 1Km/s on high thrust using LH2/LOX at Isp = 450. So a mass ratio of 1.24. So before burning the LH2/LOX we were at 37.2 tonnes. So we needed 7.2 tonnes of LH2/LOX.

Prior to this we used the MET to apply an effective delta-v of 1.4Km/s. That's a mass ratio of 1.13. And so from low Mars orbit we started with 42 tonnes.

I'll skip the working out. It turns out that the original mass in LEO was 102 tonnes with about 13 tonnes of LH2/LOX on board and about 64 tonnes of water.

This doesn't take into account consumables consumed along the way. I'm just trying to show that even something like MET with an Isp of 800 seconds and some judicial use of standard chemical propellant cuts the problem down to size.

Nice thing is you can afford to be a little generous with the water.

And yes, things do get slower on the Mars side of things with reduced solar power levels. There's a million permutations.

Now, speaking of nuclear. What's to stop us using water as a propellant and a solid core reactor and simply doing with an Isp in the high 300s but only using that for high thrust mode. Likewise with a nuclear core you can dispense with the solar panels and generate electricity to run an MET engine. There's nothing stopping you using a large tank of water as a heatsink if you run things intermittently.

Here's another out of the box thought. With lithium batteries approaching 400Whr/Kg there is an argument to be had for using battery storage to drive the power level if you're using electric thrust intermittently.

As far as gas cored reactors goes. That's no mean feat of engineering. That's way up there with fusion (almost) and there's some fusion concepts that to me seem not beyond the realms of possibility either. But put it this way. I think we'll figure out how to deploy 500KW of solar array for a couple of tonnes sooner than we'll figure out gas core.

As for liquefying hydrogen. The main thing I'm doing here is simply reducing the power level involved. You don't need as much of the stuff and you can put it off to a few months before you need it.

Btw, in case you didn't notice. The whole thing works without any refueling from Mars (you still need a Mars ascent vehicle though).

A further note about aerobraking. Provided you're patient you can keep the heating down to the point where all you need is minimal thermal blankets and a large chunk of ice. Which just happens to be what you always end up with if you put water into space. More than this, repeated passes at aerobraking gives you the opportunity to re-freeze your ice.

I left out a bunch of detail about solar arrays and other synergies, but this is enough for one post. If you really wanted to toy with this you could end up with a big store of water pre-positioned in Mars orbit but this I think only makes sense if you're going for many missions and you want to resort to even more exotic means to get large masses there (ion drives?)..

Cheers

Russel · 2013-01-02 08:29:16

Btw I love the dual mode water rocket but I'm assuming that they're not using the same actual engine for the MET and LH2/LOX side of things.

It also occurs to me that a bit of spare oxygen is rather handy on a long space flight so it may not be really necessary to get true stoichiometric ratios.

Russel · 2013-02-01 04:32:18

Here is my best attempt so far at a mission architecture for Mars. Three basic elements:

1. A transit vehicle. Multiple missions. Shuttles between Earth orbit and Mars orbit. Basic fuel - water.
2. A mars lander/ascent vehicle. Multiple missions. Singular purpose is crew transfer. Fuel - Oxygen and Carbon Monoxide.
3. Everything else landed on Mars. One way trip. Mars hab, rovers, etc.

The transit vehicle is more or less the one I pictured above. Starts off with a mass of 100+ tonnes, most of that water, but also roughly 13 tonnes of LH2/LOX provided from Earth.

Most of the delta-V is provided by a microwave electrothermal thruster. The final boost into Mars injection and likewise Earth injection is handled with a modest LH2/LOX engine.

As an additional cargo I'm now going to add an extra tonne of water.

Armed with a substantial quantity of water, and given that I've arrived into a Mars capture orbit, I can safely go about aerobraking into a low Mars orbit simply by using ice as a heat sink. This reduces the mass of my heat shield.

The waiting Mars lander/ascent is in orbit, and fueled from the surface. Before using it all we need to do is to transfer a sufficient quantity of water to the lander. My best estimates put that at under a tonne. The lander itself (see my speculation in the Landing on Mars thread) uses water (steam) as a sacrificial coolant. So in essence the cost of landing a crew is about a tonne of water.

In essence the architecture I have means landing large cargoes separately from the crew. The cargo landings can afford to be less precise but the crew lander is capable of more precise landing. In addition the crew lander does not shed anything but water vapor on its descent. It lands, it refuels and it can be tested prior to use. There will in any event be a second lander/ascent vehicle on call.

Which leaves us with the bulk of the mass travelling prior and separately. That gives you the freedom to use low thrust propulsion all the way. Even a very-compact nuclear engine is then possible - 100KW thermal power roughly speaking.

Leaving aside all of the material that is going on a one-way trip, but presumably most of which would be useful for multiple missions, the actual mass equivalent in low earth orbit per crew is now closer to 100 tonnes. Indeed, the proposal is economical enough in this regard that you could afford to be more generous and add more margins to some things.

Ok, what have I proposed that is difficult/extraordinary?

First, it requires a solar array - a real big one by today's standards. As a benchmark think 500KW in Earth orbit. Well within the realms of credibility.
Second, it requires the MET technology to become mature. That's work but its by no means exotic technology. It just needs work to be sure its efficient and above all reliable.
Third, and probably the scariest for some. Stepping away from tried and tested heat shielding technology and using a "cushion" of steam. That's going to need new theoretical work and lots of testing.
Fourth, CO/LOX engine development. Its feasible. We're not talking huge but we are talking reliable.
Fifth, the propellant production. Even though its just splitting CO2, it still means testing.

That's my wish list of new technology. What do you think?

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