New Mars Forums

Rune,

The motivation behind having a mesh surface to form drag actually has more to do with controlling the overall stream, and stability. So instead of having one solid area where the heated gas spills over the edges, it instead flows through in a controlled manner. As I said earlier it may pay to have a profile.. more density of mesh here.. less there.

Tungsten wasn't my idea. Rather What I want to see is figures that give me a rough idea of how big you'd need to build something to bring the temperature down into a range where you can use less exotic materials - even to under 500C.

Here's a half baked idea. Has anyone thought of what amounts to a water balloon? Its towed and as it heats, the water turns to steam, thus inflating it. Not sure of the materials but this would have some interesting properties as to where exactly it kicks in. Interesting question as to whether it would still have lift nearer ground level - in other words how fast does the steam cool.

Just a wild thought there

Does anyone have any idea when SpaceX will start testing its new thrusters on the Dragon? That'll answer so many questions about how to retro thrust into a hypersonic stream - at least if they test it in that regime (and I would think they'd have to).

GW,

Where I actually came from was originally a design that put a large payload (30 tonnes) on an even larger platform. One that's relatively low mass per unit area and tends to become more porous as you head towards the edge (Think 30-40m dia). To that I needed to add a drogue for stability. Then the more I thought about it the more I thought that the whole thing is better placed behind the vehicle.

Thus evolved a concept that's less like a trailed line and more like a very large "racket". A structure with a stiff edge but between that something more akin to a fabric, again with deliberate porosity this time increasing toward the center.

That then became an inverted cone. So yes, something not unlike a parachute but obviously built out of high temp materials. And when we say high temp, you build it big enough you're then dealing with 1000C not 1500C.

That then begged the question, do you need just one.. or could you build a string of these things and trail them.

And of course hangs the question, mass for mass, is a simple trailed cable a good solution. And then of course there's a million variations, including inflatable parts. I wish I had the means to do the rough sums on this, but that's why I'm mentioning it here.

My gut feeling says it can be made and made out of existing materials, and be worth the mass. One thing I'm not sure about is how it will behave as you detach it and where its likely to land. Again the question has to be asked, could a simple cable then simply be reeled in. I don't know.

My take on retro thrust into a hypersonic stream, gut feeling says it has to be angled, and it has to be well controlled. Fortunately someone's already built the right experiment and that's the dragon. Once they test that with its full abort from orbit capability, we'll have enough data. However, my original take on this was to put the thrusters far enough out on their own jib so that the control system has enough of a time constant to work with. Whether that's too hairy an idea I don't know.

Of course one thing we do know is that you have to have to mix and match technologies. So the question is, would a simple drag system get you far enough to then enable retro thrust sooner.

I suspect that if what you tow has graded porosity then it will behave quite well stability wise.

Its forcing air around the edges that buys you instability.

I'm not entirely convinced that an entirely "spineless" (for want of a better word) ballute is the way to go.

Lets consider the opposite alternative - a rigid structure, or at least one that would be under load. Here's a simple example. Build yourself a very big "racket" structure. Basically a stiffening ring supporting a mesh structure. Now attach support cables. You've now got something that will create drag and be relatively light. The nice thing about mesh is you can fiddle to your hearts content with the porosity, which lets you optimise the stability. You could even string these together in a sequence. Now, that's one extreme.

Somewhere in the middle is structures that have some backbone to them but attached to that is inflatable components.

I'm just tossing this in to point out that there's endless possibilities out there and only a few have even begun to be tested.

"But, working off the assumption of 4.3 km/s for a free-return, and an ERV amassing 32-34 tons (with LH2 for ISRU), the ERV could be sent to Mars with 3 F9H launches, two boosters and the ERV itself, though an MTV would mean an additional launch for a total of four, or even more, depending on how the packaging can or can't be arranged."

Can you expand on that? 4.3Km/s for free return? Have to know exactly what you're getting at

Ok, let me add some more random speculation here..

I've started with figures from this:

http://smartech.gatech.edu/jspui/bitstr … tories.pdf

What they say is, 4.2Km/s from LEO will get you to Mars in under 180 days.
Likewise, 3Km/s from LMO will get you to Earth in under 180 days.

Incidentally when you look at the graphs what you see is a trade-off between fuel mass and the mass of the vehicle (supplies and shielding) and it appears that the above figures are about optimal. So much for a 3 month trip but there you have it.

Now, what I'd like to suggest is the following.

First, start with a transit vehicle parked in LEO. Send a hydrogen/oxygen fueled tug to push that into a higher orbit (an extra 2.5Km/s) - in other words on par with GTO.

Second, transfer the crew, docking with the transit vehicle as it passes closest to Earth. Such a transfer requires that extra delta V but is only for the capsule.

Now, the transit vehicle is already in a higher orbit, so it needs less delta-V to achieve TMI. Now we're talking about 1.7Km/s.

Transit vehicle heads to Mars. Capsule lands. Transit vehicle aerobrakes into a high Mars orbit. This time you gain an extra 1Km/s over LMO. On a par with Deimos transfer orbit.

The capsule returns from the surface with delta-V around 5.1Km/s and again docks with the transit vehicle.

Now, the transit vehicle is already in a higher orbit, so it needs less delta-V to achieve TEI. Now we're talking about 2Km/s.

The transit vehicle returns to Earth. The capsule lands. The transit vehicle could aerobrake into the original high orbit (or it could go back to a lower orbit for maintenance).

The original hydrogen/oxygen fueled tug can gently aerobrake back into low orbit - or you could ditch it.

Now, lets do some basic figures.

Assumptions. The transit vehicle is about 20 tonnes. The propulsion parts of the vehicle another 6 tonnes. The capsule 4 tonnes. In other words about 30 tonnes all up (Yes, I did that deliberately).

To get this vehicle out of high Mars orbit and back to Earth on methane you need 2Km/s or a mass ratio of roughly 1.7.

In other words you start with a mass of 51 tonnes. 21 tonnes of propellant.

To get all of that out of high Earth orbit in the first place you need 1.7Km/s or a mass ratio of roughly 1.6 or a total mass of 81 tonnes. Total propellant (methane/oxygen) is now 51 tonnes.

And for the sake of completeness, the hydrogen/oxygen fueled tug (which strictly speaking you only have to use once, unless you have a need to service in LEO)

That requires a delta V of 2.5Km/s or a mass ratio of roughly 1.7. Now add say 4 tonnes for the vehicle itself and you need to supply it with 60 tonnes of propellant.

Now you see why its handy to to not bring the transit vehicle back to LEO.

Now I've left out some nit picky details, like how to fuel the capsule on Mars etc. But lets not worry about that too much. I'm just trying to present an example of what you can do with the benefit of aerobraking/aerocapture. And then trying to do a simple like with like comparison.

Here's the comparison.

We'll try instead a direct landing vis.

Start with a transit vehicle in high Earth orbit as before. Only this one travels to Mars, lands, and then returns directly.

Lets assume the same basic vehicle as above can land. So I'm being quite fair here. In this case the total Delta V from Mars surface is something like 4.1 + 3 = 7.1Km/s.

That's a mass ratio of roughly 6.8 on methane. So what just took off from the surface of Mars started with 204 tonnes. That's 174 tonnes of propellant..

Ouch. But hang on, you get a 16:1 mass leverage ratio if you just bring Hydrogen with you, so I'll do that.

In this case you need about 10.8 tonnes of Hydrogen to land with it. So on approach to Mars this thing weighs 40.8 tonnes.

Back tracking and you now need again a mass ratio of 1.6 so you've started in high Earth orbit with 65 tonnes. That's as opposed to 81 tonnes where you take your fuel all the way, but only to orbit.

Of course I've left out the issue of getting the capsule fueled. But then I've also left out on the flip side the issue of the extra mass the vehicle now needs to land, and by virtue of this extra structural strength to cope with actual landing.

On the basis of this crude calculation, would you save much mass? Probably. Enough to make a difference? Probably not. Its only a crude comparison and the devil is in the detail but my take on it is you can make it to Mars, safely and elegantly, with conventional propellants - and personally I'd prefer to have the safety in having something in Mars orbit at all times.

Cheers

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.

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..

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

"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 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

"Sure putting up smaller piece makes it easier to use existing launch capability but that also adds complexity to each piece depending on wait state time to mating to the next piece as well as for more dead weigh being needed to couple the stages, more fuel plus thrusters or engines, power supply parts in either solar or batteries ect...."

I think the answer to that is to use in orbit assembly judiciously. And to be clever with the way things self assemble. There's really only a few things that present a real challenge.. I'll go through those.

If its a space hab, it can launch from Earth, and simply dock in orbit.

Likewise the pieces for a propulsion vehicle don't in and of themselves have to be particularly large - at least in diameter. 

Similarly for various other things that can be launched direct.

The two main things I have an eye on are these:

1. Big heat shields. I think these will become inevitable. And lend themselves to an assembly line of sorts.

2. Mars habitat itself.

Btw, if you design a mars habitat to integrate with (structurally) and sit on top of a large heat shield (40m dia) then you suddenly have lots of design freedom (you can even throw in a central inflatable space). And you can do it on one level, and by its nature (since you have to assemble it in orbit) its compartmentalized.