New Mars Forums

louis, sorry, that wasn't clearly expressed.

What I meant was this. If you're flying home from Mars orbit you need roughly as much mass in fuel as the mass you're trying to get home. So I figured around 30 tonnes of fuel.

Now to get that fuel from the surface of Mars to orbit, you need even more fuel. In that case with a DV of 4.1 and methane/lox you're now adding twice your mass in fuel.

Add to that the ascent stage itself, leave it with enough fuel to return to surface, and add margins, and you're talking about 130 tonnes of vehicle launching from the Mars surface.

Mind you, nearly all of that mass is essentially free.

Btw, the thing that is annoying me is the sheer size of the hydrogen tank.

I'm well aware that you can cut this down by shaving mass of the space-hab. As I said before though, the minimum mass would be set by shielding requirements so I doubt you'd get that below 20 tonnes.. unless you did something real clever.. used fuel tanks for shielding.. and took some risks. You've still got the mass of the return capsule and other junk.

Oh and an aside. Since your largest threat is one off events - solar storms - I wonder if anyone's looked into taking the obvious cheat. Putting more shielding in one direction.

Ok, how about this for a bit more detail.

The return vehicle comes in 3 main parts. The space-hab itself - nothing special, its just a tin can with everything you'd expect including shielding. Then you have a propulsion module sufficient to the task of getting the space-hab and landing capsule back to Earth. Again, nothing special, and you're talking circa 40 tons of propellant. Third is a landing/ascent module - which is actually a larger and more capable rocket in its own right.

The propulsion module dovetails into the landing/ascent module. They share a heat shield and other EDL gear.

Initially you have an unmanned flight. You've got the space-hab, propulsion module and landing/ascent module mated. They arrive at mars and aerocapture into orbit The landing/ascent and propulsion module then undock and head for a landing. Somewhere you've managed to store the necessary seed hydrogen.

The propulsion module and landing/ascent module land near to the ISPP unit and (probably over time) tank up. Then the landing/ascent module carries the propulsion module into orbit. The propulsion module docks with the space hab and together the return vehicle sits there waiting as a back up. The landing/ascent module could sit around in orbit waiting for reuse. I'm going to assume that for now.

Now we're ready for humans. They turn up in another space hab. Which brings with it another propulsion module (which is handy because you need one to get there). And of course the landing capsule. The landing capsule splits off and heads for a landing. The space hab together with the propulsion module aerocapture as before. Then the space hab (you've got plenty of time here) undocks from the propulsion module. The propulsion module docks with the landing/ascent module that happened to be hanging around from the last flight, and the cycle repeats. They go down to pick up fuel, return, reassemble. All done robotically.

When the humans leave they pick up the original return vehicle, leaving a new one ready and waiting.

Point is at no stage are you left without a plan B. There's a fully fueled and ready return craft in orbit around Mars at all times. There's also an abort to surface option. And you get the most reuse out of every part (until things wear out). For instance the space-hab can aerocapture into earth orbit, refurbish, restock. So too does the propulsion module - assuming its worth mucking around with refueling in earth orbit. And of course the landing/ascent module could last you through a number of missions.

Of course you're now going to be looking for a ISPP unit that's capable beyond one mission too.

One other detail I left out. You also need to figure out how to give the landing capsule enough tankage to get it back into mars orbit. You could in theory time it right and lift it onto the landing/ascent module but I'd rather the freedom to take off at any time.

Ok well I'll just go off in all directions

I like Space X. So far they've done pretty good work. Remains to be seen how successful they will be commercially and that's important if they're keen to go any further. I'm still searching for more details on some of their stuff - especially the capabilities of the super-draco.

I like orbital assembly. It surprises me how much people shy away from it, considering we've now got a wealth of experience with the space station. In particular I dream of some things you just can't launch direct - large heat shields - large solar arrays and so on.

Lots of things done robotically.. sure.

Don't have a problem with aritifical gravity. Its not that hard to put a spin on things.

I'm not entirely convinced of the merits of trialing stuff that's really about colonisation. Exploration is one thing, but I personally think the human race needs to get a bit more civilised before we go messing up other planets.

As far as missions go, if you take a step back and try to see the thought-process involved, its sometimes easy to see a pattern. Certain paths being taken. Certain principles being emphasised over others. What I don't see is enough attention to mindless detail - enough trial and error of random ideas.

For instance. In situ propellant. On the surface sounds like a good idea. In practice, you just need to send a test mission without humans first. Does that rank against the fuel saving? Not sure here.

For instance. If you take in situ propellant production for granted, where do you produce it? Where do you refuel, and what?

I'll give you a concrete example. Lets suppose that space-faring vehicle (the space hab as some call it) hangs out in Mars orbit. But the propulsion is logically a separate issue. Ok then. How about this. You land first a propellant lab. Could be a rover in its own right. Could be nuclear. Could be solar. Doesn't matter for the purpose. Ok, next thing is the propulsion unit that gets you back home. Somewhere in the process the propulsion unit mates with the lab and gets fueled. Its all done robotically. Next the propulsion unit takes off for Mars orbit and sits there waiting.

So you could work a mission rather like this.

Everything you'd expect gets landed on Mars.. habitat.. ISPP.. propulsion unit. Propulsion unit takes on fuel, goes into orbit. All systems check out.

Skipping over the details, the humans launch, fly to Mars, leave a space hab in orbit, that robotically docks with the propulsion stage. On Mars their capsule refuels using the same propellant lab.

Anyhow, I left out a lot of details. I'm just posing this to point out the many permutations that I don't think have been considered yet.

The thing about heating is, if the heating is done well behind your craft it doesn't matter.. you're just toasting gas molecules. Its only when you're heating something near your craft that you need to worry about insulation.

More to the point, the more surface area you present the higher you start to brake and the lower the peak heating. As a consequence if you go far enough in terms of larger and larger heat shields or rather drag creating structures, the less restrictions on materials.

I just want to raise one point again, since its an important one. There's two vital services. One is heat shielding per se. The other is simply drag. They don't have to happen in the same place or structure.

The other thing I want feedback on is the idea of a deep sea anchor. Its a device designed to create drag that trails behind the craft.

The neat thing is you can make more use of structures in tension.

One very simple form of this is simply a long.. long.. cable. Along that cable you attach a succession of self inflating structures.. basically a hybrid of parachute and ballute.

You have to design it long enough so that each successive drag element isn't too shadowed by the previous one. But the essence is that its self stabilising - it will always generate a drag force in the right direction - unlike shields that are positioned below a craft.

The point is to get yourself to that desirable point where you've got a large ratio of area to mass which has knock on effects the most obvious of which is more diffuse heating plus the fact that most of the air you're heating is behind you.

I wonder if anyone's thought of this or would care to make a comment?

Aerobraking has several risks. One is the obvious - you miscalculate and you either hit the planet or fly off into space. I think they've got that one reasonably well in hand.

The more subtle point is that a smaller miscalculation can make a huge difference to the G forces. And that doesn't matter so much if its non manned.

There is a limit to how light you can make even a direct trajectory because to some extent your minimum mass is set by how much radiation shielding you have to have.

It is true though, that if you put your non manned stuff into a very slow trajectory - which is what you'd get with a low powered electric drive - then you can probably do that without any aerobraking anyhow.

There's a bunch of things that I'm intrigued about regarding Mars missions but this is the bit related to landings.

Let's suppose the following two scenarios are viable.

A: This is the "conventional" scenario. Some of your hardware is already in place on the surface of Mars, but when you land, you land seated inside some larger structure which also happens to be all or a large part of your habitat. The conventional wisdom here is some kind of biconic heat shield or perhaps a ballute or some form of large (earth orbit assembled) structure which also serves as a very large heat shield. Or some combination of the above.

B: In this scenario you land everything on Mars first except for the humans. The humans arrive in one or two dragon style capsules. You have the capability of precision landing, but just to be sure you also carry some form of lightweight transport.

To my mind, trying to land humans along with something that weighs upwards of 60 tonnes, along with all the EDL issues that comes with that is taking more risk with the crew than needed.

If you decouple the crew and the habitat then you end up with scenario B. Of course this has its own risks and complexities.

Just briefly, if you were flying to Mars, what would you bet your life on?