New Mars Forums

Ok, I'll tidy that up and email.

I've been toying with the idea of parking some part of the structure out of the way. In that sense it doesn't have to be a wing (in terms of generating much lift) though that helps.

You can augment a standard capsule with "air brake" panels - basically perforated sheet metal - which also double as an outer aeroshell. After peak heating, they fold out like petals.

Hardest thing about re-configurable wings is the practical considerations. Here's a half-way house between a capsule and a "airplane".

Start with a tube with a blunt nose. This shape has been proposed for Mars landers. I think NASA are working with this shape too. So the heat shield area is not just the nose but the belly. And as it stands, it generates a bit of lift too.

Now imagine that the upper surface of this tube unfolds, just like the payload doors of the shuttle. Unfolded they nearly double the effective drag - and there is still a bit of lift.

One design I'm fond of is just a somewhat more elegant version of the shuttle. But with no moving surfaces. Just a rigid body. Unlike the shuttle its not able to come to a rolling landing. Instead it lands under propulsion.

In the nose is a set of thrusters firing downwards through a portal protected just as you suggested with steam. Minimal solution is 3 thrusters. One direct straight down. Two canted somewhat to the side. Towards the tail are two sets of thrusters, again pointed downwards. Again, with a protected hollow.

At the tail is a set of main thrusters, just as with the shuttle.

Landing is on skids belly first. Take off happens like this. The nose thrusters fire and rotate the whole craft towards the vertical with the rear skids design to allow this. As it nears vertical the main thrusters fire and thus you begin ascent.

Descent begins with a low angle of attack. On the most critical surfaces you have a refractory metal (either a foam, or structured to lower conductivity). Or you put a small thickness of thermal insulation behind the skin. The idea here is the skin is designed to heat and lose some energy radiatively. Behind this is a tank of slush/water. Steam generated fills the hollows in which the thrusters sit, and selectively on certain areas such as the leading edges. A low angle of attack takes you through peak heating quickly with minimal usage of water.

As you descend past peak heating and lose velocity you raise the angle of attack, increasing drag and lift. You then use a small amount of thrust if necessary (I don't know if it is) to lower your rate of descent. Then as one last maneuver  you broadside to get as much drag as possible before going into retro thrust. Then its mainly the belly thrusters that do the work of positioning for landing.

Ok, that's my dream craft for the moment. I'm still not convinced its necessary to go to that much trouble when for the same effort you can always start with a regular capsule shape and instead just put the effort into making the heat shield-aeroshell as light as possible.

Returning to where I originally came into this. I'm still wary of landing crews along with large payloads. And looking at the numbers I can see there are still advantages to landing the large mass payloads and the crew separately because that leads to a more optimal design for each.

With a large payload but un-crewed you can afford to have smaller margins in terms of time and altitude. Also to some extent its not important that the habitat is a particularly accurate landing if instead the crewed landing can be more accurate as a trade off. If you do split the two then a standard capsule shape works well for the large mass. Since the heat shield is by design expendable you can oversize it (built in space in segments). Your 60 tonne lander with a 13m heat shield and a Beta of 400 now becomes a lander with an 18m heat shied and a Beta of 250 (even allowing for more mass in the heat shield).

Edit: And a 60 tonne craft with a Beta of 250 and no lift still gets you to Mach 3 at 8.7Km.. with 0.1 lift its 11Km.

What this also does is allow you to build a more squat habitat with more floor area. Or alternately the habitat ships to Earth orbit as a series of modules that interlock - a bit like a mini "space station" concept. Onto that structure you then bolt the heat shield segments. The overall structure provides some of the stiffness for the heat shield itself on descent. During landing the heat shield pieces are shed and you're basically landing a "station".. and that could extend past 20m in size. Even greater reductions in Beta are possible if you land certain other things separately like consumables.

And since its unmanned you can afford to lower margins. Ok, so you crater one now and then but noone gets hurt. But the crewed lander has a lot more margin - and that's where lift, and other fancy features, I think, comes into play.

I decided to add the convection heating model to my simulation.

With a 60 tonne lander, Beta = 400, nose radius = 6.3 I get the following.

(509 seconds) Velocity peaks at 3525m/s at 70Km altittude. This is the point where the drag starts to catch up with gravity. Heating is almost 1W/cm^2 at this point.
(597 seconds) 52Km altitude. Heating now at 2W/cm^2. Drag at 0.07 gees.
(649 seconds) 49Km altitude. Heating now at 3W/cm^2. Drag at 0.17 gees.
(691 seconds) 42.9Km altitude. Heating now at 4W/cm^2. Drag at 0.33 gees. Velocity still at 3380m/s.
(767 seconds) 30Km altitude. Heating now at its peak at 5.4W/cm^2. Drag at 1.06 gees. Velocity down to 2912m/s. So nearly half of original energy lost.
(849 seconds) 13Km altitude. Heating now down to 2.2W/cm^2. Drag now at its peak at 2.1 gees. 1574m/s.

899 seconds we reach 700m/s at 2.4Km

Total integrated heat load is 1.3KJ/cm^2.

So basically similar total heat load, but since gravity makes it all happen faster. Total time to surface = 911 seconds.

Going back to a 0.25 lift to drag...

Now it takes 1159 seconds.

700m/s at 7.9Km
Peak gees is 0.87 at 16Km.
Peak heating is 4.2W/cm^2 at 36Km

And curiously the integrated heat load is now 1.58KJ/cm^2.

On a quick inspection I think that's because of lift keeping us longer at higher altitudes where the V^3 heating term wins over the ~p^0.5 density term. In other words, more time spent cooking without the benefit of more drag to slow down. As always, I could be wrong.

Going back to a small crew lander with mass circa 5 tonnes its rather interesting. All I have to do is to add more lift - 0.5+ and I get a higher integrated heat load than with the big lander - as much as 2KJ/cm^2. Mind you, one has to multiply that by the total heat shield area which in this case I'm making to be 30 (bit of a squat shuttle shape).

If the simulator is to be believed, one conclusion that could be drawn (apart from the number one priority being low Beta) is that so long as you can handle the peak heating (say with coolant) then its better to zip through until you get to about 25-35Km and then apply as much drag/lift as you can - presumably altering your attitude (which is something I can't simulate just now).

Going back to a classic capsule shape but a bit fatter than a dragon - say 25m^2 effective drag area and a modest 0.24 lift.. Here's what I get.

Start at the "drop out of orbit" condition and you get to Mach 3 at 14Km.

But...

Kill some velocity - say down to 2800m/s and allow for 2 degree angle of entry. Doesn't change the altitude at Mach 3. Slightly higher peak heating, but the fun part is only a third of the heat load.
Using this setting (admittedly a bit hand tuned) it zips through the period of peak heating at about 11 degrees down angle, and then flattens out to about 6 degrees as it hits peak drag. And that's a totally uncontrolled descent.

Here's another wild/half baked idea. Think hovercraft here on Earth..

Imagine a large circular platform. Doesn't have to be flat. Ringed around the edge by a "skirt" or perhaps even a donut shape. Other better shapes are possible.

Important part is that instead of being convex to the oncoming stream, its concave.

In suitable places you place water or ice - within the skirt, or the platform, or both.

Steam is generated and its allowed to fill the volume contained under the platform. The pressure created keeps the heated gas from coming near the platform - at a generally greater standoff distance than with a conventional heat shield. You arrange the skirt to either contain, or be cooled by water or steam. Some steam will mix to cool the gas stream that goes up over the skirt in any case.

So basically you're talking about sub 500C temperatures on the platform and sub 1000C temperatures (remember the bigger it is, the cooler it is) at the skirt. So well within modest alloys, or even make some of that inflatable.

Some simple refinements include making the platform itself concave - rather like the bottom of a pressure cylinder.

But the pressures we're talking about aren't huge. A fraction of an atmosphere.

What do you think?

With the same simulator I tried a 5,000Kg capsule with Beta = 150 (roughly a 6m diameter heat shield.

Lift to drag 0.25.

Curiously it reaches a terminal velocity of about 200m/s in the last few Km.. but that's with the oversimplified density model.

Mach 3 at 16.7Km with a 11 degree down angle. Peak of about 0.9 gees at about 28Km.

I hope that including gravity won't be a showstopper, but we'll see..

Because all of these quantities are inter-related its difficult to express them in clear English. Yes, its L/W directly, but at a fixed L/D its getting enough D that matters. Actually its worse because L/D degrades with mach. But yeah we're saying much the same thing.

On reflection, steam might be a useful coolant. Based on the sorts of figures here we're talking a very wide ballpark of 20-200MJ/sqm. 200MJ would turn about 70Kg of ice into steam. Less for hot steam.

As for spreadsheets, the best way to publish is probably google drive. If not a basic list of formulas would be fine.

Might be of interest:

http://www.ssdl.gatech.edu/papers/confe … 3-0908.pdf

GW,

Yes, seen the profiles. Still drilling down to figure out how I would go about coding the model you've used.

There appear to be some problems that can only be resolved by trying out different scenarios with the model. For the moment I've given up on exact temperatures but instead I'm looking for proportional changes to rate of heating. Total heat is a different beast and it all depends on getting the heat to go somewhere else. But reducing the overall flow has to be a good thing.

The biggest problem I can see is getting enough lift (not just drag) at high altitudes (above 60Km). In this regime what seems to happen is you get a lot of heating (due to velocity) before you get a lot of drag. Without enough drag, lift to drag won't help you much. But on the other hand, at high velocities the effective acceleration towards the ground is much lower (since you're still sub-orbital). So which effect(s) win out?

I can feel some code coming on.

There may still be a role for some modest level of thrust even after interface but my gut feeling says its subject to the law of conservation of difficulty. Might be just as easy to burn off some velocity before interface. Again, only the computer knows.

As I said before, the reason I like having structures that are separate to the main part of the landing craft is that they don't have to be treated as kindly as far as cooling goes. Indeed they could be sacrificial. Same goes for wings. And the more I look at the profiles above the more I realise that its very thin air and very high speeds is where the problem is.

Besides even modest (shuttle like) wings perform better at lower mach.

As for a "drag net" the reason I like it so much is that it can radiate a lot of heat - and do so away from the craft you're trying to protect.

Btw, one thought I keep getting pushed to is making the main craft relatively slender with as little drag as possible (at least until you reach lower altitudes) and instead rely upon auxiliaries (temporary wings or nets or ballutes). Not much more I can say without the numbers though.

GW,

I'm wondering if you or anyone else who can do the math or the simulation can give me an answer to this.

Firstly, what order of lift to drag would be needed to keep a Mars lander high enough for long enough that one can keep the temperatures down to a modest level. I leave the definition of modest open there. It could mean low enough to be effectively non-ablative with conventional (but thinner) heat shield. Or it could mean low enough (a few hundred C) that you can afford to be relaxed about light weight materials.

Secondly, If we start from a low orbit and then a de-orbit burn, but then also just before interface apply increasingly more retro thrust, so your speed at interface is proportionally lower, what effects would this have in terms of peak heating and temperature?

As I said before, the extreme case is you simply hit the retros until you reach interface at zero, and then free fall the rest of the way. From 150Km up under Martian gravity, assuming no air and no parachute you'd reach 1100m/s by the time you hit the ground. Presumably then with ordinary ballistic drag you'd be travelling much slower. Question, what does the simulator say about this? Now, what if parachutes were used? Is it even necessary?

That's the extreme case. What I'm saying is there might be a happy optimum where you bring enough fuel for (say) 1 or 2Km/s delta v slowdown before interface and for this you're rewarded with a lighter heat shield, and more time to play with. Clearly that first 1 or 2 Km/s would have a huge effect on the total energy. And I presume also on the temperature.

Would love it if I had some numbers to play with.

Now combining the above two issues. Suppose I came up with a lander that was a bit more like a delta winged affair (it might prove totally unnecessary btw) and the said lander was able to use thrust at all stages both for stability and to control its angle and thus lift. Imagine for the moment that weight is distributed more towards the tail which is where the bulk of the thrust is, but there's also some nose thrusters.

So, from the deorbit you approach interface. As you do you apply some measure of retro thrust, losing (I don't know) 0.5 - 2Km/s. As you head into interface you use a smaller amount of thrust to optimise lift, mostly from the nose. You keep yourself as high as possible for as long as possible, losing energy as slow as possible. Then the inevitable loss of altitude as you cruise on past (again I don;t know) maybe 2Km/s. But you're still keeping the temps way down.

Finally you do a shuttle-spiral and with a bit of technique and good code you land on your tail. Well maybe. It might (just) be possible to land vertically on your belly. Whether its possible to take off (fully fueled) like that, is another matter.

Anyhow, if anyone can venture some numbers, it'd be appreciated

Josh,

I think Zubrin also proposed the idea of a mixed CO and methane engine which would to some extent preserve the Isp of methane while keeping the density advantage of CO. I saw it somewhere but I can't find the link right now. I think he did this in part as an answer to the purity issues that arise from the methane synthesis, plus wanting to limit the amount of H2 needed. Considering that H2 can be indefinitely stored in the form of H2O I'm not giving up entirely on a methane mixture.

GW,

Regarding a nuclear lander I'd love to see actual numbers regarding the shielding problem. All I have to cross check against is DRA 5 where they placed the engine a fair distance from the crew and on the crewed version also added 10 tonnes of shielding. Were they being over the top? Or is it a different problem because of the duration of burn? Just want to see the numbers there.

There does come a point where the mass of shielding and the density of hydrogen and its storage issues tends to take the fun out of things.

Speaking of lander/ascent vehicles in general. Has anyone given any thought to the absolute minimum mass you really need? Space suits and seats?

Here's another question. What if you really did consider a fully propulsive landing. Presumably you'd need to keep the speed at any point within the limits of very basic thermal protection. But the worst case is you slow yourself to zero at 150Km altitude. If you did that and went into free fall you'd potentially reach 1100m/s by the time you reached the surface. But that assumes no air resistance so in reality it would be less. And it assumes no parachute - which in this case would be perfectly feasible and would probably cut the final delta v in half. So if you budgeted for 5Km/s you'd be in the right ballpark.

All of that of course only makes sense if you go as light as possible.

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

Here's a footnote.

Just reading through DRM 5 http://www.nasa.gov/pdf/373665main_NASA-SP-2009-566.pdf

NASA want to land two payloads onto the Mars surface for each mission. One is the habitat. The other is the crew descent/ascent vehicle. Each has a landed mass of about 40 tonnes.

Here's their figures.
Entry mass 109.7 tonnes
Aeroshell structure 22.5 tonnes
TPS 18.2 tonnes

Even with the assumption of aerocapture they're apparently doing this from a 1 sol orbit, so a fair bit more energy to burn than slipping out of a low orbit.

Even so, look at their other table..

Ballistic coefficient 471Kg/m2
Altitude engine initiation 1,350m
Mach @ engine initiation 2.29

Ouch, ouch, ouch.. and then more ouch.

Now, what are the corresponding numbers for Mars direct?

Here's another interesting tidbit. Part of the reason the crew descent is so horribly heavy and doesn't go into final deceleration until you can spot the the wheel tracks of the rovers is that they throw in a bunch of supplies and other gear with the lander that carries the crew. They had to do this of course because the lander that carried the habitat was itself at max weight.

These are the sorts of decisions that detract from crew safety. And this is why I recoil from such designs. And the non NASA missions don't do much better, or simply avoid any serious EDL calculations.

All things considered a specialized crew lander is a better option. Whether that's going to be one and the same as an ascent vehicle I'm not absolutely sure yet.

Oh and btw, NASA does note that they have to throw in extra descent fuel in order to pull a maneuver that discards that 40 tonnes of aeroshell and heat shield hopefully "so
that the heatshield debris does not impact the surface near any highly valued pre-deployed assets"

One of these days Mars is going to need a garbage mission...