Landing on Mars

TwinBeam · 2012-07-02 11:44:45

Void wrote:

Using a "SkyCrane" type landing system, after ejection from an aeroshell, and after having been slowed down sufficiently, a "Cargo" of "Chain" could be released, to depend below the "SkyCrane". The upper set of links would be of the strongest matrials, and below that progressively weaker materials.
Strong metals, weak metals, plastics, edible materials.
Strapped on to the skycrane could be an inflatable shelter, or not, something that needs a greater degree of protection.
As the assembly was deployed, the chain would be too heavy for the "SkyCrane" to have any hope of landing it gently, rather the decent would be very rapid.

I'm not sure what benefit you see, of using a chain. Unless you've already expended enough fuel to effectively land, this approach will either be far up and high potential energy, or modest altitude but high horizontal velocity.

The former might represent some savings - but has already solved the basic problem of killing horizontal velocity before crashing. The latter might get the remaining lander mass low enough for a soft landing - but you'll have spread the raw materials out over at least tens if not hundreds of kilometers.

And both of them seem to require expending more time and likely fuel due to the chain taking time to unspool.

If you're going to go with crash landing materials, it seems better to just keep it simple and crash the whole craft, braking shield and all. Send a separate craft for soft-landing.

Now you might make that payload be a chain or cable. When it hits, it'll still fragment - but big chunks of it would likely remain intact, which should make it easier to find and retrieve. And "long and thin" will be easier to feed into a solar furnace's focal point.

RobS · 2012-07-02 13:13:29

Zubrin says somewhere in The Case for Mars that you do not need to despin to do mid course corrections; you just use a series of small bursts when the engines or verniers are pointed in the right direction. The inflatable he proposed for the Dragon would have 180 cubic meters, if I recall. NASA says a Mars trip needs at least 90 cubic meters per astronaut, so it would be large enough for them to move around. They'd retreat into the capsule for solar flares and perhaps to sleep.

I think I would double his proposal and use six Falcon Heavies and a crew of four. One could send one Dragon with twice as many consumables and a larger inflatable, or a pair of vehicles flying just a few kilometers apart so that if one had trouble, the other one could despin and go to the rescue. If you put too much mass into the capsule, though, you'll have more trouble with EDL. I was struck by his statement that a Dragon would have a terminal velocity without a parachute of 340 meters per second (that's about 750 mph). Does that match your calculations? I think he was using a mass of 8 tonnes and a heat shield diameter of 4 meters. That's 12 square meters of heat shield and a Beta of 670, I think.

Rune · 2012-07-02 14:00:23

RobS wrote:

Zubrin says somewhere in The Case for Mars that you do not need to despin to do mid course corrections; you just use a series of small bursts when the engines or verniers are pointed in the right direction. The inflatable he proposed for the Dragon would have 180 cubic meters, if I recall. NASA says a Mars trip needs at least 90 cubic meters per astronaut, so it would be large enough for them to move around. They'd retreat into the capsule for solar flares and perhaps to sleep.

Exactly. It may be hell to model by hand, being a vary elastic system, but that is one of the reasons we invented fancy computers, and have plenty of geeks trained to come up with fancy realtime control systems. The part I don't get is how expended expendables shield you while coming back to earth.

RobS wrote:

I was struck by his statement that a Dragon would have a terminal velocity without a parachute of 340 meters per second (that's about 750 mph). Does that match your calculations? I think he was using a mass of 8 tonnes and a heat shield diameter of 4 meters. That's 12 square meters of heat shield and a Beta of 670, I think.

I think he blundered here. Terminal velocity assumes an infinite atmosphere of uniform density. The atmosphere of Mars is anything but. Can you brake fast enough so you hit terminal speed before you hit the ground? The charts I've seen seem to indicate you really can't with a beta anywhere close to what a 11mT Dragon would have, but then again 340m/s is quite supersonic on Mars, so he seems to be obviating the issue of supersonic propulsion too.

IMO, if you double the injected payload (assemble payloads with their propulsion stages in high earth orbit from pairs of FH), then the margin stops being such a ridiculously low one, and the whole thing looks way more feasible.

Rune. Other than that, no problems with the idea

RobS · 2012-07-02 14:17:15

It will be interesting to see what Musk does with the problem of EDL.

As for the inflatable, Zubrin says a 180 cubic meter inflatable only needs to mass 250 kg because all the life support remains in the capsule. Presumably the capsule will have furniture packed in it during launch that will be rearranged in the inflatable during the voyage, then put back in the capsule for aerobraking. He says a second 250-kg inflatable can be brought along if the first inflatable has to be expended upon arrival. I find it hard to believe that in zero gee it could be deflated and compressed back into a small space for aerobraking.

I very much like this idea of a small space with a heat shield for aerobraking and a larger, expendable space for the voyage. It is very clever.

louis · 2012-07-02 15:28:24

Rune wrote:

RobS wrote:
Zubrin says somewhere in The Case for Mars that you do not need to despin to do mid course corrections; you just use a series of small bursts when the engines or verniers are pointed in the right direction. The inflatable he proposed for the Dragon would have 180 cubic meters, if I recall. NASA says a Mars trip needs at least 90 cubic meters per astronaut, so it would be large enough for them to move around. They'd retreat into the capsule for solar flares and perhaps to sleep.
Exactly. It may be hell to model by hand, being a vary elastic system, but that is one of the reasons we invented fancy computers, and have plenty of geeks trained to come up with fancy realtime control systems. The part I don't get is how expended expendables shield you while coming back to earth.
RobS wrote:
I was struck by his statement that a Dragon would have a terminal velocity without a parachute of 340 meters per second (that's about 750 mph). Does that match your calculations? I think he was using a mass of 8 tonnes and a heat shield diameter of 4 meters. That's 12 square meters of heat shield and a Beta of 670, I think.
I think he blundered here. Terminal velocity assumes an infinite atmosphere of uniform density. The atmosphere of Mars is anything but. Can you brake fast enough so you hit terminal speed before you hit the ground? The charts I've seen seem to indicate you really can't with a beta anywhere close to what a 11mT Dragon would have, but then again 340m/s is quite supersonic on Mars, so he seems to be obviating the issue of supersonic propulsion too.
IMO, if you double the injected payload (assemble payloads with their propulsion stages in high earth orbit from pairs of FH), then the margin stops being such a ridiculously low one, and the whole thing looks way more feasible.

Rune. Other than that, no problems with the idea

90 cubic metres? I make that the equivalent of a 6.7x6.7 metre room. That's a huge amount of space!

I think once again NASA have got it wrong.

I very much doubt Musk will run with that personal space budget.

RobS · 2012-07-02 15:37:49

Zubrin said the inflatable would mass only 250 kilos; he's not including all the armoring that you use (and need) in low Earth orbit, insulation, etc. If you have a crew of a dozen, you can go with less volume per person because you have more rooms (a great room, exercise room, labs, sick bay, personal quarters) but with only two, floating together for a LONG time, the more volume per person the better. I'd even toss in 250 kilos of potting soil and space to grow a bunch of plants.

JonClarke · 2012-07-02 15:57:57

There are many things wrong with Zubrin's two person Dragon based Mars mission. No margins, very tight allocations, and it isn't based on Dragon anway, but on the Falcon Heavy.

RobS · 2012-07-02 16:02:57

I think his two-person Falcon Heavy/Dragon mission (presumably a Dragon can be put on a Falcon Heavy, since it uses the same second stage as a Falcon 9) is mostly a teaching device (or perhaps I should say a rhetorical device). The margins, according to the piece I pasted in, are there, but an additional Falcon heavy launch might be sufficient to solve the mass allocations. His main beef is with 100 billion dollar "Battlestar Galactica" plans (as he is wont to call them) and he tends to bend over backwards in the other direction. But there is still a lot of valuable insight in his plan, too.

JonClarke · 2012-07-02 22:42:04

It certainly is a useful conceptual device and is pretty much the most minimalistic mission proposed, at least in terms of departure mass and crew size. It is not even a conceptual outline like MD , more like a back of the envelope sketch. I am not sure what value it is, except as a bench mark for one extreme. What you you find useful about it?

RobS · 2012-07-03 01:41:51

Precisely the idea that it is theoretically possible from the point of view of sound, professional engineering to send two human beings to Mars with three Falcon Heavies. It may not be a comfortable ride and the margins are thin. There are a few theoretical issues that remain, such as the capability of the atmosphere to decelerate the Dragon (something both Zubrin and Musk think is solvable; consider the Red Dragon proposal, which is also backed by Chris MacKay). The plan does not address precursor missions, but obviously there has to be an unmanned test of the system first. It does not go into possible missions to the moon or Phobos first, but those also are separate issues anyway. It does not mention the use of a cargo mission to provide margin on the surface, which is easy to add (strip down the Dragon and eliminate the 2.6 tonnes of methane and you could land perhaps 3 or 4 tonnes of cargo, including water that could be used to make additional methane and fuel a surface vehicle). There are all sorts of additions one can make to this basic proposal. If Zubrin had made them, though, people would have said "well, I wouldn't add that; I'd add this instead" and the debate would turn to a lot of bells and whistles, whether there should be two unmanned tests first, whether we should go to Phobos first, etc. By offering a simple, stripped down mission, Zubrin keeps the debate where he wants it to be and where I think it belongs--the basics. If you propose half a Battlestar Galactica, you'll get a Battlestar Galactica. If you propose something absolutely minimalist, you may get half a Battlestar Galactica in the end, but not the whole thing. And that is essential because we can't afford a Battlestar Galactica and will never get to Mars if we think we need one. One of the big problems NASA has is that every department and facility wants a piece of the pie, so some want VASIMR and say we have to go fast because of radiation problems, etc. A good Mars mission plan has to be straightforward and focused. That's how Musk was able to design a Falcon rocket for $300 million and a Dragon for another $300 million while NASA can't even design a launch escape system for that amount.

Rxke · 2012-07-03 01:53:05

Thanks to RobS I now have a new sig

Last edited by Rxke (2012-07-03 01:53:48)

RobS · 2012-07-03 01:57:36

I'm glad you like it. Somewhere I read that I think Musk commented that Dragon was designed for less than the current launch escape system for Orion (which may never fly). It's sad but true. NASA should pay companies like SpaceX to build the actual rockets and let its labs do a certain amount of pure engineering research, much of it related to the engineering problems that are faced in real situations.

Rxke · 2012-07-03 02:03:57

... And I bet a significant % of Musk's costs are purely cutting red tape ... Being the first non-gov, must be quite expensive to slash and hack a path through tangled legislative hindrances.

GW Johnson · 2012-07-04 09:44:20

Rxke:

Yep, cutting or avoiding government red tape is exactly why Spacex is looking for a private launch site eastward, but not at Cape Canaveral. One of the places they are considering is far south Texas, out over the Gulf of Mexico toward the Atlantic. I hope that goes through, it's not very far from the ground test station they already have in central Texas, only about 380 miles.

RobS & JonClarke:

One of the issues you've been addressing is how much more cost-effective Spacex has been vs NASA and its favorite contractor suite. There's actually two intertwined effects there: (1) government vs business efficiency, and (2) the tradeoff between being big enough to do the job vs being so big you are bloated and inefficient.

Regarding (1), I would say this: the smarts and the abilities do not lie in government labs, they lie in the contractors hired by those labs to actually do the job (pretty much true of everything, not just NASA). The folks in the government lab just need to be able to define the job, not do it. Typically, they can't even do that very well, that's where inappropriate requirements, inappropriate regulations, and excessive "red tape" come from.

Regarding (2), it doesn't take a ULA to be able to fly to Mars. Seeing things like Zubrin's minimalist mission plans is proof that it only takes outfits about like a Spacex. If a government lab is involved, it should more closely resemble the NASA of 1959 than the one we have today. I would also say that the more minimalist your mission design, the more likely it is to fail, vs the more like a "Battlestar Galactica" it is, the more likely it is never to be built for cost reasons. The one we could really do lies somewhere in between those extremes.

All:

I think the true limiting item for a manned Mars mission is a lander (or landers) capable of putting things on the surface in the 10-100 ton class. What's the point of going to all the trouble of sending people all that way, if you're not going to land? Gotta have a good lander. And you'll need a way back up, too.

Since mass more-or-less scales with dimension cubed, while (heat shield and/or lifting-surface) area more-or-less scales with dimension squared, the ballistic coefficients of vehicles in that size class are going to be much higher than anything flown to Mars before. The trend of reentry parameters in Mars's thin atmosphere with increasing ballistic coefficient is not good: you are still high supersonic when you come out of the heating phase, but at very low altitudes, even at shallow path angles. There's not time to slow down to touchdown, no matter how you do it.

I think that points toward augmenting the aero-deceleration during the hypersonics with low levels of retro thrust. That puts the end of the hypersonics at much higher altitudes, and gives you a selection of practical means to decelerate to touchdown. Retro thrust is involved no matter what, but parachutes may also be involved. Or not. That hasn't been decided yet.

There are ways to solve the very real design issues of hypersonic retro thrust. Too bad no one is yet working on them. We're going to need it.

GW

RobS · 2012-07-04 10:06:14

You've convinced me, GW Johnson, that retro thrust early in EDL is essential. It may not even amount to very much; less than 1 km/sec or less, with the atmosphere doing the rest. And I agree that the Mars mission that eventually be launched will not be minimalist. As I said, Zubrin's proposals are mostly useful rhetorical devices. Anyone who has seen him in action (and I have attended only 1 Mars Society convention) senses that. His strength is that he is a publicist with fairly strong engineering credentials and experience. His weakness is that he is a publicist.

Thanks for your comments on size and the role of government labs. Very interesting. It's a shame this issue can't be fixed. We've been spending about 15-18 billion in 2012 dollars on space exploration for 50 years now; that's approaching a trillion bucks. If it had been used efficiently, imagine what we could have accomplished! It could have created just as many jobs, too, and probably brought about a lot of additional new technology, yet we'd have a moon base and we'd be on Mars. 2001: A Space Odyssey could have happened on basically the budget we have.

GW Johnson · 2012-07-04 13:01:08

Hi RobS:

Yep, your assessment is pretty close. I saw him in action at the 14th convention in Dallas last summer. I had a paper there myself.

Mine got classed as futuristic "Battlestar Galactica" without considering bang-for-the-buck. I was guessing around $50-100B to send 6 folks to Mars, making 16 serial week-long landings (3 folks each landing) +/- 30 degrees out-of-plane to the orbit of the crew vehicle. That's most of Mars.

Most of the more-minimalist mission plans I saw were around $5-50B to send 2-4 folks and make one single landing. Those sound more like "flag-and-footprints" than they do exploration, at least to me.

Mine did need NTR propulsion in the single-stage reusable landers. I used 3 landers for the 16 landings, with probably enough propellant left over for at least one visit to Phobos. That's why I keep saying a practical lander design is the key enabling element. The other tinkertoys already exist in one form or another.

I was hoping for gas core NTR to make the manned portion a fast trip, but the backup could be solid core NTR on a slowboat trajectory. Solid NTR is a tinkertoy we could resurrect pretty quickly. Turns out gas core is further off technologically than I had hoped (although it could be done). I got to talk to some of the original NERVA guys about this at that convention.

GW

Rxke · 2012-07-04 13:31:03

Potentially very stupid interjection:

"they" say a space elevator on Mars should be possible with today's materials... Would't that solve all lander problems?

RobS · 2012-07-04 15:51:06

I'd say it's a question of timing, Rxke. No one will be building a space elevator in the first few decades of Mars exploration. Too expensive to move all those materials to Mars, too expensive to put it together. You need Mars infrastructure first.

Impaler · 2012-07-05 03:43:23

Rxke wrote:

Potentially very stupid interjection:
"they" say a space elevator on Mars should be possible with today's materials... Would't that solve all lander problems?

I think "they" are saying the Moon not Mars. Existing mass producible fibers would make a EML-Moon tether and at a mass we could conceivably launch, but the far larger issue is maneuvering the giant asteroid into position and the whole safety issue with the fact that the asteroid would drop directly onto the Earth if the tether failed.

Rxke · 2012-07-05 08:09:06

No, Mars,Luna does not rotate around her axis so a tether there will not be very plausible

GW Johnson · 2012-07-05 10:24:53

There are perils to a space elevator concept. These have already shown up in fiction written about such things. There's human sabotage and there's collisions with space debris or small bodies. Either brings down the elevator, which is a pretty harsh and widespread crash event. That's a ;lot of tonnage to fall.

I understand very little about tethers, but I do understand there's a huge point mass involved and it's a sort of centrifugal-force slingshot thing. A failure in a system like that releases both the point mass and all the sling material. Same sort of massive, widespread crash potential as the elevator.

I rather suspect we will eventually build and use these things, but we also will be very selective in where we site them, because of those dangers.

GW

GW Johnson · 2012-07-09 11:35:37

Rune:

I used the 1956-vintage entry model in one of those papers by Justus that you pointed out for me, to set up and verify an entry dynamics model (nonlifting). I've got it worked out for a sample case of Earth entry at escape speed. My profiles down the slant path look like what Justus reported for deceleration gees. His dynamical models look pretty good. But I'm also sure he is no entry heat transfer guy.

He didn't understand the hypersonic entry correlation for entry heating rate very well, and mistakenly used two different k-factors for his peak heating rate and total integrated heat closed-form estimates. As a result the actual time integral of his heating rate is factor 1.6 off from his closed form estimate equation for that same integral. Since he clearly didn't understand the heating correlation was a dimensionally inconsistent correlation equation, and that the same correlation coefficient should be used in both estimates, I do not trust the heating rates and integrals he reported in his paper.

Typical scientist, didn't pay close attention to stuff outside his narrow little specialty area of expertise. I'll correct the convective heating rate correlation later, and see if I can find a correlation for the missing radiation heating. (Which is larger!)

Meanwhile, the deceleration model looks pretty good. The peak in the gee profile is just about the right magnitude and speed as predicted closed form in the scale-height model. And it looks good enough to use for Mars and Titan. End of entry hypersonics for a blunt object is just about local Mach 3. (For a pointed object, that end is nearer Mach 5.) Below that, drag coefficient is a strong function of Mach until you are substantially subsonic. That's "ordinary" supersonic aerodynamics.

My goals here are crude first-order paper "designs" for a "universal" one-way chemical lander, a two-way chemical lander, and a reusable nuclear two-way lander. "Universal" means the very same machines work on Mars, Titan, our moon, Mercury, and all the airless moons of Jupiter and Saturn. I'm designing about 10 tons delivered "dead-head" payload to the surface of Mars from a low orbit. (My paper at the last convention in Dallas, TX, was 6 tons delivered.)

Once there's a practical lander design, there's no more excuses not to go, with big rovers or with men. To any of those locations, not just Mars.

GW

RobS · 2012-07-09 12:00:40

I am curious GW Johnson: I suspect the Titan atmosphere is just about the best one in the solar system for aerobraking, because it's dense, tall, and "fluffy," i.e., the density drops off very slowly with height. I bet you could send something from Earth to Titan at a pretty high velocity and aerobrake it just fine.

GW Johnson · 2012-07-09 13:25:20

RobS:

I haven't tried that model yet, but I suspect you are quite correct. The various data indicate aerobraking altitudes in the 800-900 km altitude class at Titan.

Surface pressure is about 1.5 that of Earth, and the densities look high quite a long way up because of the intense cold. Much colder, and the ideal gas model wouldn't apply, it's quite cryogenic, about 90 K at the surface. I've got the temperature, pressure, density, and sound-speed profiles vs altitude posted over at "exrocketman" in a recent article there (for Titan, Mars, and Earth). The Titan data are based on what the Huygens probe actually saw on the way down.

As soon as I get a bit more of this simplified entry-model stuff worked out and verified as reasonably correct, I'll post some of it at "exrocketman". I pretty much believe the dynamics right now. I think the entry convective heating is still crap, though.

GW

"exrocketman" is http://exrocketman.blogspot.com

GW Johnson · 2012-07-10 10:24:56

OK, I've got the old density/scale-height model for dynamics of deceleration in working order in a spreadsheet. Those results look quite realistic for Earth entry at lunar return speeds. The convective heating rate model is sort-of realistic, but rather crude. The closed-form total heat absorbed relation is just plain wrong, I only got believable results from numerically integrating estimated heating rate with time, as computed down the simplified straight-line slant trajectory. This is pretty close to all they could do with the original warhead re-entry problem in the 1956 time frame. It looks pretty good to me.

I have no idea if the same stagnation-point empirical heat rate equation works in Mars's "air". It ought to be ballpark, though. The dynamics look pretty believable. Heat rates are predicted lower there than here, so that's in the right direction. I have the same model set up for Titan, but the escape velocity there is not even hypersonic.

This stuff needs to "soak in" for a few more days, then I'll post the basic models over at "exrocketman". Meanwhile, I need to try some entry variations in all three locations, looking for practical deceleration gee limits vs penetration too low at Mars. I also need to look at correlating some trends of scaling up ballistic coefficient vs vehicle mass. I'm using m/CD*A = 200 kg/sq.m right now.

This is off-hours/late evenings stuff, so it'll take a while. Have patience.

GW

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