Mars Sample Return Options

[monitorlizard]

Rats! I see I'm off by a factor of ten in the weight of the MAV envisioned for an MSR. Pity. I was thinking like a Soviet engineer in the 60s.

[John Whitehead]

"Jim from NSF.com" noted earlier today that a pump-fed minature launch vehicle may not be viable due to inefficiencies. That's probably true if miniaturization is attempted for centrifugal pumps powered by turbines (aka turbopumps).

Tests on a positive displacement miniature pump indicate that only 2% of the total propellant would provide enough power to run fuel and ox pumps, which is efficient enough. See JPL's Mars Technology Program website for a synopsis at (hope it's still there):
http://marstech.jpl.nasa.gov/content/detai...amp;TaskID=2289

It's agreed that pressure-fed propulsion is nice and simple and reliable, but you end up having to make tank walls thicker and heavier than you would like them to be, and also engines larger than you would like them to be. For decades, people have envisioned pressure-fed liquid launch vehicles to get off of earth (Bob Truax, "Big Dumb Booster," Beal Aerospace, Microcosm, etc.), but so far none has succeeded. Even using the strongest materials (carbon fiber), the stages end up being closer to 80 percent propellant than 90 percent, and that makes all the difference (in displacing payload with rocket hardware, or in requiring the whole vehicle to grow huge for the same payload).

Regarding monitorlizard's comment about the 1985 ASAT test rocket, the altitude it reached does not necessarily indicate capability to attain orbit (earth or Mars). Reaching orbit requires velocity in addition to altitude. NASA's goal for Mars sample return is to park the sample package in a circular orbit at 500 km altitude. Given only a quarter of orbital energy (half the velocity), it's possible to launch straight up to 500 km (sort of like the ASAT rocket did), but then you fall straight back down again.

"Soviet engineer in the 1960's," is an especially appropriate comparison since their return vehicles that launched off the moon (Luna 16, Luna 20, and Luna 24) weighed about a metric ton. The Mars ascent vehicle has to be a tenth that mass, and just to get to Mars orbit it needs a much higher velocity (4200 m/s) than going from the moon all the way to earth (2800 m/s, which the Soviets did nicely using only one rocket stage).

Great discussion!

John W.

[nprev]

It seems that for sake of economy and simplicity a solid first stage would be ideal. However, I can't think of anything else but a liquid-fueled (hypogolic?) second stage to achieve orbit; it has to have a throttle & restart capability, esp. if the first stage under- or over-performs, and of course to circularize the orbit. Perhaps pressure-fed tanks controlled by valves alone would overcome the pump problem; pressurize the hell out them with Martian air before launch, then leave the feed pumps on the surface...

Not even touching the autonomous G&C requirements here. This will not be easy.

[monitorlizard]

I concede now that I was way off with the ASAT idea. It just seems a pity that the Air Force has so many missiles of different sizes and ranges but none seem to be usable as the basis for an MAV.

As long as I'm throwing crazy ideas to the crowd... What if instead of thinking in terms of a 1 kg or larger Mars sample , we go much smaller, say ten grams. Someone has already pointed out here that you could extract a huge amount of science from such a sample size. So you get maybe a small pebble and a little soil. Now the MAV can be smaller, though I admit you still need the same guidance systems, attitude control, radio beacon, etc. (maybe some systems could be made slightly smaller in proportion to the smaller capsule needed for the sample).

Using a balloon to raise an MAV to an altitude of several thousand feet would be a way to make the MAV even smaller, acting as a sort of first stage for the rocket. I don't know how the trade-offs would compare, but if everything were of minimal size, it might be doable. I know it probably seems unnecessarily complex and not worth the effort, yet the U.S. Defence Department studied this very same concept before Sputnik as a way to achieve Earth orbit before the Soviets.
Maybe the fact that they never did it should tell me something.

[djellison]

Using a balloon to raise an MAV to an altitude of several thousand feet would be a way to make the MAV even smaller,

And also a way to make the MAV far more complex that it ever need be. Balloon's are not great on Mars. The atmosphere is so thin that you would have to have an ENORMOUS balloon to get something like this aloft - and you still have the far from trivial issue of launching from a balloon, particularly w.r.t. orientation for launching. A comparatively simple solid fueled two stage MAV with a cube-to-nano sat sized payload with a radio beacon of some sort - keep it as simple as possible. If you send a mid-scale rover in advance to get samples - then an MSL-scale lander could house both the MAV, and a contingency sample gathering micro-rover. The hard part is the on-orbit rendezvous - and how to convince people that you've got the samples very very tightly locked up.

Doug

[John Whitehead]

You're right, nprev, that a solid first stage is very attractive. NASA's reference design concept for a MAV has been a 2-stage solid more or less since about 1999. The good news is that solid rocket motors on the scale of interest (100 kg give or take a factor of 2) are existing technology, and they are a whopping 90 percent propellant. However, there's devil is in the details. The thrust of such small solid motors is way more than is needed. It would reach high speeds while still low in the Mars atmosphere, so there's somewhat more aerodynamic drag than for a liquid MAV. Worse, perhaps, is that the high thrust also requires the directional control system to be larger and heavier than would otherwise be needed, and control must be very responsive (quick) to steer correctly for the 20 seconds or so before the first stage motor burns out.

Solid motors and their payloads are usually spinning when used for space maneuvers. Launching a spinning MAV would require the lander to have a spin table rigidly anchored to the ground so it doesn't start wiggling when the MAV is spun up. The landing orientation cannot be guaranteed, so the launch platform would require tilt adjustments on two axes, and then still be rigid when it starts spinning. How to design such a lander or estimate its weight to compare with other options? A spinning MAV was considered at NASA in 1998-1999 and ruled out.

Pressurizing the "heck" out of tanks and leaving the pumps on Mars is not a solution because the high-pressure tanks would be way heavier than pumps.

You hit 2 nails on their heads, monitorlizard.
1. There are so many different kinds of rockets and missiles out there, that it is way too easy for the "collective consciousness" to assume that it is possible to just go and buy something that can launch off of Mars. Therefore there has been no NASA (or ESA) money dedicated to aggressive technology development, most likely necessary.
2. Minimum size for avionics is really what determines the smallest MAV. Who wants to make the agonizing decision about how much telemetry to put on board? If it doesn't reach Mars orbit, how much data is needed to know why the multi-billion dollar mission failed (the painful lesson from Mars Observer 1992).

Rising through the atmosphere with a helium balloon before launching the rocket would be the ideal way to get off of Venus, if only the balloon could be kept from melting.

So for Mars ascent there are several possible solutions, none of which is existing technology. Ideally, some amount of engineering effort (building and testing things) would be affordable for each candidate, to help sort out what makes sense to pursue further.

John W.

[Jim from NSF.com]

Pressurizing the "heck" out of tanks and leaving the pumps on Mars is not a solution because the high-pressure tanks would be way heavier than pumps.

But not too heavy for a MAV

[nprev]

The good news is that solid rocket motors on the scale of interest (100 kg give or take a factor of 2) are existing technology, and they are a whopping 90 percent propellant. However, there's devil is in the details. The thrust of such small solid motors is way more than is needed. It would reach high speeds while still low in the Mars atmosphere, so there's somewhat more aerodynamic drag than for a liquid MAV.

John, I admit my ignorance with respect to propellant chemistry, but would it perhaps be possible to formulate a solid fuel mixture that would provide adequate--well, the correct amount is what I really mean--thrust for Martian conditions? Seems easier than designing the MAV for different (and possibly quite variable) atmospheric conditions with COTS booster thrust as a constant.

[John Whitehead]

John, I admit my ignorance with respect to propellant chemistry, but would it perhaps be possible to formulate a solid fuel mixture that would provide adequate--well, the correct amount is what I really mean--thrust for Martian conditions? Seems easier than designing the MAV for different (and possibly quite variable) atmospheric conditions with COTS booster thrust as a constant.

nprev, I'm happy to admit I'm not a solid rocket expert. But when I've asked solid rocket experts, there is no definitive answer to this question. I fall back on George Sutton's book. Thrust is determined by total mass flow (of burnt stuff), which is proportional to the exposed area of the propellant grain, times the regression rate of a given propellant. Smaller motors have a higher ratio of burn area to propellant mass, hence short burn times. Burn area relative to volume can be reduced by making the solid motor long and skinny, an "end burning" propellant grain. Then how does that package up as a MAV stage (bending modes, more inert wall mass, and it doesn't fit in the spacecraft on the way to Mars).

How do you make a mixture of fuel and oxidizer burn slower? The best of my understanding is that you have to dilute it with something, i.e. lower temperatures, lower Isp, lower exhaust velocity. I wish the rocket companies would publish a paper or advertise their capability to produce low-thrust solid motors. My personal bet is that it's not going to happen.

[nprev]

Thanks, John.

Hmm...sounds like a real challenge in systems engineering...so many interdependencies! I've got some former classmates still looking for thesis research topics; this sounds like a goodie. Will see if anyone's interested.

[tty]

Burn area relative to volume can be reduced by making the solid motor long and skinny, an "end burning" propellant grain. Then how does that package up as a MAV stage (bending modes, more inert wall mass, and it doesn't fit in the spacecraft on the way to Mars).

That ”long and skinny” comment is interesting. About the only existing solid rocket motors in the correct size class are for BVR AAM’s, and these tend to be “l,ong and skinny” both for aerodynamic reasons and because they do have fairly long burn times (=fairly long flight times). Perhaps a derivative AIM120 engine might be suitable for a first stage? As for control thrust-vectoring is used in modern AAM’s, though usually only in agile short-range missiles. So most of the technology does exist, but not in a form that is immediately usable.

[mcaplinger]

even a very little bit of Mars would go a long. long way in terms of answering fundamental/nagging questions such as the presence or absence of superoxides, carbon abundance/source, iridium ratios, etc...

Many of these questions could be far more cost-effectively answered with in situ measurements than by sample return, and some of the others could be answered by a much simpler SCIM-type mission. And some we more or less know already from the SNCs.

Frankly, because of the fundamental energetic difficulties and the valid-or-not "Andromeda Strain" concerns, I'm a little surprised that people are still seriously talking about Mars sample return as a likely mission for the foreseeable future.

[hendric]

There are some hybrid rockets, that have a solid fuel core and a liquid oxidizer. I believe the SpaceshipOne rockets were of this type provided by SpaceDev. They have throttleable and restartable rockets. So only one pump needed:

http://www.spacedev.com/spacedev_hybrid_prop.php

[John Whitehead]

That ”long and skinny” comment is interesting. ...control thrust-vectoring is used in modern AAM’s, though usually only in agile short-range missiles. So most of the technology does exist, but not in a form that is immediately usable.

Yes I suspect an off-the-shelf AAM motor would not have the required propellant fraction, and I agree with the intended meaning of that last line. It raises a key consideration which might sound like semantics, but bear with me. In the world of solid state advances (computer chips etc.), having "the technology" in hand often means something completely unrelated to "how heavy is the final packaged product." We are surrounded by the notion that implementation and technology are separate things. However, in the world of high performance rockets, the question of whether the hardware is lightweight enough is really not a separate issue. The weight is the main problem that needs to be solved. Most flying things that exist have already been evolved to a practical limit of least weight, given material strength versus the stress loads from internal pressure, thrust, flight vibrations, etc. Can we start with an existing solid stage that has directional control, and carve out a third or a half the weight? If we succeed at doing so, did we have to make innovations along the way that could rightly be called "new technology"?

Offered as food for thought.

In all cases of evaluating what might work for a MAV, the most concise answer to the question is a mass budget for the vehicle, initially supported by calculations showing realistic departures from proven capability, and ultimately supported by a complete design and a working vehicle that meets the need for delta V.

John W.

[nprev]

Hate to even bring this up, but it sure seems like we might need to fly a pathfinder technology demonstration mission before committing to the real deal...and the nasty part here is that there's no place to do it & gain any value at all in engineering terms but Mars itself.

Aside from the truly formidible problems of designing a MAV, there are a bunch of other systemic complexities and event dependencies to consider, far more IMHO than in any other UMSF effort to date. I wonder if a high-risk Discovery-class mission could designed to send a few grams back of any random Martian surface material as a bonus; the real value would be assessing the performance of all these subsystems.