Mars Sample Return Options

[Rakhir]

Next phase reached in definition of Mars Sample Return mission

http://www.esa.int/esaCP/SEMJAGNFGLE_index_0.html

[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.

[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.

[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.

[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.