Red Dragon Discussion Thread (1)

[go4mars]

That's an issue of the life of the Red Dragon's batteries.  Remember, after trunk/SM jettison, the spacecraft would be limited to its on-board stored power. 

The equatorial constraints are mostly solar power and thermal.

Power source? Solar arrays will need new hatches in the OML for deployment; RTGs would cost too much, perhaps not be allowed on such a new LV as FH, and anyway still need hatches for radiators.

Maybe by 2018 a big dollop of Pu-238 will be easier to come by.  It would certainly appear to solve a lot of the issues here.  Is its current scarcity the primary reason that "RTGs would cost too much"?  As to allowing it on FH, by 2018, FH might not be as new of a LV. 

[Robotbeat]

I'm sure that in 2018 plutonium is available in every corner drugstore, but in 2011 it's a little hard to come by.

[dunderwood]

Putting nuclear material on any rocket for the first time is likely a very expensive process.

[baldusi]

Putting nuclear material on any rocket for the first time is likely a very expensive process.

But perfectly specified, and there's experience for that. I think  having a nuclear certification for Falcon 9 wouldn't be too different from human rating. I'm also assuming it would require a Category 1 certification to start. Plus some extra redundancies. Jim talked about that.

[starsilk]

I'm sure that in 2018 plutonium is available in every corner drugstore, but in 2011 it's a little hard to come by.

nice quote, Doc. 

[mike robel]

This appears to be a good summary of the plutonium problem.

http://www.npr.org/2011/11/08/141931325/the-plutonium-problem-who-pays-for-space-fuel

[Kaputnik]

It's not as if an RTG would suddenly solve all the problems. It will still need a radiator piercing the pressure vessel, and an in-flight cooling system. Usually, RTGs are externally mounted where they can simply cool down by radiation.

EDL remains, of course, the biggest impediment to feasability. People seem to be assuming that the Super-Dracos are going to kill off the approach speed. Do not underestimate how much of a departure this would be from any previous attempt at atmospheric entry.

[Robotbeat]

Talking about using RTG for "Red Dragon"? Seems like a poor idea to me. Solar would be much cheaper, not just for direct costs but for all the special handling and integration required for a nuclear payload.

I like RTGs for certain things, but it's not really the inexpensive option for Red Dragon. Using an RTG could also postpone the mission if someone decides to tie up the launch with a lawsuit, etc. Not a good idea.

[hop]

Talking about using RTG for "Red Dragon"? Seems like a poor idea to me. Solar would be much cheaper, not just for direct costs but for all the special handling and integration required for a nuclear payload.

I agree, RTGs don't make sense for what is supposed to be a quick, cheap tech demo. I maybe guess there might be a small chance that it could be combined with an ASRG demo, since NASA would really like to prove one of those before committing real missions to it. Even that seems dubious though, the EDL will be high risk, and if it fails you won't really prove the durability of the ASRG. On the plus side, Dragon being designed for crew aborts should make it easier to ensure the RTG remains intact in a launch failure.

Another option for a drill mission would be small solar panels to run the avionics, and some kind expendable power source for the drill. Dragon gives you a lot volume and possibly mass to play with, so something relatively inefficient might still work.

[simonbp]

I'm sure that in 2018 plutonium is available in every corner drugstore, but in 2011 it's a little hard to come by.

Unless you're proposing for a Discovery-class NASA planetary exploration mission.

Two of the three proposals in the current Discovery round use government-supplied ASRGs (Great Scot!)...

[Nathan]

Does the dragon need to take the pressurized canister along with it? 
And would that save much mass?

[Kaputnik]

I would have thought that 'the pressurised canister' would be pretty essential to the structure of the spacecraft, acting as a sort of chassis to which all of the other components are attached.
Even if Dragon is somehow structually self-supporting without the single largest piece being present, it would be such a massive design change that the whole idea of a cheap, minimally modified Dragon has gone out the window.

As I and others have said before, the most sensible approach would be to get SpaceX to build a more conventional lander of around one or two tonnes mass, with a 5.2m heatshield, supersonic parachute, and some variant of Draco or uper-Draco for landing. In other words, repackage their existing expertise into a format that is known to work.

[simonbp]

As I and others have said before, the most sensible approach would be to get SpaceX to build a more conventional lander of around one or two tonnes mass, with a 5.2m heatshield, supersonic parachute, and some variant of Draco or uper-Draco for landing. In other words, repackage their existing expertise into a format that is known to work.

By "sensible", you mean "no way that's ever going to fit in a Discovery proposal budget". Development of the MSL EDL system was a large fraction of MSL's >$2 billion budget.

If this mission gets funded, it will be as much as an EDL demonstrator as a science mission, much like Mars Pathfinder. Except instead of proving a that mostly-passive system could work, they'd be proving that a very active all-propulsive system could work. That's plausible because SpaceX wants to land people on Mars, and so is willing to have "skin in the game" for a pressurized lander. They appear not to be as interested in just copying a Viking-style lander, as it would not advance their own goals.

And frankly, the Viking-style landing you advocate is a kludge. Viking (or rather Voyager Mars) was originally designed around using modified Apollo capsules and parachutes, but when the Mariner results reported that the atmosphere was ten times thinner than astronomical estimates, JPL had to redesign. The result was the Viking EDL system, which kept the lenticular lifting entry vehicle and supersonic parachutes, but added a propulsive stage at the end. It's not a system that you'd use if designing an EDL system from scratch, and it's the only US Mars EDL system that's ever failed (MPL).

[Zed_Noir]

As I and others have said before, the most sensible approach would be to get SpaceX to build a more conventional lander of around one or two tonnes mass, with a 5.2m heatshield, supersonic parachute, and some variant of Draco or uper-Draco for landing. In other words, repackage their existing expertise into a format that is known to work.

But that would no longer be a Discovery class mission for budgetary purposes. Never mind the time and money needed to developed yet another boutique lander design. The Red Dragon concept is to test if the Dragon capsule can make planetfall on Mars with minimum modifications.

Got a query. Can the Dragon with the trunk can have supplemental  hypergolic propellants feed from tanks inside the trunk?

[Jason1701]

Got a query. Can the Dragon with the trunk can have supplemental  hypergolic propellants feed from tanks inside the trunk?

Requires substantial redesign.

[Kaputnik]

As I and others have said before, the most sensible approach would be to get SpaceX to build a more conventional lander of around one or two tonnes mass, with a 5.2m heatshield, supersonic parachute, and some variant of Draco or uper-Draco for landing. In other words, repackage their existing expertise into a format that is known to work.

By "sensible", you mean "no way that's ever going to fit in a Discovery proposal budget". Development of the MSL EDL system was a large fraction of MSL's >$2 billion budget.

And MSL was just refining an existing concept, not doing something totally new. Still uses a heatshield to decel to supersonic, parachutes to go subsonic, and propulsion for the rest.

SpaceX are apparently relying on hypersonic retro-propulsion, a fundamentally very different means of EDL, which has never been demonstrated, even on Earth.

Does it pass the sniff test that it would somehow be cheaper to develop this new capability than it would be to clone something that is known to work?

And frankly, the Viking-style landing you advocate is a kludge. Viking (or rather Voyager Mars) was originally designed around using modified Apollo capsules and parachutes, but when the Mariner results reported that the atmosphere was ten times thinner than astronomical estimates, JPL had to redesign. The result was the Viking EDL system, which kept the lenticular lifting entry vehicle and supersonic parachutes, but added a propulsive stage at the end. It's not a system that you'd use if designing an EDL system from scratch, and it's the only US Mars EDL system that's ever failed (MPL).

Thanks for the background info, I didn't know they started with Apollo. At what point in the planning were the chutes changed to supersonic, and the heatshield from semi-spherical to sphere-cone?
If Viking was a 'kludge', what alternative design would have met the landed mass requirements with better efficiency or safety?
The studies I have seen, and my own BOTE calculations, suggest that the alternative EDL systems (all propulsive, or deployable heatshield/ballute) would all add mass and not necessarily decrease risk.


I think the mention of MPL is a bit of a red herring. For the purposes of this discussion, *all* previous US Mars EDL attempts can be lumped into the same category. They have all used the same designs and principles for entry through to subsonic flight; the individual payloads have chosen different means of actually touching down, which is trivial in comparison to the rest of EDL.

[Kaputnik]

As I and others have said before, the most sensible approach would be to get SpaceX to build a more conventional lander of around one or two tonnes mass, with a 5.2m heatshield, supersonic parachute, and some variant of Draco or uper-Draco for landing. In other words, repackage their existing expertise into a format that is known to work.

But that would no longer be a Discovery class mission for budgetary purposes. Never mind the time and money needed to developed yet another boutique lander design. The Red Dragon concept is to test if the Dragon capsule can make planetfall on Mars with minimum modifications.

Well even a rudimentary understanding of the problems at hand show that the 'minimal' modifications means the ability to fire the super-dracos into a hypersonic airstream, massively enlarged propellant capacity (at least a third of the total vehicle mass, probably a lot more), accomodation of the payload itself, accomodation of other systems for operation on the surface of Mars (external arrays), plus the minor inconvenience of making all of the systems certified for operating in the interplanetary and Martian environments.

Got a query. Can the Dragon with the trunk can have supplemental  hypergolic propellants feed from tanks inside the trunk?

Perhaps the capability could be created, but adding thrusters to the trunk itself would probably be simpler.
Of course any externally carried propellant would only be of use for MOI and de-orbit, not for descent itself.

[go4mars]

Got a query. Can the Dragon with the trunk can have supplemental  hypergolic propellants feed from tanks inside the trunk?

Requires substantial redesign.

What about big fat tanks inside the dragons interior (where the cargo currently goes)?  Seems like it might be easier to add-in (beside the drill) than having it in the trunk.  A drill that only has to penetrate the dragons hull then 1 m of ice could be pretty simple and light.  The micro-scopes, communications systems and stuff, I'm not too sure about...

Personally, I still like the giant ballute in the trunk notion (big trunk ballute does the heavy work, then LAS takes dragon off the trunk and lands it propulsively).   

Idea:
Inside dragon are 8 big spherical or cylindrical tanks (stacked 2-high near each set of super-dracos) that add extra propellant to the current system.  They can be so big that they touch each-other adjacently.   In the tall skinny middle part of the dragon (between all the tanks), a drill with a diamond coring-bit descends, cuts through the dragons hull, and 1 meter into the surface.  The core is retracted, and passes vertically up through a gauntlet of active and passive tests and imagers (with the more destructive tests near the top).  Then the data/results get transmitted back (via MRO?) before the dragon expires. 

[Jim]

How is everything installed?

[Robotbeat]

As I and others have said before, the most sensible approach would be to get SpaceX to build a more conventional lander of around one or two tonnes mass, with a 5.2m heatshield, supersonic parachute, and some variant of Draco or uper-Draco for landing. In other words, repackage their existing expertise into a format that is known to work.

By "sensible", you mean "no way that's ever going to fit in a Discovery proposal budget". Development of the MSL EDL system was a large fraction of MSL's >$2 billion budget.

And MSL was just refining an existing concept, not doing something totally new. Still uses a heatshield to decel to supersonic, parachutes to go subsonic, and propulsion for the rest.

Skycrane is just refining an existing concept? Hmmm...

SpaceX are apparently relying on hypersonic retro-propulsion

Supersonic, maybe, but where is your proof that it is hypersonic retropropulsion? A Dragon capsule, according to my calculations, has a terminal velocity of approximately the speed of sound at Mars.

, a fundamentally very different means of EDL, which has never been demonstrated, even on Earth.

It's not really that different, unless you have only a single engine pointing directly upstream. I'm not going to trivialize what needs to be done, but you seem to have only a very superficial understanding of hypersonic retropropulsion. Dragon's landing thrusters would be on the side, like crewed Dragon, and thus wouldn't be directly in the airstream... And what simulation/experiment that has been done on supersonic retropropulsion (it's not likely to be hypersonic for an unmanned spacecraft) has shown that thrusters to the side like that work quite well, keeping the vast majority of the drag. And actually, hyper- and/or supersonic retropropulsion HAS been done on Earth (remember reading about it), just not operationally, since there's exactly no reason to do it operationally at Earth since the atmosphere is far, far denser than at Mars.

Does it pass the sniff test that it would somehow be cheaper to develop this new capability than it would be to clone something that is known to work?

Something "known" to work (part of the time...), yet costing hundreds of millions of dollars for even a comparable payload and thus eating up almost all of your budget. We actually HAVEN'T proven the capability to land something weighing more than a few hundred kilograms on the surface of Mars, and that capability is rather spendy with usually a small actual payload (the MERs are only 180kg). The Skycrane concept has not been demonstrated, yet, and is pretty expensive.

SpaceX is developing much of the stuff needed for this mission, anyway. They need abort thrusters, which are already funded partly, and they will be building a version of them that will be landing with those thrusters. Why not clone that for Mars to allow a greater payload instead of copying a typically expensive EDL concept that's ultimately quite limited in its payload mass?

Propulsive landing is required either way, and will likely always be required for any payload heavier than a laptop. SpaceX's "Red Dragon" concept skips several expensive/risky steps that the older EDL concepts (for MER, MSL, etc) have: no drogues, no parachutes, no shedding the heatshield or backshell. Also, precision guidance on the entry part combined with propulsive descent and landing can also allow landing very precisely, since you aren't being blown around while hanging from parachutes. And I think a decent case can be made that the current MSL EDL with the Skycrane concept is superior for landing a large rover, but this mission isn't about landing a rover.

Really, SpaceX needs to know how to do this anyway for their propulsively landed crewed Dragon. The only difference are the different Martian entry characteristics (something we know FAR more about now than we did when Viking landed) and that the abort thrusters need to start firing when Dragon is still supersonic (not hypersonic like you claim... and the conditions near the thrusters themselves may actually be subsonic), something which can be modeled fairly easily. Aerothermodynamics are far better understood now than when the Russian lander failed due to not understanding Martian aerothermodynamic conditions, and SpaceX has proven they know how to analyze aerothermodynamics when they splashed Dragon intact within a mile or so of the predicted point on the first try.

I mean, SpaceX isn't Taco Bell in Demolition Man, but I don't understand why you're so dismissive of this idea.

Also, I'm not sure you're aware that Dragon can already take quite a high propellant load.  When it's empty, it's not that massive, either. Also, Dragon uses bipropellant landing thrusters, not monopropellant like both Viking and MSL, thus Dragon will be capable of more impulse from the same amount of fuel. And I don't see why more than 500-700m/s delta-v would be needed, thus I don't see where you get the figure of 1/3 of spacecraft mass being fuel comes from. I get a figure more like 15-25% fuel payload, and for a relatively light Dragon, that's actually a pretty typical fuel load. Remember, since parachutes wouldn't be required, you can use that mass for fuel instead.