Red Dragon Discussion Thread (1)

[Jim]

Note- I'm not sure why he insists on using the NASA and SpaceX logos in his presentations.  And I don't know why he keeps saying, "we", when he talks.  But nevertheless...

Because he managed a NASA study with Spacex.  It doesn't mean NASA is going ahead with the project nor is it a NASA endorsement.

[adrianwyard]

Now you got me interested in a topic highly on topic and at the core of the risks of this proposal!
I don't know the math, so I will ask you. If I had to think what would happen when you fire a rocket like SpaceX hopes to do with the Red Dragon, two issues come to mind:
1) If the capsule ends up going faster than the speed of sound of its surrounding fluid a couple of effects come to mind.
1.a) First, most of the exhaust and thus thermal energy could concentrate on around the nozzle and, in the Red Dragon case, around the capsule walls. Wouldn't this need a new heat shield more resistant than SPAM (I love that acronym!).
1.b) Having the huge pressure and density difference of the exhaust to the Mars atmosphere what sort of shape would the exhaust take? Intuitively I guess it should get until it sort of equalizes the dynamic pressure. But I can't wrap my mind to translate the boundary layer to a pressure number. In any case I expect it to be sort of a tear drop that rests against the Dragon walls, going even higher than the actual Super Draco nozzles.
2) My cousin is an engineer (he worked on the ArSat-1 and 2). When watching the Red Bull Stratos jump, he remarked that breaking the speed of sound at that density would barely be felt through the pressure suit, so it was not an issue, and he was right. When we saw that the free fall time was less than the record, he speculated that the boundary layer while going supersonically might actually act as a drag reduction device, thus actually allowing him to go faster and thus take longer to reach terminal velocity. Since then we learn that Kittinger's free fall included a drogue chute. But I distinctly remember that my cousin stated the drag reduction. Wouldn't this increase the thrust losses of the Super Draco firing?

That's an interesting part of the equation. But even if the plume is constrained near the capsule when it initially fires, you are in subsonic, low-pressure atmosphere within seconds. And the entire SuperDraco deceleration is over within ten seconds (according to the public info I've seen) so my guess is TPS isn't the biggest challenge.

[adrianwyard]

And continuing on the subject of aerodynamics: it was suggested further back that the Dragon backshell angle is not optimized for Mars because a steeper angle would create more drag. Let me first admit I've not ready any technical literature on this subject, so this is mainly a call for suggested reading/links, but let me throw out some thoughts in case more knowledgable people can correct me, or add something useful to this thread.

+ My understanding is at hypersonic speeds the downstream shape makes little difference to drag.
+ My guess is this remains true until you get to transonic speeds. Red Dragon spends little time in this regime.
+ You'll arrive at a steep backshell angle if you design the widest heat shield for a given payload volume/mass, so maybe this is what has driven the precedent on Mars spacecraft, not drag-related aerodynamics?

And to appease Jim: as far as I can see the most straightforward reason to be skeptical about Red Dragon is that apart from the initial proponents, other knowledgable groups do not appear to be getting on the bandwagon. Specifically Chris McKay has apparently jumped ship to a Phoenix-based ice dril...

[Kaputnik]

And continuing on the subject of aerodynamics: it was suggested further back that the Dragon backshell angle is not optimized for Mars because a steeper angle would create more drag. Let me first admit I've not ready any technical literature on this subject, so this is mainly a call for suggested reading/links, but let me throw out some thoughts in case more knowledgable people can correct me, or add something useful to this thread.

+ My understanding is at hypersonic speeds the downstream shape makes little difference to drag.
+ My guess is this remains true until you get to transonic speeds. Red Dragon spends little time in this regime.
+ You'll arrive at a steep backshell angle if you design the widest heat shield for a given payload volume/mass, so maybe this is what has driven the precedent on Mars spacecraft, not drag-related aerodynamics?

And to appease Jim: as far as I can see the most straightforward reason to be skeptical about Red Dragon is that apart from the initial proponents, other knowledgable groups do not appear to be getting on the bandwagon. Specifically Chris McKay has apparently jumped ship to a Phoenix-based ice dril...

Interesting thoughts. I'll admit I wasn't fully aware of the differences between hypersonic drag and drag at lower speeds. It might have been myself who was flagging up the higher drag of a flatter afterbody shape. I will defer to others who know the subject better.

As an aside, I'll be attending an event tomorrow where Doug McCuiston is speaking- if I get a chance, I'll see what he thinks about Red Dragon...

[malu5531]

Also, how deep can it be throttled for the last meters of approach? Hard to do a soft landing with deceleration of more than 20m/s² on mars.

If super draco support pulsed fires, which seems to be the plan based on the naming of a video covering testing, you can have very large throttle range.

For example, if they can vary the pulse duration to as little as say 0.1s and the pause between pulses any amount of time, with the right software and control system they can have a very large throttle range.

For these landing engines it's not required to have throttling of a continuous fire, instead rapid pulses at say 50% thrust with the right software, would do just fine and enable hovering on earth, moon or mars.

This would also mean some really spectacular landing noise.

Perhaps something like this;

[guckyfan]

That rapid fire looks and sounds spectacular.

But it will be a rough shaking for the payload.

[krytek]

Interesting tidbit from a recent interview with Elon.

Anderson: And Dragon, the spacecraft you berthed with the ISS in May, has features that might eventually prepare it for a manned Mars mission.

Musk: Eventually, yes. The thrusters on Dragon are sized so they’ll be able to do launch escape—which means being able to move away from the rocket at a force of approximately 6 g’s. That same thrust level happens to be kind of a good number for supersonic retro-propulsion for landing on Mars.

Anderson: Could you have sent Dragon to Mars instead of the ISS?

Musk: Well, it would have gone very slowly—and when it arrived, it couldn’t have landed. It would have made a crater.

Anderson: The issue is stopping once you get there.

Musk: Version two of Dragon, which should be ready in three years, should be able to do it. But really, if humanity is to become multi-planetary, the fundamental breakthrough that needs to occur in rocketry is a rapidly and completely reusable rocket. In the absence of that, space transportation will remain two orders of magnitude more expensive than it should be.

full article http://www.wired.com/wiredscience/2012/10/ff-elon-musk-qa/all/

[Dalhousie]

Specifically Chris McKay has apparently jumped ship to a Phoenix-based ice dril...

Not at all, there has been no slackening of interest from the dozen or so people at Ames working on the concept, or the people at langley or JSC.

The Phoenix-based ice drill is simply a parallel approach to get a similar (though less capable) drill to Mars with the same analytical suite for the samples.  The Red-dragon mounted drill would be able to drill to about twice the depth of the one on Phoenix, so it is, from a science return point of view, the preferred platform.

[meekGee]

I see I missed a lot of excitement yesterday....

I don't think anyone ever suggested that Red Dragon is simply "Dragon going to Mars", like a title of a children's book...  "Where is Dragon going to go today?"

Clearly "As is" Dragon can't go to Mars (or moon). Is anyone suggesting that SpaceX is too incompetent to know that changes need to be made?

So arguing that Red dragon "isn't Dragon" because it is modified is silly.  It was already a modified Dragon the minute it was proposed.

How modified?  This is simply going to be an optimization between cost and performance.  Clearly even a modified Dragon is less optimal than a purpose-built Mars Lander.  Clearly it really depends on what you're going to be carrying.

My guess - with SpaceX's low R&D cost, quite a lot.

But they are completely justified IMO in calling any capsule vehicle using PICA-X, Dracos and super-Dracos in a pusher configuration, a derived control package, derived avionics, etc "XXX Dragon".

[go4mars]

Musk: Version two of Dragon, which should be ready in three years, should be able to do it. ...full article http://www.wired.com/wiredscience/2012/10/ff-elon-musk-qa/all/

Assuming that Elon is referring to the 5 meter version, purported by the MarsOne thread

"January 2016 is only three years one month and a couple of weeks away.   Can SpaceX really have the 5m Dragon variant ready to go by then?"     ~"Pretty much"

, which may or may not be one and the same as crew dragon, how would downmass capability for red dragon (Mars) increase?  Linearly, etc.?   It will have a crappier mass to heat shield surface area ratio (I suspect), but could potentially haul a lot more dragon juice along (and/or more potent NOFBX).

I assume we haven't been talking about red dragon as a 5 meter capsule all along, but am not certain given the timeframe.  Might be analogous to the "merlin block 2 performance" that we kept reading about...  Is Red Dragon = dragon 2?  Might explain a few things about current dragon's deficiencies highlighted on this thread. 

[guckyfan]

The study that suggested Red Dragon, clearly implied a stock Dragon for the task with barely the minimum changes required.

That would be:
No docking mechanism and no parachutes to save weight.

Adding a deep space communication device, probably off the shelf from NASA.

Software upgrades for orientation in space and trajectory adjustment burns, probably also using NASA experience.

Upgraded solar arrays using radiation hardened cells with high efficiency. Long life batteries. Some of these changes may already be in the manned dragon if they are to be used for extended stays at the ISS for emergency evacuation.

Changes to the metal frame of the pressure cabin, with a bigger exit door as required by the payload. That requires less pressure in the pressure cabin.

Adjust the casing and cooling of the flight electronics to the lower cabin pressure.

Development of the software for descent and landing on Mars including appropriate attitude control during entry and aerobraking. That's the most critical development IMO.

A radar sensor to measure ground distance for timing of the deceleration and landing burn. That may be part of the development for landing on earth but may require changes as the entry and descent mode would be very different on Mars.

If the electronics and sensors built into the Dragon are to be used on the surface, there would have to be a power supply and temperature control that works on the surface as the trunk is gone. But these would be part of the payload development, not basic requirements for a Red Dragon.

Any other necessary changes?

[Dalhousie]

Musk: Version two of Dragon, which should be ready in three years, should be able to do it. ...full article http://www.wired.com/wiredscience/2012/10/ff-elon-musk-qa/all/

Assuming that Elon is referring to the 5 meter version, purported by the MarsOne thread

"January 2016 is only three years one month and a couple of weeks away.   Can SpaceX really have the 5m Dragon variant ready to go by then?"     ~"Pretty much"

, which may or may not be one and the same as crew dragon, how would downmass capability for red dragon (Mars) increase?  Linearly, etc.?   It will have a crappier mass to heat shield surface area ratio (I suspect), but could potentially haul a lot more dragon juice along (and/or more potent NOFBX).

I assume we haven't been talking about red dragon as a 5 meter capsule all along, but am not certain given the timeframe.  Might be analogous to the "merlin block 2 performance" that we kept reading about...  Is Red Dragon = dragon 2?  Might explain a few things about current dragon's deficiencies highlighted on this thread. 

The scale drawings I have seen all show current Dragon dimensions.  I am not aware of any suggestion of a scaled up version in connection with Red Dragon.

[mlindner]

Upgraded solar arrays using radiation hardened cells with high efficiency. Long life batteries. Some of these changes may already be in the manned dragon if they are to be used for extended stays at the ISS for emergency evacuation.

Are those even needed? They currently use cheap silicon cells from my understanding, yes they're less efficient, but if you're not power/mass constrained to a large extent isn't it fine to go with regular cells? It's not in my area of expertise, but what can radiation do to a solar cell? It's not much more than a slab of multicrystaline (in the cheap cells) doped silicon with some wires running across it. It's not like you have to worry about SEEs.

[guckyfan]

Upgraded solar arrays using radiation hardened cells with high efficiency. Long life batteries. Some of these changes may already be in the manned dragon if they are to be used for extended stays at the ISS for emergency evacuation.

Are those even needed? They currently use cheap silicon cells from my understanding, yes they're less efficient, but if you're not power/mass constrained to a large extent isn't it fine to go with regular cells? It's not in my area of expertise, but what can radiation do to a solar cell? It's not much more than a slab of multicrystaline (in the cheap cells) doped silicon with some wires running across it. It's not like you have to worry about SEEs.

An upgrade is required as the intensity of solar energy is only ~40% of earth level at Mars. I think it would be easier to go for high efficiency cells rather than a much larger array.

Solar cells do degrade under radiation. About the mechanism of degradation and what is done for hardening I don't know. So communications satellites with their high life expectancy need the hardened version. Only 6 months for transfer to Mars may not.

[dcporter]

By the time this happens, the efficiency of stock solar panels will have increased to cover the 40% difference. (Yes you need hardened panels for LD flight however.)

[Kaputnik]

Red Dragon, in cruise mode, won't need a huge amount of power, compared to the manned LEO version. It will not spend half its time in shadow, nor will it have to support a power-hungry ECLSS.

Surface power, on the other hand, remains- IMHO- a potentially big problem to solve.

[guckyfan]

Red Dragon, in cruise mode, won't need a huge amount of power, compared to the manned LEO version. It will not spend half its time in shadow, nor will it have to support a power-hungry ECLSS.

Surface power, on the other hand, remains- IMHO- a potentially big problem to solve.

I would not see surface power as part of Red Dragon development. The Dragon has done its part as soon as it touches down and opened the door for surface access. Everything else is the job of the payload.

If any resources of Dragon would be utilized on the ground enabling its continued function would be part of payload/mission development.

[go4mars]

A quote from that article:

"Dragon's structure is designed to contain the high
forces of an Earth sea-level atmosphere; for unpressurized
missions, the structure can accommodate large
doors, fairings, or other routes to the outside environment."


Interesting capability.

But can the avionics handle it?

I don't know.  But it doesn't make a lot of sense to boast about the structure if it's completely irrelevant.

[ClaytonBirchenough]

How much pressurized volume will the Red Dragon provide? Says 7m3 payload here but am still unsure if this volume is pressurized or not.

EDIT: Been thinking about this, and I suppose it could be pressurized because the standard Dragon has this capability and modifying it would defeat the purpose of the Red Dragon mission...

[IRobot]

If Red Dragon is to carry some sort of rover for sample harvesting, it could share the RTGs power with the rover.