Author Topic: Red Dragon Discussion Thread (1)  (Read 559055 times)

Offline Lobo

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Re: Red Dragon
« Reply #1140 on: 12/03/2013 07:10 pm »
May I ask is the PICA-X heat shield of Dragon designed and expected to survive a high-speed reentry of the Martian atmosphere?
That's a key question, and yes it is.

There's a lot of information upthread, and places like http://en.wikipedia.org/wiki/Red_Dragon_(spacecraft)

For those that don't know, Red Dragon EDL was evaluated by NASA Ames who added some insight from MSL. Their concerns were not mass or heatshield, but things like backshell heating, firing the engines while supersonic, and landing leg strength. (Aspects I addressed in my cartoon design above.)

Seems to me, from my limited knowledge of such things, that the heating on Earth reentry would be greater than for Mars reentry because of higher gravity and thicker atmosphere.  The atmosphere on Earth provides more dV, which means more heat loading on the shield.  I'd think a Mars heat shield could be much thinner/lighter because of the lower dV provided by the Martian atmosphere.  Which is why much more dV much be provided by supersonic parachutes and retro rockets than for an Earth landing.
So I'd think a heatshield designed to reentry Earth's atmosphere from BLEO speeds, should be able to handle reentry in Mars's atmosphere from BLMO speeds easily.  Is that incorrect?

As a side note, it's too bad Mars isn't more like Titan.  That would be a much easier target to put humans on I think if it were closer and warmer.  A thicker atmosphere than Earth means a small heatshield and awesome dV for EDL.  Same with parachutes.  Thicker atmosphere and lighter gravity should mean parachutes can be deployed subsonically and don't need ot be very large.
With a thick atmosphere like that, the EVA suits would be much more simple and easy to work in, as they don't need to be pressurized.  They just need to be insulated/heated, with a scuba like system for the astronaut to breathe.  THey'd need to be more "environmental suits" than space suits.
If they were insulated well enough, theoretically the astronauts own body heat would be enough to heat the suit without any active heating required.  They could use something like a Scuba dry suit, but with a full contained helmet.  As long as they can reduce the heat transfer of body heat out through the suit material enough on the severely Titan atmosphere.  If Titan were where Mars is, it'd be much warmer and thus much easier to insulate than Titan's temperatures.
Not to mention an atmosphere full of nitrigen, and lots of water ice on the surface, as well as methane in the atmosphere would make for great in-situ refueling. 

Yes, I know if Titan were where MArs is it probably wouldn't have that big atmosphere as it wouldn't have the protection of Saturn's magnetosphere.  But just such a cool though...don't ruin it for me.  ;-)




Offline newpylong

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Re: Red Dragon
« Reply #1141 on: 12/04/2013 12:04 am »
It will be the the manned Dragon without full ECLSS. That will be capable of landing through the earth atmosphere. It will have landing legs that support the landing in earth gravity. It has a pressure cabin, so what backshell? Why additional batteries? It may need better, not bigger solar panels if even that. So except the com equipment for interplanetary distances what else would add weight?

I just listed this. I think if you do some research on the items I mentioned it will be helpful.
« Last Edit: 12/04/2013 12:20 am by newpylong »

Offline guckyfan

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Re: Red Dragon
« Reply #1142 on: 12/04/2013 05:51 am »
It will be the the manned Dragon without full ECLSS. That will be capable of landing through the earth atmosphere. It will have landing legs that support the landing in earth gravity. It has a pressure cabin, so what backshell? Why additional batteries? It may need better, not bigger solar panels if even that. So except the com equipment for interplanetary distances what else would add weight?

I just listed this. I think if you do some research on the items I mentioned it will be helpful.

Could you give some pointers? After all your claims are a lot different to everything discussed here on Red Dragon.


Offline go4mars

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Re: Red Dragon
« Reply #1143 on: 12/04/2013 03:09 pm »
Lobo, I think the heat shield would still need to be beefy.  It isn't just total heat.  Duration of heating is an important factor.  I don't have anything to back that up, but read something along those lines somewhere years ago.  Feel free anyone if you can shed light or add a link.
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Offline Lobo

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Re: Red Dragon
« Reply #1144 on: 12/04/2013 04:30 pm »
Lobo, I think the heat shield would still need to be beefy.  It isn't just total heat.  Duration of heating is an important factor.  I don't have anything to back that up, but read something along those lines somewhere years ago.  Feel free anyone if you can shed light or add a link.

Well, perhaps I don't understand ablative technology correctly.  But I would think that the rate of the ablasion of the shield depends both on the total time it's being heated as well as the actual head load...i.e. the heat transfer from the atmospheric drag friction to the ablative material.  And it will ablate away as needed to shed that energy.  Whether that's a shorter duration with a much higher friction rate as on Earth EDL, or a longer duration at a much lower friction rate, as on Mars EDL.  The PICA-X or whatever material will ablate as needed to shed that energy. 

To look at it another way, MRO would dip into the upper atmosphere in order to change it's orbit into a stable orbit via aero braking.  And although it had several passes through the atmosphere for a fairly long total duration I imagine, it was exposed to so thin of atmosphere that it didn't need a heat shield of any sort.  HAd it plunged deep into the atmosphere and done all of it's orbital correction in one or two passes, it would damaged or destroyed the satellite.
I guess in my mind, I imagine Mars EDL and Earth EDL being like that.  Even doing a direct descent to the surface, with a much taller atmosphere, it's so thin that there's just not a lot of friction compared to Earth. 
Especially if you have a smaller heat shield area like a Dragon capsule would for it's volume and mass vs. more traditional shapes which have been used in Mars in the past, or some concepts which use biconics and inflatable decelerators to greatly increase the drag and thus friction. 
Dragon has a shield really designed for Earth EDL.  It'll work on Mars, but it won't decelerate itself as much as a larger surface area shape would like a biconic or large diameter shield.   It'll have a faster terminal velocity, and thus need more dV provided by some other means.  IN the case of Dragon, retropropulsion. 

Besides, isn't Dragon's heatshield designed for multiple uses for Earth EDL?  On Mars, such a lander would only need to get through Mars's atmophere once.  So I think if it's thick enough for multiple Earth EDL's, it could do one EDL.  And, unless my understanding of ablatives is completely wrong (which is a possibility), I think an ablative heatshield designed for just one EDL on earth would still be thick enough for one EDL on Mars as the atmophere imparts less energy visa vi friction to the ablative material due to it's lower density.

But if that's incorrect, someone feel free to correct me.  I'm here to learn!  :-)

 

Offline guckyfan

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Re: Red Dragon
« Reply #1145 on: 12/04/2013 05:58 pm »
I do remember the discussion of the reentry of the Inspiration Mars capsule. It was said NASA did extensive studies because the reentry speed is extreme. So they checked skip reentry against direct reentry and came to the conclusion that the hard direct reentry is more suited to the characteristics of PicaX (or ablative heatshields in general?).

In any case with the thin atmosphere and the low reentry speed on Mars a Red Dragon should be fine. Unless newpylong can point us to studies that show otherwise. He did not yet do that above his claims there would be problems.


Offline newpylong

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Re: Red Dragon
« Reply #1146 on: 12/04/2013 06:26 pm »
I do remember the discussion of the reentry of the Inspiration Mars capsule. It was said NASA did extensive studies because the reentry speed is extreme. So they checked skip reentry against direct reentry and came to the conclusion that the hard direct reentry is more suited to the characteristics of PicaX (or ablative heatshields in general?).

In any case with the thin atmosphere and the low reentry speed on Mars a Red Dragon should be fine. Unless newpylong can point us to studies that show otherwise. He did not yet do that above his claims there would be problems.

Why does there need to be a study? Common sense goes a long way.

If the atmosphere is so thin, what is going to slow the capsule down? Is there enough fuel for the super dracos at that speed? What about supersonic chutes to start with? They aren't light...  What is going to provide power to the capsule once it separates from the trunk? There are no panels, it's supposed to separate and die within hours. The list goes on and on... (I have already provided numerous examples).

My point was a group of people at Ames did research with SpaceX 3 years ago on this. Much has changed since then. It is a theoretical idea and it would not work with an off the shelf Dragon. Every modification would add mass. If you are not accepting of my opinion, that is fine.
« Last Edit: 12/04/2013 06:27 pm by newpylong »

Offline guckyfan

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Re: Red Dragon
« Reply #1147 on: 12/04/2013 08:47 pm »
I do remember the discussion of the reentry of the Inspiration Mars capsule. It was said NASA did extensive studies because the reentry speed is extreme. So they checked skip reentry against direct reentry and came to the conclusion that the hard direct reentry is more suited to the characteristics of PicaX (or ablative heatshields in general?).

In any case with the thin atmosphere and the low reentry speed on Mars a Red Dragon should be fine. Unless newpylong can point us to studies that show otherwise. He did not yet do that above his claims there would be problems.

Why does there need to be a study?

In other words you have nothing to back up your claims.


My point was a group of people at Ames did research with SpaceX 3 years ago on this. Much has changed since then. It is a theoretical idea and it would not work with an off the shelf Dragon. Every modification would add mass. If you are not accepting of my opinion, that is fine.

Point out what has changed. I really don't know of anything. Dragon is being developed exactly the way that study assumed. You keep bringing up modifications that add weight, many tons to make the concept non viable. Name any.

OK you have an opinion, that I don't see as valid and you make no argument to back it up. But it's yours, so let's agree to disagree.


Offline Kaputnik

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Re: Red Dragon
« Reply #1148 on: 12/04/2013 09:19 pm »
What about supersonic chutes to start with? They aren't light... 

I believe the plan was to have no chutes at all.

FWIW, I have been very sceptical of the Red Dragon idea all along. Seems to be right at the margin of what is technically possible within the mass limits. We don't know enough about the super-dracos to really say, either way, but IMHO it's very close.
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Offline guckyfan

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Re: Red Dragon
« Reply #1149 on: 12/04/2013 10:04 pm »
I believe the plan was to have no chutes at all.

FWIW, I have been very sceptical of the Red Dragon idea all along. Seems to be right at the margin of what is technically possible within the mass limits. We don't know enough about the super-dracos to really say, either way, but IMHO it's very close.

I don't know why the SuperDraco would be the weak point. They have a lot of thrust to work as LAS. Fuel might be the limiting factor. It may be necessary to increase fuel load. But not too much could be added because aerobraking would go down also with extra weight. So maybe this would be where the case for Red Dragon does not close.


Offline adrianwyard

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Re: Red Dragon
« Reply #1150 on: 12/04/2013 10:20 pm »
That seems correct to me. The rule of thumb with heavier Mars EDL seems to be go for the lowest ballistic coefficient available, i.e. a large heat shield area per mass resulting in low density. And (Red) Dragon does not do this, and so on the face of it looks like a poor solution.

If you take the public numbers to be correct then you can add up to ~5 tonnes of additional fuel and/or heat-shield area to the Red Dragon concept and still be within the capability of FH. So there is room to maneuver if you think it won't close as is.

But remember the entry trajectory of the Red Dragon proposal was evaluated by NASA Ames, and they evidently thought it could land.

Additionally, I don't see what has materially changed since this Ames analysis: the performance of the Super Dracos for example are still rated 8x15Klb which is ~ dictated by their abort role on crewed ISS missions. If there's a problem with their performance I could see SpaceX pulling out all the stops to solve it in order to win the commercial crew competition. Red Dragon benefits from the company-critical nature of this work.
« Last Edit: 12/04/2013 10:47 pm by adrianwyard »

Offline Jcc

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Re: Red Dragon
« Reply #1151 on: 12/07/2013 01:18 am »
To me, one of the long poles for Mars human exploration and colonization is precision landing. Habitations, equipment, supplies and crew capsules all need be landed in close proximity to one another. An EDL system that relies on propulsion should also have a lot of control in terms of landing in a precise location (once there is some infrastructure in place to provide guidance, such as radio beacons,etc.).

Maybe even the ability to land initially in a "drop zone" then propulsivley relocate to the final position. Sure if you have ground based equipment to pick up modules and relocate them that is more energy efficient, but otherwise you need to use the resources you have, so if the Red Dragon capsule has engines powerful enough to raise it up, and put it down precisely a few hundred meters away, so it can connect with others a la Mars One, then it would be practical to do that.

Offline adrianwyard

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Re: Red Dragon
« Reply #1152 on: 12/07/2013 05:49 pm »
The ATHLETE and TRI-ATHLETE projects have always struck me as pretty sensible ways of relocating items so they are close together.

http://athlete.jpl.nasa.gov

These may take an age to do the relocating if it's a long distance, but that's of little consquence if it's just cargo - it's already taken 6-9 months. If EDL is accomplished without significant use of parachutes, i.e. an aerodynamic shape that has some lift/crossrange, followed by landing rockets, all with accurate awareness of current atmospheric condictions, then you can hope to land quite close to your target.

ATHLETE works well if integrated into the payload itself. Or if you have a couple of TRI-ATHLETEs around already they can naviagte to your landed compatible payload, and join together to move it before moving on to other tasks.

The problem of rocket-sandblasting the neighborhood during landing is a severe one. Even a short hop is a problem as you have minimal air resistance (near vacuum at 1% of Earth pressure) and substantial gravity (1/3 g) to thrust against.

What makes sense to me here is landing in a crater, or more likely a robotically created and maintained berm such that your existing assets are in the blast shadow of arriving and departing craft (debris flies safely overhead). With such a thin atmosphere the safe region can be predicted very accurately.

Such a berm need not require heavy excavation equipment. It could be constructed by small rovers out of small rocks, it would just take a bazillion of them, and take months if not years to create. But again, a slow pace does not really count against it.

Once a craft has landed you go and retrieve it, and move it into the blast shadow before the next one arrives. As more craft land they are likely to dig a deeper hole, increasing the safe shadow area.
« Last Edit: 12/07/2013 06:01 pm by adrianwyard »

Offline Kaputnik

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Re: Red Dragon
« Reply #1153 on: 12/08/2013 07:00 pm »
My comment re: the Super-Dracos and our lack of information is not so much about the thrust, as it is about their specific impulse, and their mass.

They are optimised for sea level on Earth, not the near-vacuum of Mars. It is impractical to lengthen the nozzles at all to change this. Further, they are canted significantly from vertical and will suffer large cosine losses.
This all adds up to a significant degree of inefficiency. Granted, they do not need to fire for very long, but regardless of the duration of the burn, its purpose is to translate propellant into a sizable delta-v (I cannot remember the likely figure these days, apologies). If they are half as efficient as an optimised propulsion solution might be, then you need more than twice the propellant. The margins are thin enough that this is an issue. IMHO.

Secondly, we do not know what the mass of each super-draco is. There is no reason to believe that thrusters of this output will be particuarly light. If there are indeed eight of them (nobody has suggested a smaller number will be used) then that is a significant extra mass on top of the 4.5t capsule.

A final point. Just because some people at NASA like an idea, does not automatically mean it is a good idea. X-33, anyone...?
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

Offline IRobot

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Re: Red Dragon
« Reply #1154 on: 12/08/2013 10:32 pm »
Further, they are canted significantly from vertical and will suffer large cosine losses.
Are you sure about that? I don't think the losses are very significant, maybe 5-10%.

Offline Jcc

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Re: Red Dragon
« Reply #1155 on: 12/08/2013 11:27 pm »
Further, they are canted significantly from vertical and will suffer large cosine losses.
Are you sure about that? I don't think the losses are very significant, maybe 5-10%.

I don't think the actual SD angles have been revealed yet;  the animation of propulsive landing is understood to be inaccurate, the SD pods will stick out more.

Offline QuantumG

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Re: Red Dragon
« Reply #1156 on: 12/08/2013 11:33 pm »
SpaceX is being very tight lipped about the SuperDracos. The last time I had anyone tell me anything about them, they said the on-orbit specific impulse was going to be at least 320 seconds, which suggests some sort of adjustable nozzle. There's been no public statements to the same effect, and we've seen no pictures to suggest this is true. Everything we have seen suggests over-expansion.

Dragon version 2 was supposed to be revealed this year, but it didn't happen. Presumably it has to happen before the abort test next year.
Human spaceflight is basically just LARPing now.

Offline Borklund

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Re: Red Dragon
« Reply #1157 on: 12/08/2013 11:49 pm »
SpaceX is being very tight lipped about the SuperDracos. The last time I had anyone tell me anything about them, they said the on-orbit specific impulse was going to be at least 320 seconds, which suggests some sort of adjustable nozzle. There's been no public statements to the same effect, and we've seen no pictures to suggest this is true. Everything we have seen suggests over-expansion.

Dragon version 2 was supposed to be revealed this year, but it didn't happen. Presumably it has to happen before the abort test next year.
Huh, it's already 2014?

Offline QuantumG

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Re: Red Dragon
« Reply #1158 on: 12/09/2013 12:00 am »
SpaceX is being very tight lipped about the SuperDracos. The last time I had anyone tell me anything about them, they said the on-orbit specific impulse was going to be at least 320 seconds, which suggests some sort of adjustable nozzle. There's been no public statements to the same effect, and we've seen no pictures to suggest this is true. Everything we have seen suggests over-expansion.

Dragon version 2 was supposed to be revealed this year, but it didn't happen. Presumably it has to happen before the abort test next year.

Huh, it's already 2014?

Back in May, Elon said they were working on the unveiling with NASA, hopefully for this year.

By September the unveiling had become "next year". The pad abort test for Dragon v2 is supposed to be in Q2 next year, so presumably before then.

Human spaceflight is basically just LARPing now.

Offline Kaputnik

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Re: Red Dragon
« Reply #1159 on: 12/11/2013 07:09 pm »
Further, they are canted significantly from vertical and will suffer large cosine losses.
Are you sure about that? I don't think the losses are very significant, maybe 5-10%.

Well if my trig is correct, to get down to that sort of loss you need the thrusters to be firing at 20-25deg from vertical.
If they are actually at 45deg, which the artists' impressions suggest, the losses are about 29%.
"I don't care what anything was DESIGNED to do, I care about what it CAN do"- Gene Kranz

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