Red Dragon Discussion Thread (1)

[Robotbeat]

...What's very interesting is this:

Fire launch abort motors supersonically and use them for remainder
of descent.

They are stating that they can fire their SuperDracos while in supersonic flow? Wasn't that completely out of question?

No, of course not. You weren't paying that close attention when reading about supersonic retropropulsion... Shame on you.

Firing supersonically poses really big problems only if you have a single rocket engine in the center of your heat shield pointing straight down-flow. Multiple thrusters on the outside and shielded from the direct supersonic flow shouldn't have problems firing.

[Blackstar]

Looks like Dragon can land a ton on Mars

Pigs can also fly.

[Robotbeat]

Looks like Dragon can land a ton on Mars

Pigs can also fly.

The document you posted says this: "The analysis indicates that Dragon would be able to deliver more than one tonne to our landing sites." My comment is basically just a rewording of that statement.

[kch]

Looks like Dragon can land a ton on Mars

Pigs can also fly.

Of course they can -- we call them "pig-eons" ...

[baldusi]

Looks like Dragon can land a ton on Mars

Pigs can also fly.

And are called F-4 Phantom, for example. I'm not saying that this is will happen. I'm just saying that it's worth doing the analysis. Mor like to know what's wrong.

[Kaputnik]

The big bit of the puzzle that is missing is the isp of the Super Draco.
Time for an educated guess!
IIRC, the Dracos use N204/MMH, so presumably Super Draco will be the same. About the highest isp achievable with a pressure fed system is the 316s achieved by the OME on the Shuttle. Thus far, SpaceX haven't quite been able to match the best engine manufacturers in terms of isp, but, hey we can give them the benefit of the doubt.
Then we need to factor in the cosine losses from the thrust angle. Again, I don't think this information is known, and all we have to go on are artists' impressions, which suggest an angle of nearly 45 degrees- again, giving the benefit of the doubt, perhaps it is only 35? If I can remember my high school trig that brings effective isp down to about 260s...

Anybody else care to take a stab at this one?

[Robotbeat]

The big bit of the puzzle that is missing is the isp of the Super Draco.
Time for an educated guess!
IIRC, the Dracos use N204/MMH, so presumably Super Draco will be the same. About the highest isp achievable with a pressure fed system is the 316s achieved by the OME on the Shuttle. Thus far, SpaceX haven't quite been able to match the best engine manufacturers in terms of isp, but, hey we can give them the benefit of the doubt.
Then we need to factor in the cosine losses from the thrust angle. Again, I don't think this information is known, and all we have to go on are artists' impressions, which suggest an angle of nearly 45 degrees- again, giving the benefit of the doubt, perhaps it is only 35? If I can remember my high school trig that brings effective isp down to about 260s...

Anybody else care to take a stab at this one?

With your very optimistic figures, and giving a roughly 1250m/s delta-v needed and an initial mass of 6.5mT (10 tons is max that Falcon Heavy can send to Mars... some of that is needed for mid-course correction, some will probably be shed when the trunk is discarded)...

6500kg*(1-1/e^(1250/(260*9.8 )))=~2500kg

That's a LOT of fuel. But it really depends on the details of the analysis.

[manboy]

I found it. Here is the presentation.

Thanks for this.

[go4mars]

Pigs can also fly.

The martians will appreciate getting a tonne of bacon.  mmm...

Or a couple of these...
http://www.foxnews.com/story/0,2933,275524,00.html#

[simonbp]

I found it. Here is the presentation.

Thanks!

Slide 10 shows it's a lifting entry, dipping down to 20 km altitude before climbing to 45 km again, which partially explains how they shed so much velocity before igniting the rockets. This is all based on NASA Ames's trajectory simulations; one of the "next steps" is to get independent confirmation.

The instrument payload mix is interesting: drill to collect samples and a range of direct analysis instruments and ISRU demos. Plus, a reflight of the Phoenix SSI "camera-on-a-stick".

The target is a cold northern site like Phoenix. It would be pretty cool if it actually went to the same site, but I doubt it.

[MP99]

I found it. Here is the presentation.

Many thanks for posting that.

cheers, Martin

[Nathan]

I found it. Here is the presentation.

Many thanks for posting that.

cheers, Martin

Yes brilliant - thank you. Confirms much of the speculation and back of envelope calculations on this thread. High lift trajectory, supersonic retropropulsion, low landing site, no parachute, payload on the order of one tonne.

Love it!

[as58]

It seems pretty risky as in trying to do quite a lot of things never done before. I wonder if NASA would be willing to accept that much risk in a Discovery mission, which are supposed to be about science, not tech development however valuable that could be.

[baldusi]

I have a few questions:
Are Dragon's legs rated for unleveled terrain?
Do they have autonomous landing capability with the ability to choose leveled terrain?
How are they going to let the instruments out?
Will they keep the pressure vessel with environmental control, or will they have to vacuum rate everything?
How are they going to do thermal control if the go with a vacuum system?
Doesn't the pressure vessel use the internal pressure as a structural reinforcement?
Aren't the avionic computers not rad hard, but with multiple redundancies? I understood this was acceptable for LEO but a complete no go for deep space.

This are just some of the questions that popped in my mind in five minutes I took to think about it.

[Ben the Space Brit]

@ baldusi,

At the risk of sounding cynical, the answer to those issues is probably: "Send money; we'll work it out".

[apace]

I have a few questions:
Are Dragon's legs rated for unleveled terrain?

As the paper plays with the numbers and features of the enhanced crew version of Dragon (with propulsive landing), your questions can only be answered from one inside Dragon development. But about autonomous landing capability, there should be enough systems around now which can be used for this.

[baldusi]

I have a few questions:
Are Dragon's legs rated for unleveled terrain?

As the paper plays with the numbers and features of the enhanced crew version of Dragon (with propulsive landing), your questions can only be answered from one inside Dragon development. But about autonomous landing capability, there should be enough systems around now which can be used for this.

Even the robotic Dragon with full return and pin point landing capability, will make use of GPS and will land on a cement landing pad. Here we are talking about flying by inertia, recognizing the terrain, and deciding, with no external aid whatsoever (save the MRO maps), where there's a patch of terrain clean and leveled enough to actually land.
We are talking about 3D terrain recognition on real time. That's stuff that even the Google car is not ready for. There's a reason that Curiosity uses those huge wheels and suspension, and the Sky Crane works the way it does. Its to land on very unfriendly terrain.

[Robotbeat]

They may be looking for an exceptionally flat spot with ice under it. Less flexibility is the price you pay for not going for something like MSL's system.

[Zed_Noir]

One other point- any 'drop tanks' or attempts to enter with the trunk attached will completely invalidate the entry characteristics of the vehicle.

A pop off drop tank should not alter the entry characteristics. It would look like the currently discarded nosecone, only slightly larger.

[Kaputnik]

One other point- any 'drop tanks' or attempts to enter with the trunk attached will completely invalidate the entry characteristics of the vehicle.

A pop off drop tank should not alter the entry characteristics. It would look like the currently discarded nosecone, only slightly larger.

True, but it's not just about shape, it is about weight distribution. Putting propellant up on top of the capsule is going to destabilise it.