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Red Dragon
Mongo
post Sep 17 2012, 05:17 PM
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That's the big question -- whether it would be more cost-effective to use a nearly stock Dragon, with its very low cost, to deliver a science cargo of several tonnes to the Martian surface, or to design a vessel specifically for Mars, which could deliver a substantially larger payload to Mars surface but at a much greater cost.

My guess is that a specially designed landing vehicle plus Falcon Heavy launcher could deliver maybe six tonnes of scientific payload to the Martian surface in a single package, but would cost as much as three separate two-tonne payload Red Dragon landers plus launchers.
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Kaputnik
post Sep 17 2012, 08:39 PM
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A 'nearly stock' Dragon would be aiming to deliver about 1t to the surface (not 'several tonnes'). The papers from AMES suggest an entry mass of about 7t, with 2t of that being the propellant burned in the last few seconds to reduce speed from Mach 2 to a soft landing. That leaves 5t to be landed, of which 4t is the capsule itself.
Now, that 4t figure for an empty Dragon equipped with landing gear and Super-Dracos is one that I find a little hard to digest. Bear in mind the far smaller Soyuz capsule, which has no large liquid fuelled engines or landing legs etc, is 3t. The Super Draco thrusters are going to have to pretty special indeed (i.e. have exceptionally high thrust:weight).
Not saying that this cannot work, obviously SpaceX and AMES think it can, but I think we should be aware of the margins that the system has.

Entry mass is proportional to the amount of drag that the vehicle can generate. A dedicated Mars lander would use the full 4.6m payload capacity of the FH, allowing, in theory, up to 60% greater entry mass than Red Dragon (i.e. over 11t). In addition, a dedicated platform could offer wider scope for different payloads, and could probably offer some mass savings by not carrying unnecessary hardware all the way to the surface.

I'm not saying that Red Dragon isn't a cool proposal- just I think some people are pinning too many hopes on it.
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Mongo
post Sep 17 2012, 09:30 PM
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Acording to SpaceX, the dry mass of a Dragon is 3,180 kg (albeit without landing structure).

The landed mass according to this proposal for Red Dragon EDL is stated as 5,180 kg

5,180 kg minus 3,180 kg is 2,000 kg

The "1,000 kg" figure being thrown about is a conservative estimate of landed payload, with a considerable safety margin. Depending on reentry trajectory and altitude of the surface, it can go higher (perhaps considerably higher, as seen in the above report).
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Kaputnik
post Sep 17 2012, 11:29 PM
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A 3180kg Dragon does not have, as far as I am aware, the Super-Draco engines included in that mass.

No data has been released on the mass of these engines. However, we do know that they will provide an a total axial thrust of 120,000lb; given that they are canted at an angle of at least 35 degrees, and probably more, the actual thrust produced must be in the region of 150,000lb or higher. A generous figure for thrust:weight ratio is about 50:1 (IMHO) which gives an engine mass of 1363kg, plus tank mass. The engine mass could easily be greater than this if they are canted at a steeper angle and/or if the T:W is lower. Admittedly the mass could be lower as well, but, again IMHO, not by much.

As you say, the landing gear and its deployment system also has to be accounted for.

If SpaceX have managed to squeeze the engines, tanks, and landing gear into a capsule the same weight as a Soyuz (which has none of those things), yet at the same time made it twice as big.... then I take my hat off to them!
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