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So SpaceX's Red Dragon is unlikely to ever fly. But if it had, what would it's realistic payload to the surface be? How significant would the payload penalty be if the capsule itself was built to maintain an atmosphere, to allow use as a room, when people arrived? (Life support being brought later)

I ask this because the initial payloads talked about were 1T (in 2016)... But curiosity was 900kg and landed by a sky crane. So surely Dragon would be better? Especially after Falcon Heavy had a 20% capacity upgrade.

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  • $\begingroup$ What is Red Dragon? Google shows me a lot of hits not related to space or rockets at all. $\endgroup$ – Suma Aug 14 '20 at 15:15
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    $\begingroup$ en.m.wikipedia.org/wiki/SpaceX_Red_Dragon $\endgroup$ – user2702772 Aug 14 '20 at 15:19
  • $\begingroup$ Falcon (Heavy or otherwise) isn't used to land the capsule $\endgroup$ – JCRM Aug 14 '20 at 15:26
  • $\begingroup$ FH demo with the roadster has shown that FH is technically capable of trans-martian injection for small payloads however if a crew dragon capsule can aerocapture and deorbit is a big "?" $\endgroup$ – Dragongeek Aug 14 '20 at 16:59
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    $\begingroup$ Red Dragon was dropped when the parachutes decision was made. Developing a completely separate system, at their own cost, just for Mars was an undesirable expense. $\endgroup$ – user2702772 Aug 20 '20 at 18:58
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I wasn't planning on answering my own question, but I've been obsessing about the problem.

This paper https://www.lpi.usra.edu/meetings/marsconcepts2012/pdf/4216.pdf describes a landing with an entry mass of 7200kg, 1900KG of fuel, 5180kg of landed mass and 1000kg of actual payload. Launched by a Delta Heavy variant (it was written in 2012), it left about 7m3 of space remaining. That allows for 1900kG of fuel, given the SuperDraco fuel is on average about 1g/cm3, that's about 2m3 of space refunded, for 9m3.

Wikipedia lists draco as an Isp of 300 (vac) and SuperDraco at 265 (SL), so that translates to between 750-1000m/s of DeltaV to land. Upgrading to a Falcon Heavy gives us a maximum entry mass of 16T ((TMI payload 16.8T, rounded down).

Maintaining the requirement to have 1km/s of DeltaV, that gives a maximum landing mass of 11500KG, 4500KG fuel. The capsule mass was given as 4180KG. Increasing that by 25% (heat shielding, upgraded engines, kerbal paintjob, rounding errors) is 5500KG. 11500KG - 5500KG = 6000KG of payload. Probably 5500KG, because you'd want to have some fuel left over...

Volume wise, we had 9m3 to start. SuperDraco fuel has a density of 1.4 and 0.8 g/cm3, so average that out as water for 6000KG tonnes of fuel is 6m3. So about 3m3 of payload.

So if you don't mind sticking to cargo with an average density above 1.83g/cm3, you have a payload of 5.5T. For less dense payloads, as the fuel requirement decreases, you get more volume reclaimed from fuel. Special case payload, you only brought fuel. 9m3 of volume, 9000KG+5500KG=14,000KG TMI load. Landing mass of 10,000KG. 5500KG for the capsule, or about 4500KG of fuel. Two full payloads would refuel a Red Dragon. That would, assuming Vac Isp of 300, get you 2.7km/s of DeltaV. Or not enough to make Mars orbit at 3.8km/s, but interesting.

Final answer: Between 0-5.5T of payload, with harsh density/volume constraints. Works for machine parts, dissembled machines, food etc - but doesn't allow, under any circumstances, for a manned mission. Would be sufficient for cargo support to a manned mission – which was the intent behind the question.

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  • $\begingroup$ It could work as a Mars decent vehicle, intended to take humans to the surface of Mars, but not get them up. $\endgroup$ – PearsonArtPhoto♦ Aug 20 '20 at 16:15
  • $\begingroup$ This assumes the detrimental change to the ballistic coefficient has no impact. $\endgroup$ – JCRM Aug 20 '20 at 16:20
  • $\begingroup$ @JCRM There are so many assumptions that I'm making that the list is longer than my answer. It's why I asked the question - I wanted something more accurate than my own maths! $\endgroup$ – user2702772 Aug 20 '20 at 17:24
  • $\begingroup$ doubling the mass of your craft increased it's terminal velocity by 40%, sou you'll need another 40% delta V. so more of that payload needs to be fuel. And now you need to start burning earlier, to slow down before you hit the floor, so you lose less speed to aero braking - so you need more fuel to cover the extra speed. Bbecause you're burning for longer, you've more gravity to fight, so you need more fuel for that $\endgroup$ – JCRM Aug 20 '20 at 17:29
  • $\begingroup$ @JCRM All valid criticisms. It sounds like you want to submit a better answer. This one was written on the back of an envelope and can be improved in many ways. $\endgroup$ – user2702772 Aug 20 '20 at 18:19
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Realistically, I expect it would have been around 1 tonne. As it stood, it would only be able to land at altitudes below -1.5km (which isn't that bad, it's most of the northern hemisphere.) If the craft were heavier it would require lower landing altitudes, making it increasing less useful.

In my opinion the design point are the EDL phases of the journey, once that is modelled the next step is working out how to get it to entry. The paper "Red Dragon-MSL Hybrid Landing Architecture for 2018" states the vehicle is "at or above" the limit for Delta V Heavy, so isn't proposing using that launch vehicle, instead mentioning he F9 Heavy which has "significant margin" which suggests they could have made the launch payload larger, but didn't because launch mass wasn't the limiting factor.

Had it been capable of significantly more, but limited by launch capability, there's have been talk about on orbit fuelling/cargo loading -- because Musk is a great guy for ideas.

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  • $\begingroup$ The paper I linked to was a NASA paper outlining a 1T payload for an atlas heavy launch. Given a doubling in available mass, can you see any space for improvement? $\endgroup$ – user2702772 Aug 21 '20 at 10:25
  • $\begingroup$ not without detailed EDL information, and EDL is hard @user2702772 $\endgroup$ – JCRM Aug 21 '20 at 10:32
  • $\begingroup$ EDL is traditionally hard. However, even if the DeltaV requirement just doubled, you'd still manage a payload of 3T. I understand your desire to stick to accurate, factual answers - but speculate a little. $\endgroup$ – user2702772 Aug 22 '20 at 19:45
  • $\begingroup$ see the final sentence @user2702772 $\endgroup$ – JCRM Aug 22 '20 at 23:36
  • $\begingroup$ About your edit - therey listed TMI payload for a delta heavy is 8T, the quoted paper lists an entry mass of about 7T. Once you add the mass of the dragon trunk you're only slightly over the mass of the Delta heavy. So it is very much a launch mass limited system. $\endgroup$ – user2702772 Sep 6 '20 at 9:12

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