According to this Wikipedia article, Falcon Heavy is claimed to be able to deliver a payload of 63,800 kilograms to LEO. According to this article, the Tesla Roadster has a curb weight of 1,305 kg. Now there is also the Starman dummy and his suit, the support frame, and camera booms. And there would have also been the extra second stage fuel to push it all out of Earth orbit. How does everything add up as far as "payload to Earth orbit" is concerned for the test flight? Was it a true test of Falcon Heavy's advertised capacity? Was there unused capacity? Was there additional ballast to bring the actual payload weight up to something close to the Falcon Heavy's "advertised capacity"?

  • $\begingroup$ Minor side point - the "rocket motor and fuel to push it all out of Earth orbit" is the second stage, there was no additional kick motor or third stage, or payload propulsion. $\endgroup$ – Saiboogu Feb 11 '18 at 16:08

The advertised 63.8 tons payload capacity is for low Earth orbit (LEO). The further you want to send the payload, the less payload mass you can send.

The Roadster/Starman test payload went far beyond LEO, winding up on a trajectory that reaches beyond Mars' orbit, to the inner reaches of the asteroid belt. The rocket motor and fuel used is the Falcon second stage, which is not part of the advertised payload capacity.

In order to get 63.8 tons into LEO, the second stage would use all its fuel.

Getting the comparatively tiny Roadster into LEO took only a small portion of the second-stage fuel. The rest of the second-stage fuel was then used to send it out of Earth's orbit.

Finally, the 63.8 tons figure advertised by SpaceX is almost certainly for the fully expendable mode, where none of the boosters are recovered (because no fuel is saved to land them). Their payload figures include the potential of sending 3.5 tons to Pluto - twice the mass of the Roadster and a substantially greater velocity requirement.

  • $\begingroup$ I get that payload to LEO is only a "benchmark" figure, and that the farther/faster you want to push something, the more fuel it takes, which deducts from the LEO payload number. My question was whether the payload as publicly described (car/dummy/support frame etc.) all added up to the limit that Falcon Heavy could put on the trajactory it now has, or conversely, the payload as described is now going as far/as fast as Falcon Heavy could push it. I.e. no unused payload capacity, no added ballast. $\endgroup$ – Anthony X Feb 10 '18 at 16:44
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    $\begingroup$ As far as I know, there's no additional ballast, and the second stage burned to depletion for the final trajectory. I don't know that they loaded the launcher with its maximum amount of propellant; I can see incentives both to demonstrate its full capability and to keep some performance in reserve for the future. For this launch, they may have been somewhat conservative in budgeting fuel for the booster and first stage recovery. There are a lot of variables to play with, and any "x tons to y destination" figure is hiding a lot of parameters. $\endgroup$ – Russell Borogove Feb 10 '18 at 16:56
  • $\begingroup$ I don't know but I don't think it's wise or even plausible to launch with less than a full load of fuel. All of their simulations would be based around a given mass at each point in flight, adjusting throttle position to give their desired G loading and velocity for that air thickness. Payload masses are a tiny percent of the total and would have minimal influence, but fuel loading could cause large discrepancies. Loading the same fuel each time eliminates another variable in flight planning and fuel cost is minimal (estimated as $200k on a F9 flight) in the total launch costs. $\endgroup$ – Saiboogu Feb 11 '18 at 16:07
  • $\begingroup$ I know other launchers have historically varied their fuel loads from launch-to-launch; SpaceX needs to run completely separate simulations for every mission, so there's no additional burden in doing so with varying fuel loads. (I'm talking about the difference between 95% and 99% full tanks, here, not like half-empty.) Their mission trajectories are surprisingly diverse: i.redd.it/xaisqxao5ef01.png $\endgroup$ – Russell Borogove Feb 11 '18 at 16:30

This was a test flight, one that was to demonstrate that the rocket worked. You don't do your first test flight at full capacity, as even a small error could cause you to be falling in to the ocean without success. The payload actually could have been launched to the same trajectory on a reusable Falcon 9, without the extra boosters, as the Falcon 9 payload to Mars is 4,000 kg, and the payload was only 1350 kg. Of course, this assumes a direct to Mars trajectory.

Furthermore, they deliberately did a 6 hour coast. As the coast goes on, the amount of fuel remaining in the rocket drops. The fuel is cryogenic, and slowly will evaporate, which generates pressure in the tank, which must be released to keep the rocket from exploding. Some of the spare capacity was lost there.

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    $\begingroup$ "The payload actually could have been launched to the same trajectory on a reusable Falcon 9, without the extra boosters." Eh? $\endgroup$ – Russell Borogove Feb 10 '18 at 21:57
  • $\begingroup$ There have been a few people who have crunched the numbers, and found that Falcon 9 could have actually carried Starman/ Roadster to a trans-Mars injection burn, and had enough capacity to land the booster core. There was a lot of unused capacity in this launch. $\endgroup$ – PearsonArtPhoto Feb 11 '18 at 1:50
  • $\begingroup$ I think I can guess why, but fuel levels dropping during a coast phase is not intuitive and didn't even occur to me until you said it. An explanation of why that is and why it matters for the FH second stage fuel system as opposed to fuels that get sent on deep space missions would improve this answer. $\endgroup$ – Caleb Feb 11 '18 at 6:49
  • $\begingroup$ Good point, added more info. $\endgroup$ – PearsonArtPhoto Feb 11 '18 at 11:30

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