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I've seen the question "Why don't SpaceX just use Raptors in the F9 now since their better" asked here and there, and I understand why it isn't possible to do that- they're not a drop-in replacement and methane is stored differently and used in different ratios with LOX to RP1.

That having been said; if one ignores the fact that SpaceX is phasing out medium/heavy semi-reusable launch vehicles, consider if a vehicle were built in the basic form factor of the F9 (so 9 1st stage engines, and 1 vac optimised 2nd stage engine) from the ground up to accommodate raptors instead of merlins.

  • What kind of performance could be expected from such a vehicle?
  • And to expand further, if the need to use the F9 framework were completely relaxed, what would the optimum configuration be for a semi-reusable medium lift raptor-powered launch vehicle?
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  • $\begingroup$ This is a good question! I've modified the phrasing somewhat to make it sound less inviting of opinion-based answers. I think it can be answered based on facts and well-reasoned arguments. $\endgroup$ – uhoh Nov 9 '19 at 0:20
  • $\begingroup$ Question 1: For "basic form factor of the F9" are you asking for the same size, or just the 2-stage, 9-and-1 layout? Between the higher performance of the Raptor and the lower density of methane, the latter will be a much bigger rocket. $\endgroup$ – Russell Borogove Nov 9 '19 at 1:09
  • $\begingroup$ Question 2: "optimum" by what metric? Launchers are never optimized for a single characteristic. $\endgroup$ – Russell Borogove Nov 9 '19 at 1:09
  • $\begingroup$ @RussellBorogove Form factor is an aspect of hardware design which defines and prescribes the size, shape, and other physical specifications of components... How about the 9 + 1 engines and a similar size and shape to the F9; would the result be able to lift a payload to LEO of similar mass to an F9's capability? $\endgroup$ – uhoh Nov 9 '19 at 2:41
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    $\begingroup$ hi- to clarify by form factor i really just mean configuration of 9+1 engines. the size of the rocket itself would probably change of course. $\endgroup$ – Eoin O'Kelly Nov 9 '19 at 16:36
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If you're trying to preserve the stage dimensions of the Falcon 9, like if you want to maintain as much of the existing tooling as possible, then you definitely won't want a 9+1 Raptor configuration. Methane is substantially less dense than kerosene, so you'll get less propellant mass in a stage of the same size -- you'll end up with a very overpowered rocket.

In fact, 3 Raptors on the first stage would be sufficient, and you can get 24 tons to LEO with a 15% fuel reserve for landing the first stage, as compared to about 13 tons for Falcon 9. (I've seen conflicting information on whether 15% is the required reserve for barge landing or for RTLS).

If you want a 9+1 Raptor configuration, you want to go to a much larger rocket -- weighing in at 1380 tons (about 2.5 times the mass of the Falcon 9), something like 5 or 6 meter diameter instead of 3.7 meters, and you send 40 tons to LEO with a 15% fuel reserve for landing in the first stage. This is using slightly more conservative values for dry structural mass than Falcon demonstrates.

These figures come from a simulation I've been developing; please take them with a big grain of salt. One of the big issues I'm having is in optimizing ascent trajectories; small differences in my guidance algorithm lead to huge differences in where the rocket ends up. I've assumed that the first-stage Raptor has a sea level Isp of 330 seconds and sea level thrust of 1760kN, the vacuum-optimized Raptor has an Isp of 375 seconds in vacuum, and produces 2000kN thrust in vac.

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