4
$\begingroup$

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?
$\endgroup$
6
  • $\begingroup$ 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, 2019 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. $\endgroup$ Nov 9, 2019 at 1:09
  • $\begingroup$ Question 2: "optimum" by what metric? Launchers are never optimized for a single characteristic. $\endgroup$ Nov 9, 2019 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, 2019 at 2:41
  • 1
    $\begingroup$ 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. $\endgroup$ Nov 9, 2019 at 16:36

1 Answer 1

3
$\begingroup$

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.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.