Why doesn't SpaceX use solid rocket fuel?

Question

I am curious to why SpaceX wouldn't use more efficient solid rocket boosters instead of the liquid booster they currently have for the first stage of their Starships. At first, I thought this was due to the fact that catching the superheavy booster requires careful tunning of the thrust as demonstrated by their Ship 30 flight video. This seemed impossible with solid fuel, as it would have to burn all the way through and could not allow for such fine tuned adjustments. However, from analyzing the flight trajectory below, I do not see why the solid fuel couldn't be separated into two compartments:

The first compartment of the main stage would bring the Starship up into an ascent at a high delta V. It would then end the burn after Hot Staging, the flip back maneuver, and the boost back burn. Then, the booster would fall through the atmosphere, naturally deaccelerate, and then it could stop its velocity with a small second compartment through a retrograde burn. I understand that the timing would be extremely hard to pull off, but I feel like this could be feasible in the future, either in this form or some other system.

Answer 1

Solid rockets are a good choice for cheap, compact high thrust, but they are significantly less mass-efficient than liquids.

SpaceX in particular has developed their rockets around reuse via powered landing, and solids are completely unsuited to that; you need rapid, precise throttling of thrust to do a SpaceX style landing.

SpaceX's economics also depend on economies of scale, using the same engines in first and second stages. Going to a combination of solid boosters and liquid sustainers would go against that strategy.

Answer 2

More reasons not to use solid rockets:

• they have a low mass fraction: they require a heavy casing to withstand the internal pressure. Liquid-fueled rockets can be made much lighter.
• a solid rocket cannot be rapidly refueled. The propellant has to be cast in the casing, using a slow, carefully controlled process to prevent cracks from forming in the propellant. A liquid rocket can be refueled in a few hours.

Answer 3

In addition to Russell’s excellent answer, I suspect solid rockets would be a thermal impossibility in a re-usable booster the size of Starship.

For a long cylindrical solid fuel rocket, most waste heat must be dispersed through the side walls of the rocket after shut-down. With increasing size, the square/cube relationship of surface to volume means there is more heat per area which needs to be dispersed. If the heat cannot be dispersed, the rocket structure risks being heated above its service temperature limit by residual heat after shutdown. This could potentially prevent re-use.

If the proposed solid fuel Starship were a bundle of 33 solid rockets (picture a bundle of Shuttle SRBs) , the 60% of the rockets on the outside of the circumference could only dispose of heat from half their surface. Even worse, the remaining "inside" rockets would be completely surrounded by other hot rockets, so would be unable to dispose of any waste heat.

Combustion temperatures in solid fuel rockets typically reaches 5000F https://secwww.jhuapl.edu/techdigest/Content/techdigest/pdf/APL-V13-N04/APL-13-04-McClure_Comb.pdf

Maximum service temperature of 304L SS is 1650F https://www.sunmach.net/Alloys/Stainless-Steel-304-or-304L-or-304H-Manufacturers.html#:~:text=Stainless%20Steel%20Grade%20304L%20has,F%20(816%C2%B0C)

Following engine shutdown, residual heat would destroy the central rockets in this “bundle of soda straws” architecture.

Alternatively, a single solid fuel rocket 30ft in diameter would have its own heat dissipation and hoop tension problems.

Related: Why do the walls of a solid rocket booster not glow red hot?

Answer 4

In addition to the points raised by others a key issue of solid rockets is that once you've lit them they burn and keep burning until they run out of fuel, you can control the burn rate with the internal geometry of the fuel but you can't dynamically control the thrust.

With some hackery it might be possible to land a solid rocket but at orbital rocket scales the lack of true rapid throttle control would make landing nearly impossible.

Answer 5

The goal is to go to Mars with Starship — and come back. That goal is much easier to achieve if fuel can be produced on Mars. The principle ingredients for both hydrogen and methane production are there (H₂O and CO₂). While hydrogen would be a good choice because it comes for free with oxygen production by electrolysis from water, its storage temperature would only be 20K, as opposed to the much less demanding 110K for methane. And long-term storage is an issue: The larger temperature differential to the environment for hydrogen would require continuous cooling to prevent boil-off, given the probably slow production rate with the restricted resources and correspondingly long production and hence accumulating storage time.