Would it be possible to get a modified SprHvy booster to orbit, then send a full stack with Starship on a "standard" config, then mate them in orbit?

Question

The idea is to get a modified Superheavy booster to orbit minus Starship. Then send a Starship to orbit on a standard configuration, and mate the first booster with the recently arrived Starship. So now numerous refueling tankers fill both vehicles. (I know, it's a lot). With the fully fueled booster and ship, what would be possible in terms of reduced travel time to the outer solar system? I would think Starship would have sufficient fuel left to slow down and land on, let's say Ceres, Ganymede, Titan, or Triton. 150 tons of infrastructure could be a good start for a fuel depot.

Answer 1

An earth to orbit rocket is not automatically a good choice for flying interplanetary.

The major problem is that thrust to weight ratio has to be higher than one to take off, but often departure maneuvers are better if you remove engines and add fuel for same overall mass, though there are limits.

The nozzles are normally optimized for atmospheric pressures, so would be inefficient.

The structure will be rated for multiple G and landing, far heavier than it needs to be (so less fuel available).

And the fuel combinations, plumbing and ignition systems may not be optimized for zero G.

All that aside answers to questions like this can be eyeballed with charts like this.

It takes around 9400 meters per second of Delta V to reach LEO, so we can assume a Starship combination can do that with standard payload. From there Jupiter insertion is around 7000, so you can get there but are probably having to aerobrake or do gravity assist to reach the inner moons, and Saturn is another 1000ms away so pretty thin margins there. In neither case do you get your Super Heavy or Starship back.

So that part is possible, though probably not very useful.

Depending on whose numbers you believe it may be possible to get a stripped back Super heavy into orbit with no payload, so that may also be possible.

The issue then is you have an empty super heavy in orbit needing somewhere north of 3000 tonnes of fuel, so lots and lots of ordinary Starship tanker flights, it is a question if you could launch the ~30 tanker missions fast enough to beat evaporation losses from the minimally insulated Super Heavy (the ground side logistics would be slightly mind bending as well).

A more likely mission profile would be to design a dedicated light weight/low thrust craft optimized for the mission at hand and construct and/or fuel that via Starship and use the available super heavies in their intended role.

Answer 2

It might be just barely feasible, …

… given a generous dose of handwavery and wishful thinking.

SpaceX can't be bothered to release detailed design docs, so we will have to resort to rough guesstimates.

According to Wikipedia, the Super Heavy has a dry mass of 160–200 tons holding 3600 tons of propellants. That translates to a mass ratio (MR) of 19–23.5, or a propellant mass fraction of roughly 95%.

Assuming the sea level specific impulse of 330 s stays constant throughout your SSTO flight (that's the mother of all assumptions, see below) and plugging it into the rocket equation:

$\Delta v = I_{sp} g_0 \ln(MR)$

We get roughly 9500 m/s of $\Delta v$, which is just about enough to get into LEO. Yay…

… but it's likely not such a great idea.

Your Super Heavy is going to need quite some modifications to pull this off:

• Reusability goes out the window. There's absolutely no way this empty soda can is going to survive reentry from orbit, so you can get rid of the corresponding hardware to shave some weight off (and put on a nose cone instead). It's not going to be much, as those landing legs, thrusters, and thermal protection aren't super heavy to begin with.
• The engines are a problem.
  • Those 33 Raptors are way too much thrust once the vehicle is mostly empty, so you'll have to shut most of them down. (Outside KSP, engines can't be throttled down to 1%.)
  • Those tiny sea level nozzles are going to run horribly underexpanded towards the end of the launch, negatively impacting the specific impulse. They might even blow up from the stress (assuming they're built to barely withstand the design loads, since that's pretty much the norm in aerospace).
  • You could just design some compromise nozzles, strenghtened to withstand vacuum but still kinda working across the range. If at all feasible, those are going to be inefficient, putting a question mark next to the Isp assumed above.
  • Or you can add a couple of Raptors with those big honking vacuum nozzles. The whole aft end will have to be redesigned to make them fit. So not a SSTO anymore, but an ordinary two-stage orbital launcher. And those 33 sea-level engines? You're not going to use them ever again once you cut over to second stage, so you might as well toss them. Except that you can't, unless you redesign the whole thing Atlas-style. So you're left with a whole bunch of dead weight to carry along your interplanetary journeys.
• Finally, if your big empty soda can makes it into LEO, you will have to hurry up with all the mating and refueling missions to GTFO before atmospheric drag makes it reenter again. Once you start filling up, you'll have to deal with having a minimally insulated tank of cryogenics sitting there baking in the sun.