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I'm thinking about orbital fuel tanks for low earth orbit, and even Mars orbit in the event fuel farms on Mars take longer to build than anticipated.

That said, Can SpaceX put a nosecone on its Super Heavy Booster instead of a Starship on the booster?

Some Thoughts:

  • An entire Booster in orbit could make a great orbital storage unit because its larger than Starship.
  • It doesn't need heat shields or grid fins because it won't re-enter the atmosphere.
  • It will probably need some raptor vacuum engines, but is there enough room?

If a booster can be used as a space fuel tank, can it be moved to Mars' or Moon's orbit for future return missions?

Or does it make more sense to make a slightly larger Starship with no heat shields, flaps and only vacuum raptor engines.

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    $\begingroup$ I'm not sure how useful such an orbital tank would be. A standard Starship could be used as a propellant depot much more easily. $\endgroup$
    – Slarty
    Nov 10, 2021 at 18:15
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    $\begingroup$ Yes, it can. No it's a horrible idea. $\endgroup$ Nov 10, 2021 at 18:25
  • $\begingroup$ related: Is SpaceX's Starship single stage to orbit? $\endgroup$
    – uhoh
    Nov 11, 2021 at 11:03
  • $\begingroup$ There were a lot of ideas around using empty tanks as building blocks during the very early stages of American space stations, aka "wet workstation". Here the more practical option should be putting the center stage of a 1.5-stage vehicle (one center stage plus 2 or 4 boosters) into orbit, like CZ-5B, rather than the lower stage of a 2-stage vehicle. $\endgroup$ Nov 27, 2021 at 5:18
  • $\begingroup$ Wet workshop schemes always treat pressurized volume as some precious resource that is horribly expensive in orbit, and ignoring the many other components and systems needed to make an airtight hull into a habitable volume. It seems more productive to look at the payload mass and volume capacity of a Starship, and start thinking about what you'd need to take the construction techniques that SpaceX is executing in a Texas coastal swamp, and reproduce them in vacuum and freefall. $\endgroup$ Nov 27, 2021 at 20:44

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Have not attempted to math it out, but some rocket first stages can make orbit alone. The difficulty is that they have a payload of zero when doing so, because they are taking all the empty tank mass and un-needed engines all the way to orbit as a single stage. Dumping mass on the way up improves mass fraction of the parts left trying to make orbit.

This is why all currently flying rockets have multiple stages.

As you note, there is a hypothetical situation if you wanted a really large empty storage tank, that a first stage body would be an option to do that by dumping engines on the way up as fuel burns off to make a one point five stage rocket.

This of course still presumes that a large metal body contaminated with propellants is useful for the final purpose (noting that in LEO reboost is required, so heavy things are still not 'free'). In many cases it may be easier and cheaper long term to use the rocket conventionally to loft a purpose designed balloon structure.

Certainly trying to move a tank structure strong enough to stand up on the pad by itself to the moon or mars is not the most efficient option, since it will be much stronger than needed for the job forcing you to burn more fuel to move it than a purpose built solution would. The other payload that this would deliver to orbit would be lots of engines, but it is hard to visualize a use for lots of sealevel methalox engines, where you could instead use that same superheavy re-useably to launch starships and pull the vacuum engines off them three at a time before sending them home for a more useful result.

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    $\begingroup$ +1 Good discussion of why this is not a good idea $\endgroup$
    – antlersoft
    Nov 11, 2021 at 17:48
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I think it theoretically could. Superheavy has by itself a delta V of ~9.9 km/s, which roughly corresponds with ~10.8 km/s for the SH stacked with Ship - which obviously is designed to reach orbit.

Here is how I came up with those numbers:

Formula for delta V:

Delta V formula from Wikipedia

and so given:

v_exhaust_sl v_exhaust_vac payload
3,300 m/s 3,800 m/s 100,000 kg

I get

As separate vehicles, no payload Stacked with payload
SS SS
m_dry 120,000 kg 220,000 kg
m_wet 1,320,000 kg 1,420,000 kg
Δv 9,112 m/s 7,086 m/s
SH SH + SS
m_dry 180,000 kg 1,600,000 kg
m_wet 3,580,000 kg 5,000,000 kg
Δv 9,868 m/s 3,760 m/s

(SS = Starship upper stage, SH = Superheavy)

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