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Starship/Super heavy second stage and fuel depot concept.


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Can the super heavy booster carry a fully starship and a expandable second stage to LEO with minimal refueling in orbit? Also can a super heavy carry a expandable fuel depot into orbit with enough propellent to fuel starships etc? 

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In theory, it could carry a solid fuel upper stage with some probe(s) attached to it. It could also carry a liquid fuel upper stage, but this would require new fueling equipment to be developed and would suffer from boil off.

SpaceX plans for a fuel depot but it will be used over and over again instead of expending it.

The fuel depot might be able to carry some fuel into orbit on launch, but to be fully fueled it would likely require a Starship tanker flight(s).

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The cargo Starship would be able to carry a substantial solid or hypergolic 3rd stage without too much issue. This 3rd stage plus payload could weigh up to 150t or so whilst fully reusing Starship

 

Alternatively, an expendable-variant Starship could arrive in LEO with about 265t of payload/fuel. The expendable Starship would then manage about 6km/s for a 12t payload.

If Superheavy was expended as well, that'd be about 525t of payload and fuel. That's about 6km/s to a 52t payload.

 

Of course both of these beg the question - why not just do a couple of refilling flights and save the cost of expending anything at all? With a top up in LEO starship can shoot a JWST-sized object through 4.2km/s and still propulsively return.

 

There are use cases where the Starship itself can be counted as part of the payload.

Depot starship for instance. It's effectively an expendable starship mission (although reused in orbit). Counting both hull and fuel, that's ~300t of payload to a useful parking orbit.

Ditto Lunar Starship. All of that can be counted as useful payload to LEO.

 

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2 hours ago, RCgothic said:

The cargo Starship would be able to carry a substantial solid or hypergolic 3rd stage without too much issue. This 3rd stage plus payload could weigh up to 150t or so whilst fully reusing Starship

 

Alternatively, an expendable-variant Starship could arrive in LEO with about 265t of payload/fuel. The expendable Starship would then manage about 6km/s for a 12t payload.

If Superheavy was expended as well, that'd be about 525t of payload and fuel. That's about 6km/s to a 52t payload.

 

Of course both of these beg the question - why not just do a couple of refilling flights and save the cost of expending anything at all? With a top up in LEO starship can shoot a JWST-sized object through 4.2km/s and still propulsively return.

 

There are use cases where the Starship itself can be counted as part of the payload.

Depot starship for instance. It's effectively an expendable starship mission (although reused in orbit). Counting both hull and fuel, that's ~300t of payload to a useful parking orbit.

Ditto Lunar Starship. All of that can be counted as useful payload to LEO.

 

People say multiple refueling’s is risky and time consuming while during peak transfer orbit times.

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28 minutes ago, Cloakedwand72 said:

People say multiple refueling’s is risky and time consuming while during peak transfer orbit times.

I don't think any mission is expecting multiple refuelings. Missions will launch when a depot is full and ready to refuel a mission in one docking.

Filling the Depot will take multiple tanker flights, yes. 

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4 hours ago, Cloakedwand72 said:

People say multiple refueling’s is risky and time consuming while during peak transfer orbit times.

Those people were mostly National Team wilfully misrepresenting the process. There just one docking between the mission ship and the depot when it's ready, and every other HLS system required at least some on orbit assembly too.

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I don't know enough about rocket fuels - but would some kind of flexible hose between the two ships work (like mid-air or under-way refueling of aircraft or ships)?

Or is the cryo aspect such that they need to figure out how to transfer cryogenic fuels through a hard-docked mechanism to keep the fuels at the proper temps?

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20 minutes ago, JoeSchmuckatelli said:

I don't know enough about rocket fuels - but would some kind of flexible hose between the two ships work (like mid-air or under-way refueling of aircraft or ships)?

Or is the cryo aspect such that they need to figure out how to transfer cryogenic fuels through a hard-docked mechanism to keep the fuels at the proper temps?

Cryogenics are a known difficulty with the process, yes.  But unlike zero G, we can test cryo docking  fittings in vaccum here on earth, which makes it cheap to iterate.

Edited by Rakaydos
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1 hour ago, Rakaydos said:

Cryogenics are a known difficulty with the process, yes.  But unlike zero G, we can test cryo docking  fittings in vaccum here on earth, which makes it cheap to iterate.

Thanks!

I guess what I'm asking is whether the two SS's can just pull up alongside one another and have a flex tube or arm extend between the two... or if they're going to have to mate both together every time with some hard coupling?  Any guesses / info on that?

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One issue I see is that the quick disconnect adapter is not an docking adapter.
It would probably need redesign for this. And the depot would need an separate adapter who is more like the one on the tower but would also have to be an docking adapter. 
Yes you could also make an docking adapter in addition to the quick disconnect adapter. The docking adapter would not need lots of the features the QDA need for one but you would want some sort of guidance and an hard lock on who can support some forces. 

As for flexibility, well the pipes to the engines must have some flex because you can gimbal the engines. 

Edited by magnemoe
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10 hours ago, JoeSchmuckatelli said:

I guess what I'm asking is whether the two SS's can just pull up alongside one another and have a flex tube or arm extend between the two... or if they're going to have to mate both together every time with some hard coupling?  Any guesses / info on that?

A flexible tube would probably require an EVA, and I believe the current plan for loading up the fuel depot only involves unmanned flights.

Extending a prehensile fuel line sounds like a significant engineering challenge, and no relevant results pop up on google for the term.

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19 minutes ago, Terwin said:

A flexible tube would probably require an EVA, and I believe the current plan for loading up the fuel depot only involves unmanned flights.

Extending a prehensile fuel line sounds like a significant engineering challenge, and no relevant results pop up on google for the term.

The arm on the launch tower sort of does this.

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The issue with a flexible connection is that unless the two are hard-docked, station keeping becomes extremely challenging, particularly whilst exchanging large quantities of fuel.

Unless the centres of masses of the two vehicles are exactly along and remain exactly along prograde/retrograde, the two vehicles will effectively want to exchange altitudes over the course of an orbit.

Plus one vehicle passing fuel aft will want to accelerate and gain altitude, and the one receiving will want to decelerate and lose altitude.

Hard docking seems necessary IMO.

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37 minutes ago, RCgothic said:

The issue with a flexible connection is that unless the two are hard-docked, station keeping becomes extremely challenging, particularly whilst exchanging large quantities of fuel.

Unless the centres of masses of the two vehicles are exactly along and remain exactly along prograde/retrograde, the two vehicles will effectively want to exchange altitudes over the course of an orbit.

Plus one vehicle passing fuel aft will want to accelerate and gain altitude, and the one receiving will want to decelerate and lose altitude.

Hard docking seems necessary IMO.

This is kind of what I was afraid of.  I'm guessing that with vehicles of the masses the depot and the SS FuelTruck will have that minor velocity can have a big impact.  Being able to soft-dock or have an "Expanse"-style docking tube between vessels seemed like a good solution; but apparently orbital mechanics hasn't gotten the memo from Hollywood!

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If you look at the QD stuff on the tower, it has a flex hose, but also a hard connect. Seems like such a system could do both. You'd want 2, widely separated (the second might only be the hard connector, no hose needed).

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Do the QD connections require human assistance to connect?

 

Any thoughts on the propellent aggregator keeping the fuel super-cooled?  

Would it likely even keep the fuel cooled aside from venting to keep the pressure reasonable?

Presumably they would like to keep at least some fuel for extended periods(such as a buffer in case of losses during fueling), but I would expect cryogenic fuels to require active cooling and I do not recall hearing anything about that.

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20 minutes ago, Terwin said:

Do the QD connections require human assistance to connect?

Remotely they might, the QD has cameras on it... course cameras can steer cars.

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2 hours ago, JoeSchmuckatelli said:

This is kind of what I was afraid of.  I'm guessing that with vehicles of the masses the depot and the SS FuelTruck will have that minor velocity can have a big impact.  Being able to soft-dock or have an "Expanse"-style docking tube between vessels seemed like a good solution; but apparently orbital mechanics hasn't gotten the memo from Hollywood!

Soft docking becomes more reasonable in orbits with longer periods or in deep space. Around Earth, Mars or Lunar low orbits it's probably not advisable.

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On 3/14/2022 at 8:42 AM, StrandedonEarth said:

Filling the Depot will take multiple tanker flights, yes. 

That would mean the number of flights for a fuel depot increases (since your need to bring up the depot itself), and you have to deal with boil-off right?

This is in comparison with a direct refuel in orbit with the primary Starship that is going somewhere. 

I guess the main advantage of this fuel depot is your primary Starship (which is probably crewed) can get all the fuel from the depot and be off, rather than wait for multiple re-fueling Starships to come to it. 

 

Also, I'm not sure if this is a concept or the actual plan. Is SpaceX still planning multiple fights to fuel a single Starship, or using this fuel depot concept or something else? They change plans pretty often so I wont be surprised if things have already changed. 
 

 

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The current plan is 1 Depot, 4 refilling flights, and 1 HLS.

Subsequent missions would require more refilling flights as the depot will launch with fuel that will be expended in the first mission, assuming the depot is reused.

More refillings may be needed for heavier payloads and more exotic missions.

Also a reasonable depot upgrade would be to include a sunshade/earthshade. In that instance it could easily be a case that keeping the fuel *warm* would be the issue, not boil off. Nice problem to have.

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