Starship On-orbit refueling - Options and Discussion

[Nevyn72]

Rather than clutter up the Off Earth modification and Assembly thread it's probably time that the In-orbit refueling concept gets it's own thread.

My own thoughts are that some concepts are getting a bit complicated and convoluted for their own good, especially at such an early stage in the process.

The first stage is and should be SS to SS refueling. This has the advantages of;
- simplest hardware
- you are only manufacturing one type of orbital hardware
- greatest flexibility (of mission profile)
- proof of concept with the most capable vehicle (ie. a fully maneuverable space ship)

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I envisige you would replace the SH thrust structure with a SS thrust structure. The SH (tank) could even supply fuel to the SH (launch) to help with the increased takeoff weight. You would then be using the proven SS attachment system for In-Orbit fuel transfers.

You could have a small nose section containing header tanks for the RCS system, batteries, avionics and even a deployable solar array. The existing SH design (with a SS thrust structure) is already devoid unnecessary external fittings found on the SS such as landing legs, flaps and thermal tiles.

[Coastal Ron]

In my opinion the second stage of development of the concept should be a permanently orbiting 'tank' which can be repositioned in orbit as required.
The prime candidate for this would be a modified SH which would be comparatively easily developed given it's an iteration of an existing design.

I like the simplicity of this idea, and as you pointed out, it doesn't require any new hardware design.

Initially you might have to use a Super Heavy that is fairly new, but over time you could use older Super Heavies that need to be retired, and retiring them in space to use as a tanker might be an option.

[xvel]

the problem of too little thrust at lift off is not due to the fact that there is too little fuel, but to the physical limitations of the materials of which the engine is built, so there is a limit of maximum thrust per unit of surface area of the rocket base, which implies the maximum height of the rocket and the starship is already close to this limit, so adding more tanks on top makes t/w ratio worse not better

[Nevyn72]

the problem of too little thrust at lift off is not due to the fact that there is too little fuel, but to the physical limitations of the materials of which the engine is built, so there is a limit of maximum thrust per unit of surface area of the rocket base, which implies the maximum height of the rocket and the starship is already close to this limit, so adding more tanks on top makes t/w ratio worse not better

My thought was to use a partly filled SH (tank) section, thus reducing total takeoff weight ratio to an amount similar to a SS+SH combo.

Using the connection between the SH components you could feed some additional fuel to the SH (launch) section to allow it to burn for a longer duration thus imparting a greater Delta V to the SH (tank) section.

I'm not a rocket engineer so this might just be a pointless thought bubble.....

EDIT: Clarity & grammar

[fael097]

You think the tanker variant will have just two huge fuel/ox tanks (plus header tanks) or separate regular tanks and storage tanks?

[xvel]

if super heavy had more dv and used it, it would cause problems with entering the atmosphere and landing
I'm not a rocket engineer either, I'm just an engineer 

[Nevyn72]

You think the tanker variant will have just two huge fuel/ox tanks (plus header tanks) or separate regular tanks and storage tanks?

My concept is literally a standard SH with SS thrust structure and a small nosecone for support equipment.....

[Nevyn72]

if super heavy had more dv and used it, it would cause problems with entering the atmosphere and landing
I'm not a rocket engineer either, I'm just an engineer 

Would extra fuel for extra boost back help?

[fael097]

You think the tanker variant will have just two huge fuel/ox tanks (plus header tanks) or separate regular tanks and storage tanks?

My concept is literally a standard SH with SS thrust structure and a small nosecone for support equipment.....

I guess I mean the SS tanker variant.
On that note, I don't think a SH could be a tanker, SH wouldn't survive orbital reentry, would it?

[xvel]

if super heavy had more dv and used it, it would cause problems with entering the atmosphere and landing
I'm not a rocket engineer either, I'm just an engineer 

Would extra fuel for extra boost back help?

Yes, but you couldn't take it

[xvel]

Current design of starship barely works on paper, there really is no room for modifications, and I don't see any need for the tanker to be bigger than a starship. Refueling anyway will probably be done with one tanker parked in orbit which will be refueled several times from Earth and then it will refuel the starship parked in orbit (thanks to this the starship will be fully refueled with one refueling which will increase safety)

[BrianPeterson]

Current design of starship barely works on paper, there really is no room for modifications, and I don't see any need for the tanker to be bigger than a starship. Refueling anyway will probably be done with one tanker parked in orbit which will be refueled several times from Earth and then it will refuel the starship parked in orbit (thanks to this the starship will be fully refueled with one refueling which will increase safety)

Musk hates the idea of an orbiting tanker. What has been stated is one Starship fueled up by an additional 3 to 4 other Starship launches.

Don't have any idea what lOX or Liquid methane weight, but payload bay converted to a tank is probably how this will work. .

[rakaydos]

From what we've seen of the starship thrust structure, there is a single pipe from the CH4 header tank, to the engines, and down to  the skirt where it will presumably form the refueling system.

A single pipe means the connector will need to be androgonus.

I'm less familiar with airticht androgonus designs- any suggestions for the pipe cap?

[fael097]

How much propellant mass a tanker Starship could haul into LEO? Regular SS carries 1200 t of prop and 100 t of payload, so tanker SS would only carry 1300 t of prop? I mean, since it can carry more prop it can burn more prop, and it has internal space for carrying 2100 t of propellant without compromising fairing space for header tanks, batteries, actuators, etc. but could a booster even lift that up? and in the end, would a 2100 t prop tanker have more fuel left when it gets to LEO than say a 1300 t prop tanker?

This is a tanker with capacity for 2100 t:

[joek]

...
Musk hates the idea of an orbiting tanker. What has been stated is one Starship fueled up by an additional 3 to 4 other Starship launches.
...

"Hate" seems a bit strong; do you have a cite?  Pragmatic is likely a more appropriate description.  Not to mention that Musk appears to be trying to stay out of the "no depots" debate.

[Nevyn72]

You think the tanker variant will have just two huge fuel/ox tanks (plus header tanks) or separate regular tanks and storage tanks?

My concept is literally a standard SH with SS thrust structure and a small nosecone for support equipment.....

I guess I mean the SS tanker variant.
On that note, I don't think a SH could be a tanker, SH wouldn't survive orbital reentry, would it?

It wouldn't need to survive re-entry, it stays up there. 

When it finishes the current mission (could be multiple refueling events for a particular synod) it can reposition to new orbit as required/desired using the onboard SS thrust structure and surplus onboard fuel.

Note: this is all based on the concept that you don't a crewed SS sitting around waiting for multiple refueling events. You want a tanker up there waiting for it so refueling and departure can occur as quickly as possible. Having a SH based unit means you have surplus fuel available to top up the departing SS as much as possible.

[Nevyn72]

if super heavy had more dv and used it, it would cause problems with entering the atmosphere and landing
I'm not a rocket engineer either, I'm just an engineer 

Would extra fuel for extra boost back help?

Yes, but you couldn't take it

Well you could, the extra fuel for a longer boost back is part of the transfer from the SH (tank) being launched.

The only variable is how much fuel is left in the SH (tank) when it reaches the desired orbit.

[rakaydos]

if super heavy had more dv and used it, it would cause problems with entering the atmosphere and landing
I'm not a rocket engineer either, I'm just an engineer 

Would extra fuel for extra boost back help?

Yes, but you couldn't take it

Well you could, the extra fuel for a longer boost back is part of the transfer from the SH (tank) being launched.

The only variable is how much fuel is left in the SH (tank) when it reaches the desired orbit.

are you trying to SSTO? Because that never works out as well as you think it does.

[BrianPeterson]

"Hate" seems a bit strong; do you have a cite?  Pragmatic is likely a more appropriate description.  Not to mention that Musk appears to be trying to stay out of the "no depots" debate.

It was a video interview, where you could say he scoffed at the idea. Derided the idea. He doesn't like the idea much. I've watched so many interviews with him, finding the exact one is a needle in a haystack. 

I on the other hand think an orbital platform could be the best idea. What happens if one of the four launches required to refuel fails? Or weather suddenly makes it impossible to launch. You could have to scrub the mission or have a starship and crew hang out for days/weeks/months waiting for fueling. Having a platform that is constantly stocked up means you can launch knowing the required fuel is there and get the mission under way.

Musk has other ideas: this was easy to find.
<blockquote class="twitter-tweet"><p lang="en" dir="ltr">Orbital refilling is vital to humanity’s future in space. More likely spacecraft to spacecraft (as aircraft do aerial refueling), than a dedicated depot, at least at first.</p>&mdash; Elon Musk (@elonmusk) August 1, 2019 <script async src="https://platform.twitter.com/widgets.js" charset="utf-8"></script>

[Nevyn72]

...
Musk hates the idea of an orbiting tanker. What has been stated is one Starship fueled up by an additional 3 to 4 other Starship launches.
...

"Hate" seems a bit strong; do you have a cite?  Pragmatic is likely a more appropriate description.  Not to mention that Musk appears to be trying to stay out of the "no depots" debate.

I believe (my opinion, no citation) that the issue Musk has with an orbiting tanker is that it's located for where you need it initially but the desired location changes with each mission so quickly becomes useless.

My suggestion means that when needed you can reposition the unit to the next ideal staging orbit.

You will be leaving a SS in orbit in this role anyway but with the SS(tanker)-SS you are launching a whole lot of additional weight to allow the SS(tanker) to return to Earth, just so you can then re-launch the whole lot again to a different orbit for the next mission.