Starship Expendable Upper Stage and other FrankendRocket Concepts

[TheRadicalModerate]

, and some way to mitigate the 2-3x MMOD risk.

Wouldn't a HEEO spend the vast majority of its time at higher altitudes where there's less artificial debris, so the risk would be less than in lower orbits?

Yes, but the risk should scale roughly with the number of orbits.¹  That doesn't say anything about the single-orbit risk.  I agree that it's lower.  It should also be lower as the HEEO apogee increases, because it'll transit the high-risk altitudes faster.

Note that this says nothing about the risk generated if a Starship breaks up.

____________
¹It'll actually scale as 1-(1-pMMOD)ⁿ, where n is the number of orbits and pMMOD is the single-orbit risk.

[Vultur]

, and some way to mitigate the 2-3x MMOD risk.

Wouldn't a HEEO spend the vast majority of its time at higher altitudes where there's less artificial debris, so the risk would be less than in lower orbits?

Yes, but the risk should scale roughly with the number of orbits.¹  That doesn't say anything about the single-orbit risk.  I agree that it's lower.  It should also be lower as the HEEO apogee increases, because it'll transit the high-risk altitudes faster.

I was more thinking that if the risk is significantly less than LEO, then spending more time in HEEO probably isn't a meaningful MMOD concern. So I'm not sure MMOD risk is relevant to consider in this context.

Note that this says nothing about the risk generated if a Starship breaks up.

That's an interesting question. I'd think it would depend heavily on how it breaks up. My first thought, given the Starship test failures we've seen even with aerodynamic forces acting, is that "a significant part of the Ship's mass blown into small pieces" is probably a pretty unlikely outcome. A non-functional but largely intact Starship is probably a lot more likely result of trouble, and would be easily trackable and as a single object in a colossal volume of space is incredibly unlikely to hit anything.

This is also a much larger region of space than LEO, so even if/when it starts getting littered with debris the debris density will be low.

[TheRadicalModerate]

That's an interesting question. I'd think it would depend heavily on how it breaks up. My first thought, given the Starship test failures we've seen even with aerodynamic forces acting, is that "a significant part of the Ship's mass blown into small pieces" is probably a pretty unlikely outcome. A non-functional but largely intact Starship is probably a lot more likely result of trouble, and would be easily trackable and as a single object in a colossal volume of space is incredibly unlikely to hit anything.

I suspect the chance of MMOD penetrating one fairing wall and the common dome is non-trivial, and likely to lead to an explosion.  Stuff that spends any amount of time in HEEO really needs a Whipple shield and a bunch of MLI, at least in some areas.

There are any number of ways that transferring LOX and LCH4 in close proximity to one another leads to an explosion.  Whether that explosion is big enough to cause a cascade that breaches the common dome is a good question.

This is also a much larger region of space than LEO, so even if/when it starts getting littered with debris the debris density will be low.

I don't worry about generated debris at high altitudes, but when it comes through altitudes <600km, it's gonna have a lot of kinetic energy.  And when it's at high altitudes, identifying and tracking it is going to require better tech than the way we track stuff in LEO.

[Vultur]

Some kind of explosion is one thing, but effectively turning a Starship into scattered debris is something else. Even the FTS explosion on IFT-1 didn't immediately do that; without aero forces, a similar size explosion would probably rupture tanks but still leave it basically in one piece.

And when it's at high altitudes, identifying and tracking it is going to require better tech than the way we track stuff in LEO.

Well, I was saying a largely intact (nonfunctional, maybe tanks ruptured, but still more or less in one piece) Starship would be "easily trackable", not debris in general. Its such a huge object that it should be very visible even at greater distances.

[Twark_Main]

As regular readers of Space Debris Quarterly know well, big derelict debris objects have a bad habit of having periodic "shedding events" caused by either internal explosions (batteries, tanks) or minor collisions.

https://orbitaldebris.jsc.nasa.gov/quarterly-news/

The main problem is that 1 cm debris can shatter a vehicle, whereas we can only track >10 cm debris. Unfortunately that means autonomous collision avoidance isn't the "magic bullet" fix that most people think it is.

[TheRadicalModerate]

As regular readers of Space Debris Quarterly know well, big derelict debris objects have a bad habit of having periodic "shedding events" caused by either internal explosions (batteries, tanks) or minor collisions.

https://orbitaldebris.jsc.nasa.gov/quarterly-news/

The main problem is that 1 cm debris can shatter a vehicle, whereas we can only track >10 cm debris. Unfortunately that means autonomous collision avoidance isn't the "magic bullet" fix that most people think it is.

I sooooo wish they'd actually called it Space Debris Quarterly.

[Twark_Main]

As regular readers of Space Debris Quarterly know well, big derelict debris objects have a bad habit of having periodic "shedding events" caused by either internal explosions (batteries, tanks) or minor collisions.

https://orbitaldebris.jsc.nasa.gov/quarterly-news/

The main problem is that 1 cm debris can shatter a vehicle, whereas we can only track >10 cm debris. Unfortunately that means autonomous collision avoidance isn't the "magic bullet" fix that most people think it is.

I sooooo wish they'd actually called it Space Debris Quarterly.

It is a better name, isn't it? NASA seems allergic to names that just roll off the tongue.

Mostly though, I wish I could get a nice glossy print edition delivered for my coffee table!   Guess it's back to the local print shop for me...

[crandles57]

If side by side refuelling is held rigidly together ...

Is it complete madness to imagine 12 raptor engine TMI(TLI) burn at LEO perigee while pumping fuel from depot to mars (/lunar) ship to keep the mars ship full, pause the burn and separate ships then mars ship relights engines to complete TMI burn?

Presumably you would have to throttle the engines on the lighter depot to minimise stresses on the connections between the ships and even then I imagine you would need orders of magnitude more structural strength than just needed for fuel transfers?

Is this completely mad for tiny, no, or negative benefit compared to nose to tail pusher/depot concepts? Less development effort or crazily worse?

[InterestedEngineer]

If side by side refuelling is held rigidly together ...

Is it complete madness to imagine 12 raptor engine TMI(TLI) burn at LEO perigee while pumping fuel from depot to mars (/lunar) ship to keep the mars ship full, pause the burn and separate ships then mars ship relights engines to complete TMI burn?

Presumably you would have to throttle the engines on the lighter depot to minimise stresses on the connections between the ships and even then I imagine you would need orders of magnitude more structural strength than just needed for fuel transfers?

Is this completely mad for tiny, no, or negative benefit compared to nose to tail pusher/depot concepts? Less development effort or crazily worse?

mad.  Utterly mad.

Why didn't you just transfer the fuel at apogee?.  The empty tank can aerobrake / HDL with a tiny bit of deltaV, and thus be reusable, and the outer-planets or Moon bound Ship can do its burn at perigee.

That's enough deltaV to (barely) escape the solar system.

Now if one wants multiple ships full of fuel to do a perigee burn, they can all do it and meet up later on.  Any subsequent burn won't get an oberth boost (unless you do it at Jupiter or Saturn).  This is called "laddering", for which there are entire threads.

You do have a limited time to do an Oberth burn.  At  15km/sec and a 7 minute burn time you've traversed 6,300km.  That's close to the limit, you are starting to leave the gravity well after that.

If you ran halve the engines with double the fuel, it's a 28 minute burn.  You'll traverse 25,000km by the end of the burn, which means a lot of energy went into boosting fuel to a higher orbit.

[crandles57]

If side by side refuelling is held rigidly together ...

Is it complete madness to imagine 12 raptor engine TMI(TLI) burn at LEO perigee while pumping fuel from depot to mars (/lunar) ship to keep the mars ship full, pause the burn and separate ships then mars ship relights engines to complete TMI burn?

Presumably you would have to throttle the engines on the lighter depot to minimise stresses on the connections between the ships and even then I imagine you would need orders of magnitude more structural strength than just needed for fuel transfers?

Is this completely mad for tiny, no, or negative benefit compared to nose to tail pusher/depot concepts? Less development effort or crazily worse?

mad.  Utterly mad.

Why didn't you just transfer the fuel at apogee?.  The empty tank can aerobrake / HDL with a tiny bit of deltaV, and thus be reusable, and the outer-planets or Moon bound Ship can do its burn at perigee.

That's enough deltaV to (barely) escape the solar system.

Now if one wants multiple ships full of fuel to do a perigee burn, they can all do it and meet up later on.  Any subsequent burn won't get an oberth boost (unless you do it at Jupiter or Saturn).  This is called "laddering", for which there are entire threads.

You do have a limited time to do an Oberth burn.  At  15km/sec and a 7 minute burn time you've traversed 6,300km.  That's close to the limit, you are starting to leave the gravity well after that.

If you ran halve the engines with double the fuel, it's a 28 minute burn.  You'll traverse 25,000km by the end of the burn, which means a lot of energy went into boosting fuel to a higher orbit.

The ship would be full at the start of the TMI/TLI burn so it can't take any more but more fuel is needed for return from moon or whatever. During the first part while attached to depot it would be refilled as it uses fuel. So like the pusher/depot concept the lunar ship is full at beginning and at end of burn while attached to depot. There are more engines lit at start of the burn so this enhances the oberth effect. The time to undock and drift far enough away to relight lunar ship engines might be more than for a pusher/depot? Depot is not in direct path of immediate relight of ship engines so earlier relight of engines after undocking may be possible but then it starts to go past the depot and plume expands outward to affect depot. The direction of separation might be better? Anyway if it does take longer to sufficiently separate this might reduce or eliminate the oberth benefit gained earlier.

Not sure if it might get a shorter transit time though VA belts by accelerating more earlier compared to pusher/depot but it probably isn't as fast as one crewed ship and a separate depot going to NRHO to provide return fuel.

I am assuming it is mad for the extra structural strength required of the docking mechanism, but I could be confused on other issues as well.

[TheRadicalModerate]

It is a better name, isn't it? NASA seems allergic to names that just roll off the tongue.

Mostly though, I wish I could get a nice glossy print edition delivered for my coffee table!   Guess it's back to the local print shop for me...

If you're gonna do a mock-up, make sure it has a centerfold.

[MickQ]

It is a better name, isn't it? NASA seems allergic to names that just roll off the tongue.

Mostly though, I wish I could get a nice glossy print edition delivered for my coffee table!   Guess it's back to the local print shop for me...

If you're gonna do a mock-up, make sure it has a centerfold.

AND make sure that the staples don’t obscure anything picturesque 😁😁😁

[TomH]

The ship would be full at the start of the TMI/TLI burn so it can't take any more but more fuel is needed for return from moon or whatever. During the first part while attached to depot it would be refilled as it uses fuel. So like the pusher/depot concept the lunar ship is full at beginning and at end of burn while attached to depot. There are more engines lit at start of the burn so this enhances the oberth effect. The time to undock and drift far enough away to relight lunar ship engines might be more than for a pusher/depot? Depot is not in direct path of immediate relight of ship engines so earlier relight of engines after undocking may be possible but then it starts to go past the depot and plume expands outward to affect depot. The direction of separation might be better? Anyway if it does take longer to sufficiently separate this might reduce or eliminate the oberth benefit gained earlier.

In essence, you want to create a dual core Starship Heavy with cross feed. And the cross feed plumbing is somehow plugged in after rendezvous and docking and in space.

I don't even know where to begin.

Firstly, the risks of setting up and utilizing cross-feed while under full thrust is....just wow. In theory, tri-core heavy LVs would be geometrically mass balanced. A twin-core would start out unbalanced and as one core depleted, the thrust asymmetry would get worse and worse. Also, tri-core heavy LVs require high mass strong back joinery. Elon said of FH that it's like flying three cores independently in perfect formation, even in spite of the connecting frames. The thrust-mass asymmetry of what you describes is just......VERY bad.

I would say that mating a circular cluster of drop tanks completely around the SS while in space and then filling the core vehicle (and the tanks through the core) until all are full might work better. Then, when the SS departs for deep space, you burn solely from the drop tanks until all are depleted. They then peel off like multiple boosters from the side of an Atlas V, and the engines switch to the main tanks. It's mass-thrust balanced. I know it violates Elon's tenet of complete reusability, but I do not think the tanks themselves would be insanely expensive; finding a way to rendezvous and attach all of them while in space would be the long-pole. What you have proposed would be more likely to work with a tri-core while draining the outer cores at the same rate. You have tripple the number of engines and you're also pouring an awful lot of ΔV into the side cores (which have to decelerate and return afterwards). With drop tanks, you're wasting much less ΔV on the lower mass of those tanks and you don't need extra engines involved. You also don't have to worry so much about gravity losses from the T/W like you would at sub-orbital velocity.

Edit to add: penetrating the TPS with drop tanks that encircle the ship is problematic. This might work best on Lunar SS which has no TPS and will never need to perform an atmospheric entry.

Drop thanks could also be lower mass than SH/SS skin, possibly thinner and perhaps aluminum. You could also simplify their design by making some solely fuel and some solely oxidizer. You'd balance the mass by geometrically placing them appropriately. Separate fuel/LOX tanks also reduces the plumbing connections.

[crandles57]

Firstly, the risks of setting up and utilizing cross-feed while under full thrust is....just wow. In theory, tri-core heavy LVs would be geometrically mass balanced. A twin-core would start out unbalanced and as one core depleted, the thrust asymmetry would get worse and worse. Also, tri-core heavy LVs require high mass strong back joinery. Elon said of FH that it's like flying three cores independently in perfect formation, even in spite of the connecting frames. The thrust-mass asymmetry of what you describes is just......VERY bad.

Yeah asymmetry is very bad.

'just wow' - as in way out there, ridiculous, I assume. At least there is thrust to settle the fuel  What does the cross feed require more than the intended refuelling? More powerful pumps? Higher flow rate pipes? Or is it a whole different ball game due to the vibrations under full thrust?

I would say that mating a circular cluster of drop tanks completely around the SS while in space and then filling the core vehicle (and the tanks through the core) until all are full might work better.

If you are just doing drop tanks then a single pair of large tanks on the nose would seem to work better than a circular cluster.

I was just thinking (or probably failing to think sensibly though I did suggest it was mad at the start) that if the tanks you send up have engines on them, why not use those engines on the TLI for improved oberth effect. Removing the engines to reduce the tank mass might be better but seems like extra development work though probably not as ridiculously difficult development work as higher flow rate refuelling under lots of vibrations from full thrust.

How difficult SpaceX found it to develop falcon heavy after F9 probably shows the idea is a non starter without much thought being needed.

[Overtone]

Single drop tank inline and ahead of the ship coupled using the same hardpoints as the chopsticks and the megabay lifter.  Coupling structure may look a lot like the megabay lifter though would be structurally different because stress is compression not tension (plus needs to unfold after riding to orbit inside a ship payload bay).

Propellant flows down into the ship due to acceleration thrust so no pumps needed - keeps the discarded tank simpler and cheaper. Plumb the connections into the header tank lines just below the header tanks. Pierce the leeward side then cross inside the ship to the propellant lines on the windward side.

Would be an interesting analysis to figure out if an additional hardpoint on the leeward side is needed to adequately handle thrust vectoring stresses.

[TheRadicalModerate]

Single drop tank inline and ahead of the ship coupled using the same hardpoints as the chopsticks and the megabay lifter.  Coupling structure may look a lot like the megabay lifter though would be structurally different because stress is compression not tension (plus needs to unfold after riding to orbit inside a ship payload bay).

Propellant flows down into the ship due to acceleration thrust so no pumps needed - keeps the discarded tank simpler and cheaper. Plumb the connections into the header tank lines just below the header tanks. Pierce the leeward side then cross inside the ship to the propellant lines on the windward side.

Would be an interesting analysis to figure out if an additional hardpoint on the leeward side is needed to adequately handle thrust vectoring stresses.

This seems like it's a lot harder to dock than a pusher would be.  Mind you, a pusher isn't easy to dock either, but its hard-capture system is just the existing Starship staging latches.  Soft-capture is still a problem, but it's less of a problem than guiding something on top of the pointy end.

Furthermore, a drop tank is inherently expendable, since it has no propulsion to get home.  A pusher, on the other hand, is reusable:  it just shuts down and stages with enough prop to return to LEO at the next perigee.  Fill it up at a depot and it's ready for the next mission.

[Overtone]

Single drop tank inline and ahead of the ship coupled using the same hardpoints as the chopsticks and the megabay lifter.  Coupling structure may look a lot like the megabay lifter though would be structurally different because stress is compression not tension (plus needs to unfold after riding to orbit inside a ship payload bay).

Propellant flows down into the ship due to acceleration thrust so no pumps needed - keeps the discarded tank simpler and cheaper. Plumb the connections into the header tank lines just below the header tanks. Pierce the leeward side then cross inside the ship to the propellant lines on the windward side.

Would be an interesting analysis to figure out if an additional hardpoint on the leeward side is needed to adequately handle thrust vectoring stresses.

This seems like it's a lot harder to dock than a pusher would be.  Mind you, a pusher isn't easy to dock either, but its hard-capture system is just the existing Starship staging latches.  Soft-capture is still a problem, but it's less of a problem than guiding something on top of the pointy end.

Furthermore, a drop tank is inherently expendable, since it has no propulsion to get home.  A pusher, on the other hand, is reusable:  it just shuts down and stages with enough prop to return to LEO at the next perigee.  Fill it up at a depot and it's ready for the next mission.

Both are excellent points.  Thanks TRM.  I was reacting to crandles57.  A more complete introduction to my post would have been "if you're going to do a drop tank at all, use a single tank inline rather than trying to balance/fly several tanks arranged around the ship."

On reflection, the drop tank concept may still make sense if the mission profile requires a higher propellant mass fraction than a recoverable pusher can provide. Interesting option would be to 3D print the drop tank in orbit.  It's just a big lightweight tank with minimal rigidity except in the base plate, no insulation needed.  Building it in orbit eliminates structure needed to survive launch stress, enables making a tank bigger than you can launch, and using a dumb tank avoids the need to launch/maintain the engines electronics and valves in the pusher.   CONOPS: dock the ship to an empty tank while the tank is still held by the orbital factory, which provides the attitude thrusters and control needed for RPOD. Ship carries the tank away.  Depot/ship interact and transfer propellant as before, but the ship routes propellant into the drop tank in addition to the ship's internal tanks so it can accept several depot loads of propellant before departure from LEO.

Fast transfer to Jupiter?

[TheRadicalModerate]

On reflection, the drop tank concept may still make sense if the mission profile requires a higher propellant mass fraction than a recoverable pusher can provide.

You don't have to recover the pusher.  There's no question that a drop tank will result in a lower dry mass, but you need really high energy (like, well past heliocentric escape speed) before you care very much.  So you'd need a very compelling case before figuring out how to do a drop tank.  And it would likely be easier just starting from a very energetic HEEO.  That would take a buttload of tankers, but there's no real development involved.

Fast transfer to Jupiter?

FWIW, a v3 Starship with no tiles, flaps, header tanks, and a jettisonable nose fairing can do a direct transfer to Jupiter from VLEO (200 x 200).  For that matter, a stripped-down v3, even with fairly conservative dry mass estimates, can get more than 40t of payload from VLEO to heliocentric escape.

It's really non-intuitive that landing a Starship on the Moon and returning it to EDL takes more delta-v than escaping the solar system, but that is the case.  So if a fully-fueled pusher can manage that (and it can, as long as the target Starship is refueled as well), I don't think you need anything fancier than that.

ETA:  HEEO refueing can obviously also be made to work.  The pusher would just make things all LEO-based, which is handy.  I've more-or-less given up on the idea of a combined pusher/depot, simply because the refueling interface is kludgy.  It'd lovely to remove an RPOD from the system, but it's not necessary.  Both the pusher and an HEEO refueling require the same number of RPODs for the target system.  (A dedicated pusher would require one more RPOD with the depot than just having the depot boost to HEEO.)

[rsdavis9]

It's really non-intuitive that landing a Starship on the Moon and returning it to EDL takes more delta-v than escaping the solar system, but that is the case.  So if a fully-fueled pusher can manage that (and it can, as long as the target Starship is refueled as well), I don't think you need anything fancier than that.

This can't be emphasized enough.
Everyday people say "but the moon is closer". But the fact of the matter in terms of fuel the rest of the solar system(one way) is closer. The moon round trip takes more.

[Vultur]

It's really non-intuitive that landing a Starship on the Moon and returning it to EDL takes more delta-v than escaping the solar system, but that is the case.  So if a fully-fueled pusher can manage that (and it can, as long as the target Starship is refueled as well), I don't think you need anything fancier than that.

This can't be emphasized enough.
Everyday people say "but the moon is closer". But the fact of the matter in terms of fuel the rest of the solar system(one way) is closer. The moon round trip takes more.

The Moon having a significant gravity well, but no atmosphere to aerobrake, makes a big difference. I think only Mercury has more gravity without atmosphere than the Moon (though Ganymede is close).