Author Topic: A question of maximum Kinetic Energy  (Read 1706 times)

Offline aero

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A question of maximum Kinetic Energy
« on: 04/17/2019 08:21 am »
Maximum KE for asteroid re-directing or maximum velocity just to "get out of Dodge" quickly.

Did I read on this forum that the Starship booster (BFB) could reach orbit if launched without a Starship but with a fairing or cap over the innerstage? If that is so, then how many tanker loads would it take to refuel the BFB on orbit, plus the 6 tanker loads needed for the Starship itself (launched separately)? The Starship would then be mated to the BFB on orbit. What would be the performance of such a system? Maybe a quick round trip to Jupiter or Saturn's moons, maybe a seriously hefty asteroid deflector. Maybe actually catch up to that extra-solar object ('Oumuamua) which passed through the solar system last year. I'm sure we can dream up a lot of missions that would be enabled by such a configuration.

As an aside, would the BFB be able to carry itself and a completely dry Starship to orbit? If so, the guidance and control problem of getting the BFB to orbit could be easily solved with software.
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Offline speedevil

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Re: A question of maximum Kinetic Energy
« Reply #1 on: 04/17/2019 10:18 am »
Maximum KE for asteroid re-directing or maximum velocity just to "get out of Dodge" quickly.

Did I read on this forum that the Starship booster (BFB) could reach orbit if launched without a Starship but with a fairing or cap over the innerstage? If that is so, then how many tanker loads would it take to refuel the BFB on orbit, plus the 6 tanker loads needed for the Starship itself (launched separately)? The Starship would then be mated to the BFB on orbit. What would be the performance of such a system?
They probably don't want to do that even if it is theoretically possible.
This is as BFB cannot reenter.
With Starship, you can do things like refuel another Starship in a high energy orbit at ~10km/s velocity at perigee, boost another 2.5km/s just before perigee, then transfer all propellant over to one, and then boost another 10km/s at perigee.

And you can recover the tanker by aerobraking 'free'.
Using the booster means you expend a booster every time, as it can't aerobrake to recover.

Online rakaydos

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Re: A question of maximum Kinetic Energy
« Reply #2 on: 04/17/2019 12:03 pm »
Maximum KE for asteroid re-directing or maximum velocity just to "get out of Dodge" quickly.

Did I read on this forum that the Starship booster (BFB) could reach orbit if launched without a Starship but with a fairing or cap over the innerstage? If that is so, then how many tanker loads would it take to refuel the BFB on orbit, plus the 6 tanker loads needed for the Starship itself (launched separately)? The Starship would then be mated to the BFB on orbit. What would be the performance of such a system?
They probably don't want to do that even if it is theoretically possible.
This is as BFB cannot reenter.
With Starship, you can do things like refuel another Starship in a high energy orbit at ~10km/s velocity at perigee, boost another 2.5km/s just before perigee, then transfer all propellant over to one, and then boost another 10km/s at perigee.

And you can recover the tanker by aerobraking 'free'.
Using the booster means you expend a booster every time, as it can't aerobrake to recover.
In theory, there's no reason you couldn't tile the 301 stainless booster with the same 310 stainless tiles the starship uses. Getting it to orbit may require some absurd "reverse staging" (using the refueling lines to drain the upper stage of fuel first, then "abort" the upper stage on header tanks and continue to orbit with superheavy)

I've mentioned in the past using an orbital superheavy as a Titan lifter, using ISRU to refuel a superheavy/starship stack for cargo return to the inner system. But the initial poster is right, a superheavy after dismounting most the engines is an absurdly powerful orbital tug.

Offline Steve D

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Re: A question of maximum Kinetic Energy
« Reply #3 on: 04/17/2019 01:41 pm »
Could it be used as a fuel depot?

Online rakaydos

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Re: A question of maximum Kinetic Energy
« Reply #4 on: 04/17/2019 01:48 pm »
Could it be used as a fuel depot?

Might need better thermal shielding for long term storage, but that's a customization that can be made on the ground.

Offline aero

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Re: A question of maximum Kinetic Energy
« Reply #5 on: 04/17/2019 03:43 pm »
Maximum KE for asteroid re-directing or maximum velocity just to "get out of Dodge" quickly.

Did I read on this forum that the Starship booster (BFB) could reach orbit if launched without a Starship but with a fairing or cap over the innerstage? If that is so, then how many tanker loads would it take to refuel the BFB on orbit, plus the 6 tanker loads needed for the Starship itself (launched separately)? The Starship would then be mated to the BFB on orbit. What would be the performance of such a system?
They probably don't want to do that even if it is theoretically possible.
This is as BFB cannot reenter.
With Starship, you can do things like refuel another Starship in a high energy orbit at ~10km/s velocity at perigee, boost another 2.5km/s just before perigee, then transfer all propellant over to one, and then boost another 10km/s at perigee.

And you can recover the tanker by aerobraking 'free'.
Using the booster means you expend a booster every time, as it can't aerobrake to recover.

Yes, I assumed that it goes without saying that I am contemplating "One-off" expendable missions. Max KE would burn the BFB to depletion, as would minimum time trajectories to the outer planet's moons. And I don't think SpaceX would mind expending some BFB's if it were profitable enough. In particular, the KE impactor to divert a large, late-discovered asteroid that was assured to strike Earth could be priceless. Because the asteroid was discovered late, there would not be enough time to develop the capability, rather the capability needs to be developed, proven and available.

Edit add:

There is a post in the SpaceX General section that overlaps this thread. Maybe the moderators would like to adjust topics?
https://forum.nasaspaceflight.com/index.php?topic=47880.msg1937330#new
« Last Edit: 04/17/2019 04:31 pm by aero »
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Offline Oersted

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Re: A question of maximum Kinetic Energy
« Reply #6 on: 04/17/2019 04:38 pm »
The BFB doesn't have vacuum Raptors so it would be hugely inefficient in space.

Online PADave

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Re: A question of maximum Kinetic Energy
« Reply #7 on: 04/17/2019 04:51 pm »
We don't really have enough information to give a good answer. A Starship like Elon tweeted about in https://twitter.com/elonmusk/status/1111798912141017089 would give about 12.7 km/s to 40 tons. Just guessing Superheavy adds another 5.3 km/s (a wild guess) that would give a velocity change of 18 km/s. Which implies a KE of about 6.5 * 10^12 joules (1/2 * 40,000 kg * (18,000 m/s)^2). But then you have to take in orbital mechanics so it depends on what directions it's coming from and the velocity of the asteroid you are trying to move.

So there's really no way to give a good answer. Possibly someone can correct my math or give a better estimate?

Offline alexterrell

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Re: A question of maximum Kinetic Energy
« Reply #8 on: 04/17/2019 04:52 pm »
Maximum KE for asteroid re-directing or maximum velocity just to "get out of Dodge" quickly.

Did I read on this forum that the Starship booster (BFB) could reach orbit if launched without a Starship but with a fairing or cap over the innerstage? If that is so, then how many tanker loads would it take to refuel the BFB on orbit, plus the 6 tanker loads needed for the Starship itself (launched separately)? The Starship would then be mated to the BFB on orbit. What would be the performance of such a system?
They probably don't want to do that even if it is theoretically possible.
This is as BFB cannot reenter.
With Starship, you can do things like refuel another Starship in a high energy orbit at ~10km/s velocity at perigee, boost another 2.5km/s just before perigee, then transfer all propellant over to one, and then boost another 10km/s at perigee.

And you can recover the tanker by aerobraking 'free'.
Using the booster means you expend a booster every time, as it can't aerobrake to recover.

It might be easier to launch a custom built disposable fuel tank. This could then be docked in front of Starship, or have two strapped on the sides. Some sort of propellant transfer is needed, but no extra engines. A bit like in Falcon Heavy configuration, but without engines on the side boosters. 

Online rakaydos

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Re: A question of maximum Kinetic Energy
« Reply #9 on: 04/17/2019 04:53 pm »
The BFB doesn't have vacuum Raptors so it would be hugely inefficient in space.
Neither does Starship, these days. It's not THAT inefficient.

Online Keldor

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Re: A question of maximum Kinetic Energy
« Reply #10 on: 04/17/2019 08:32 pm »
With inflight refueling, they can have a topped off second stage in orbit regardless of whether they recover first stages - it's just a matter of how many flights it will take to bring up enough fuel.

The question then is if there are any circumstances which would make expending let's say 4 boosters to refuel a second stage is better than flying 10 reusable flights to refuel.  Or else where a single flight expendable rocket with an only partially filled second stage makes sense.

Offline aero

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Re: A question of maximum Kinetic Energy
« Reply #11 on: 04/18/2019 12:18 am »
With inflight refueling, they can have a topped off second stage in orbit regardless of whether they recover first stages - it's just a matter of how many flights it will take to bring up enough fuel.

The question then is if there are any circumstances which would make expending let's say 4 boosters to refuel a second stage is better than flying 10 reusable flights to refuel.  Or else where a single flight expendable rocket with an only partially filled second stage makes sense.

I don't know where this is going. There is no mention in the OP of expending the booster during on-orbit refueling operations. Refueling is well covered by the existing plans using recovered boosters. Expending the booster was introduced to maximize delta-V for a quick trip to the moons of the outer planets or to maximize the momentum of a kinetic impactor. It might even be usable in order to launch a mission to Mars to relieve an emergency situation at a time where orbital alignments are not ideal. But I can't justify expending a booster simply to carry a bit more fuel to orbit. The plan is for the boosters to be a somewhat scarce resource after all, so there may not even be 4 boosters in the whole fleet.
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Online Keldor

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Re: A question of maximum Kinetic Energy
« Reply #12 on: 04/18/2019 01:43 am »
With inflight refueling, they can have a topped off second stage in orbit regardless of whether they recover first stages - it's just a matter of how many flights it will take to bring up enough fuel.

The question then is if there are any circumstances which would make expending let's say 4 boosters to refuel a second stage is better than flying 10 reusable flights to refuel.  Or else where a single flight expendable rocket with an only partially filled second stage makes sense.

I don't know where this is going. There is no mention in the OP of expending the booster during on-orbit refueling operations. Refueling is well covered by the existing plans using recovered boosters. Expending the booster was introduced to maximize delta-V for a quick trip to the moons of the outer planets or to maximize the momentum of a kinetic impactor. It might even be usable in order to launch a mission to Mars to relieve an emergency situation at a time where orbital alignments are not ideal. But I can't justify expending a booster simply to carry a bit more fuel to orbit. The plan is for the boosters to be a somewhat scarce resource after all, so there may not even be 4 boosters in the whole fleet.

What I'm saying is that if you can refuel in orbit, expending the booster won't give you any extra delta-V.  You leave it behind part way to orbit either way, so the only thing that changes is how much extra fuel the second stage has once it gets there.  But it's going to be refueled!

Emergency situations would make sense.  Probably not Mars since you're going to be sending stuff every launch window anyway, so all the tankers for refueling will already be in place, but maybe a Moon mission gone wrong?

Online rakaydos

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Re: A question of maximum Kinetic Energy
« Reply #13 on: 04/18/2019 02:16 am »
With inflight refueling, they can have a topped off second stage in orbit regardless of whether they recover first stages - it's just a matter of how many flights it will take to bring up enough fuel.

The question then is if there are any circumstances which would make expending let's say 4 boosters to refuel a second stage is better than flying 10 reusable flights to refuel.  Or else where a single flight expendable rocket with an only partially filled second stage makes sense.

I don't know where this is going. There is no mention in the OP of expending the booster during on-orbit refueling operations. Refueling is well covered by the existing plans using recovered boosters. Expending the booster was introduced to maximize delta-V for a quick trip to the moons of the outer planets or to maximize the momentum of a kinetic impactor. It might even be usable in order to launch a mission to Mars to relieve an emergency situation at a time where orbital alignments are not ideal. But I can't justify expending a booster simply to carry a bit more fuel to orbit. The plan is for the boosters to be a somewhat scarce resource after all, so there may not even be 4 boosters in the whole fleet.

What I'm saying is that if you can refuel in orbit, expending the booster won't give you any extra delta-V.  You leave it behind part way to orbit either way, so the only thing that changes is how much extra fuel the second stage has once it gets there.  But it's going to be refueled!

Emergency situations would make sense.  Probably not Mars since you're going to be sending stuff every launch window anyway, so all the tankers for refueling will already be in place, but maybe a Moon mission gone wrong?
I think you misunderstand the topic. It isnt about expending a booster on launch. It's about putting an entire first stage into orbit, then refueling it there, to be used as a super high DV tug.

Offline Ludus

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Re: A question of maximum Kinetic Energy
« Reply #14 on: 04/18/2019 06:59 am »
All those sea level raptor engines on SH would be kind of a waste for this purpose. A modified Tanker Starship could also be a depot/deep space booster.

Just make a Starship that basically looks like a Superheavy, extend the propellant tanks well beyond the point they could be launched full, it launches partly empty. Strip away the fins-legs and heatshield. End it with a Superheavy fore end. Itís now a cylinder shaped Starship that can dock and transfer propellant at either end. Put an expendable nosecone on it for launch.

It never re-enters but it holds more propellant than a regular Starship and can dock to the aft end of one and boost it. Itís got vacuum raptors. For this purpose there isnít much point in burning a lot of them, a few burning longer work as well and mass less.

It can boost starships and come back on a free return. It can accumulate propellant for transfer to Starships. It doesnít need to be able to re-enter to have a lot of utility.
« Last Edit: 04/18/2019 07:02 am by Ludus »

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Re: A question of maximum Kinetic Energy
« Reply #15 on: 04/18/2019 12:15 pm »
Haven’t we seen that in the field construction is more viable than people thought? Why not orbital assembly of your reusable, in space only, high delta v or whatever super heavy booster (or call it a “pusher”)?

Take the sheet metal pieces and welding gear in on launch. Bring up whatever rocket engines and bells you want in a later launch. Use a SS as a construction shack.

Never sees ground ops, but can push payload to higher speed and , if desired, retro burn to return to orbit, refuel and go again.

Online Keldor

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Re: A question of maximum Kinetic Energy
« Reply #16 on: 04/18/2019 05:46 pm »

What I'm saying is that if you can refuel in orbit, expending the booster won't give you any extra delta-V.  You leave it behind part way to orbit either way, so the only thing that changes is how much extra fuel the second stage has once it gets there.  But it's going to be refueled!

Emergency situations would make sense.  Probably not Mars since you're going to be sending stuff every launch window anyway, so all the tankers for refueling will already be in place, but maybe a Moon mission gone wrong?
I think you misunderstand the topic. It isnt about expending a booster on launch. It's about putting an entire first stage into orbit, then refueling it there, to be used as a super high DV tug.

Hmm.  Consider that it would take ~30 flights to refill a first stage.  I'm not convinced of the practicality, but maybe eventually when they're flying very cheaply and NASA has a flagship class mission?

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Re: A question of maximum Kinetic Energy
« Reply #17 on: 04/18/2019 07:30 pm »
I recall Tsiolkovsky deriving a solution: kinetic energy efficiency is maximized for a rocket having a mass ratio of exactly four, having an energy efficiency of 65%. See "Coefficient of Useful Work (Utilization) of the Rocket on Ascent" in his Exploration of Outer Space by Means of Reaction Devices (1903, Public Domain NASA translation available here, see PDF page 83).

So astrodynamically, is it as simple as burning until C3=0 to the asteroid, noting the total rocket mass, then continue burning until your total mass is 1/4 that value (maximizing KE to the asteroid), then iterate that trajectory planning over different impact directions and intercept times to find the plan which maximizes deflection? Or is it more complex than that?

Oops, not quite. I just realized that I'm trying to maximize KE, but asteroid deflection calls for maximizing momentum transfer instead. This is maximized at a mass ratio of exactly e ~= 2.72 instead of 4, at which it achieves a momentum transfer efficiency of 1/e ~= 38% of m0 * v_exhaust (ie soft-landing and pushing directly on the asteroid, or using the stage as a gravity tractor, ignoring dry mass of course).

This math all leads me to the conclusion that multiple Starships (ideally the stripped down legless no heatshield variant) would be better for asteroid redirection than using a Super Heavy booster in LEO. Ideally you want your mass ratio from LEO as close to e as possible (to maximize asteroid-imparted momentum per tanker flight), and StarTug is closest to that ideal. Obviously the cheapest source of ballast is propellant.

This seems to answer the original question. Can someone sanity check my math and logic?

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