Author Topic: Proposal - second stage recovery  (Read 98058 times)

Online meekGee

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Proposal - second stage recovery
« on: 03/18/2015 06:04 pm »
I'm not putting this in the recovery thread since it's not based on any SpaceX data, and is not even speculation.  It is a proposal at best.

I've been pondering this for a while now - how to get the second stage back.  The problem is that all the factors that come together and make propulsive RTLS such a no-brainer for the first stage, are ass-backwards for the second stage:


                       FS             US         Comment
Insta-9:1-throttle     Yes            No                   
Sea-level engines      Yes            No                     
Slow reentry           Yes            No                   
Mass penalty           4:1            1:1
Entry point            downrange      launch site
Aspect ratio           slender        squat
Mass                   higher         lower
Drag/mass              lower          higher


For these reasons:
- 1st stage does not need separate landing engines
- 1st stage can enter tail first
- 1st stage does not need a heat shield
- 1st stage needs a reentry burn so as not to break up

OTOH, mid-air helicopter recovery which was practically impossible for the first stage, becomes almost natural for the second stage:

- It is much lighter
- Recovery can be performed right at the launch site, no need to try to operate 1000 miles off-shore
- The second stage is short, therefore more rigid, and should be easier to take through reentry
- A heat shield is much lighter than propellant, and for the second stage, it's a 1:1 mass penalty

The tricky part is to keep the stage stable while entering head-first, which would require some aerodynamic drag device on its back end, one that can survive reentry.

I know Musk said they're not working on it, but I think it might become a priority again in a few years, as launch rate increases.
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Offline Jim

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Re: Proposal - second stage recovery
« Reply #1 on: 03/18/2015 06:10 pm »

OTOH, mid-air helicopter recovery which was practically impossible for the first stage, becomes almost natural for the second stage:


How do you know it is light enough to be mid air recovered?

Online meekGee

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Re: Proposal - second stage recovery
« Reply #2 on: 03/18/2015 06:19 pm »

OTOH, mid-air helicopter recovery which was practically impossible for the first stage, becomes almost natural for the second stage:


How do you know it is light enough to be mid air recovered?

An empty second stage?  I'd put it at around 5 tons.  Maybe 6 after modifications.  Heavy helicopters can lift well over 10 tons.

Everything is in your favor: you're operating near sea-level, humid air, short flight...  Since you control the timing, you can avoid the hottest part of the day, or just bad weather.   You need to have the second stage stay alive until de-orbit time, but that's not very hard to do.
« Last Edit: 03/18/2015 06:20 pm by meekGee »
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Offline GreenShrike

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Re: Proposal - second stage recovery
« Reply #3 on: 03/18/2015 06:49 pm »
How do you know it is light enough to be mid air recovered?

An empty second stage?  I'd put it at around 5 tons.  Maybe 6 after modifications.  Heavy helicopters can lift well over 10 tons.

What would terminal velocity of the stage be? Do you think a helicopter could catch an object falling at that speed? Would the second stage need a parachute of some sort to be slowed enough to be successfully caught?
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Online meekGee

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Re: Proposal - second stage recovery
« Reply #4 on: 03/18/2015 06:53 pm »
How do you know it is light enough to be mid air recovered?

An empty second stage?  I'd put it at around 5 tons.  Maybe 6 after modifications.  Heavy helicopters can lift well over 10 tons.

What would terminal velocity of the stage be? Do you think a helicopter could catch an object falling at that speed? Would the second stage need a parachute of some sort to be slowed enough to be successfully caught?
Oh, parachute for sure.
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Offline TrevorMonty

Re: Proposal - second stage recovery
« Reply #5 on: 03/18/2015 07:01 pm »
The Russians had some papers on helicopter recovery, of boosters.

Offline cdleonard

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Re: Proposal - second stage recovery
« Reply #6 on: 03/18/2015 07:06 pm »
According to this page the second stage has a dry mass of ~4,900kg. I found a paper which claims that mid-air recovery could work for up to 22000lb "without any technological breakthroughs".

It's worth noting that ULA is looking at recovering the first stage engine module with a helicopter. That thing weighs ~25000lb.

A heat shield and parachute would add to the second stage mass. But it would still be easier than recovering the Atlas 5 engine pod.
« Last Edit: 03/18/2015 07:07 pm by cdleonard »

Offline spacenut

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Re: Proposal - second stage recovery
« Reply #7 on: 03/18/2015 07:14 pm »
Seems to me the 2nd stage could land on legs by parachute in the desert somewhere.  Then be picked up via a large helicopter or if it had enough fuel, land like the first stage.  If it slowed down enough, it could almost drop straight out of orbit and land like the first stage. 

Online meekGee

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Re: Proposal - second stage recovery
« Reply #8 on: 03/18/2015 08:47 pm »
Seems to me the 2nd stage could land on legs by parachute in the desert somewhere.  Then be picked up via a large helicopter or if it had enough fuel, land like the first stage.  If it slowed down enough, it could almost drop straight out of orbit and land like the first stage.

The problem with landing "like the first stage" is that the existing motor can't be used, so you need a new motor, or more likely a set of them.

Touching down with a parachute always has horizontal velocity, so it's hard to do it on legs.  Not impossible, but not straight forward.

Once you're on parachute though, air-capture is very gentle, and the stage never interacts with the ground or water.  It's back on base within minutes, and you're done.  For all practical purposes, it's RTLS.
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Offline groundbound

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Re: Proposal - second stage recovery
« Reply #9 on: 03/18/2015 09:32 pm »
Seems to me the 2nd stage could land on legs by parachute in the desert somewhere.  Then be picked up via a large helicopter or if it had enough fuel, land like the first stage.  If it slowed down enough, it could almost drop straight out of orbit and land like the first stage.

The problem with landing "like the first stage" is that the existing motor can't be used, so you need a new motor, or more likely a set of them.

Touching down with a parachute always has horizontal velocity, so it's hard to do it on legs.  Not impossible, but not straight forward.

Once you're on parachute though, air-capture is very gentle, and the stage never interacts with the ground or water.  It's back on base within minutes, and you're done. For all practical purposes, it's RTLS.

Is that assuming once-around?

Online meekGee

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Re: Proposal - second stage recovery
« Reply #10 on: 03/18/2015 10:32 pm »
Seems to me the 2nd stage could land on legs by parachute in the desert somewhere.  Then be picked up via a large helicopter or if it had enough fuel, land like the first stage.  If it slowed down enough, it could almost drop straight out of orbit and land like the first stage.

The problem with landing "like the first stage" is that the existing motor can't be used, so you need a new motor, or more likely a set of them.

Touching down with a parachute always has horizontal velocity, so it's hard to do it on legs.  Not impossible, but not straight forward.

Once you're on parachute though, air-capture is very gentle, and the stage never interacts with the ground or water.  It's back on base within minutes, and you're done. For all practical purposes, it's RTLS.

Is that assuming once-around?
Several-around.  The second stage is assumed to be alive, and with very little impulse you can get the ground track to overfly the launch site.  Whether you want it back in 90 minutes or 6 hours is up to you.

By "minutes" I meant from reentry to landing.
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Offline Jim

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Re: Proposal - second stage recovery
« Reply #11 on: 03/18/2015 11:24 pm »

Several-around.  The second stage is assumed to be alive, and with very little impulse you can get the ground track to overfly the launch site.  Whether you want it back in 90 minutes or 6 hours is up to you.



Quite wrong.  90 minutes is not doable, especially for higher inclinations at low altitudes.  Takes a very lot of impulse.

And ones in GTO are going to take days to get in the right phasing for return to launch site.
« Last Edit: 03/18/2015 11:27 pm by Jim »

Online meekGee

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Re: Proposal - second stage recovery
« Reply #12 on: 03/18/2015 11:36 pm »

Several-around.  The second stage is assumed to be alive, and with very little impulse you can get the ground track to overfly the launch site.  Whether you want it back in 90 minutes or 6 hours is up to you.



Quite wrong.  90 minutes is not doable, especially for higher inclinations at low altitudes.  Takes a very lot of impulse.

And ones in GTO are going to take days to get in the right phasing for return to launch site.
A very rough estimate:

90 minutes is 5400 seconds.  If you gave the stage 100 m/s, it will have shifted by 540 km.  This is good for small angle approximations, but you get the drift.

If you're willing to wait two revolutions, you need half the impulse.  So It's up to you to decide how much you want to spend.  I doubt it will make sense to try to get it back after one LEO orbit, nor do I see a motivation to do so, but if someone really wanted to, they have a merlin to do it with.

For a GTO trajectory, each revolution takes much longer, and so there's more time for your correction to have an effect.

My guess is that they will want it back within a day or two, to keep the size of the fleet down...

* if and when they start reusing the second stages
« Last Edit: 03/18/2015 11:40 pm by meekGee »
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Offline Jim

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Re: Proposal - second stage recovery
« Reply #13 on: 03/18/2015 11:44 pm »
1.  A very rough estimate:


2. For a GTO trajectory, each revolution takes much longer, and so there's more time for your correction to have an effect.


1.  Not even close.  Changing inclination takes much more delta V

2. No, after the first orbit, the perigee over Africa.

Online meekGee

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Re: Proposal - second stage recovery
« Reply #14 on: 03/18/2015 11:51 pm »
1.  A very rough estimate:


2. For a GTO trajectory, each revolution takes much longer, and so there's more time for your correction to have an effect.


1.  Not even close.  Changing inclination takes much more delta V

2. No, after the first orbit, the perigee over Africa.
1. This is where you need to apply more rigorous methodologies and put up better numbers.

2. Yes, that's true.  But again, it should be trivial to simulate the effect of 100 m/s on the location of the perigee one orbit later.
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Offline Burninate

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Re: Proposal - second stage recovery
« Reply #15 on: 03/18/2015 11:55 pm »
If they conceded a small orbital upper stage for BLEO missions, it would provide them with the M1Dvac-carrying upper stage in a standardized LEO altitude after every mission, and the mass added to permit reentry wouldn't sting quite so much.

After that, a mild parachute & helicopter recovery is half of the solution.  The other half is reentry.  SSRP is probably not an acceptable solution here, given there's only one engine - though if that could be engineered, even at low Isp, it would be a great achievement.

Some combination of PICA-X, HIAD, and MAC might be deployed to save the whole stage, or possibly just the engine & avionics.  I think reentering the whole vehicle nose-forward on a conventional cylindrical heatshield gliding orientation might be problematic given the center of mass will be way back.

Offline Burninate

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Re: Proposal - second stage recovery
« Reply #16 on: 03/19/2015 12:04 am »
A perfect polar launch LEO once-around will be ~22.5 degrees of longitude offset.  Assume 90 minute orbits, 16 orbits per day, 360 degrees rotation per day.

This converts to distance thusly according to latitude:
0N        2504.7   km
15N      2419.9   km
30N      2170.1      km
45N      1774.1   km
60N      1255.5      km
75N      650.3      km
90N      0      km


A perfect equatorial launch LEO once-around, on the other hand, will reach the launchsite every single orbit without effort.

LEO-28.5 will be somewhere in between.
« Last Edit: 03/19/2015 12:06 am by Burninate »

Offline Jim

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Re: Proposal - second stage recovery
« Reply #17 on: 03/19/2015 12:09 am »

1. This is where you need to apply more rigorous methodologies and put up better numbers.

2. Yes, that's true.  But again, it should be trivial to simulate the effect of 100 m/s on the location of the perigee one orbit later.

1.  Not when totally nonsensical and plainly wrong methodologies are used.

2. 100 m/s is not going to change a GTO enough to matter, because it will be over Asia one orbit later, and then the Pacific and then in the same hemisphere as the launch site 24 hours after launch.

Online meekGee

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Re: Proposal - second stage recovery
« Reply #18 on: 03/19/2015 12:19 am »

1. This is where you need to apply more rigorous methodologies and put up better numbers.

2. Yes, that's true.  But again, it should be trivial to simulate the effect of 100 m/s on the location of the perigee one orbit later.

1.  Not when totally nonsensical and plainly wrong methodologies are used.

2. 100 m/s is not going to change a GTO enough to matter, because it will be over Asia one orbit later, and then the Pacific and then in the same hemisphere as the launch site 24 hours after launch.
Small angle approximations might be more accurate (or less inaccurate, rather) than you think.

You seem confident, you should be able to come up with something more rigorous, no?
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Offline Burninate

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Re: Proposal - second stage recovery
« Reply #19 on: 03/19/2015 12:25 am »
Rather than *once again* breaking rules 1, 3, and 6 of the NSF forums, going on for half a day of nonsensical flaming between Jim and $whoever'sturnitistoday, and getting the thread locked or deleted with the question unanswered... how about we bring a little math into this and not rely on Jim presenting compelling, articulate proofs of his arguments in a pleasant nonconfrontational tone?

Plane change maneuvers cost:
Delta V = 2*V*sin((change in incl)/2)

That means a 2 degree change in inclination at 300km altitude (about 7730.3m/s orbital velocity) costs around 2 * 7730.3 * 0.017452 = 269.8m/s.

So the next question is: How many degrees inclination change, and when, is required to rendezvous with KSC's coordinates after one orbit?
« Last Edit: 03/19/2015 01:21 am by Burninate »

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