Author Topic: F9 Second Stage Reusability  (Read 66504 times)

Offline meekGee

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F9 Second Stage Reusability
« on: 03/31/2017 12:55 AM »
Biggest news of the day, other than that bit about the flight-proven stage being reproven, is that "Next thing is Second Stage reusability".

This should have its own thread
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Offline meekGee

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Re: F9 Second Stage Reusability
« Reply #1 on: 03/31/2017 12:59 AM »
To me, the main argument against second stage reusability was that most launches are commercial GTO launches, and this makes recovering the second stage very difficult.

But as the BFC became real, and now the vBFC tripled the number of LEO/vLEO satellites that need to be launched (12,000), we're looking at LEO launch rates of several per week.

It is inconceivable that they'll throw away second stages at that rate.

It is also such a repeating problem that I can't imagine it won't get its own custom engineering solution.

So my prediction is either an integrated S2/deployer, or a reusable S2 with an expendable deployer/cover combo that does not require a full-scope fairing.

EDIT:

I should add, I think landing vertically is not the right approach for S2.

Since it can orbit multiple times, it takes negligible dV to make sure the deorbit pass is near the launch site.  Then deorbit, re-entry (possibly ballistic, since there's nobody on board), parachute deployment, and helicopter capture.

All the factors that make helicopter capture difficult for the fairing (range, duration, etc) are non-issues with S2.
« Last Edit: 03/31/2017 01:32 AM by meekGee »
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Offline Bubbinski

Re: F9 Second Stage Reusability
« Reply #2 on: 03/31/2017 01:23 AM »
That is big news indeed, that they will try for 2nd stage recovery.

I recall the video shown a few years back where the first stage landed, the 2nd stage deployed a Dragon, then deorbited with heat shielding material covering part of the stage, then the 2nd stage landing on a land landing pad. Would SpaceX still plan it this way, or have they come up with something else in the intervening years? If they do land the 2nd stage would they need extra barges, or try to land at a landing pad somewhere away from populated areas (like say at Vandenberg AFB)?

Edited to add: how much would second stage recovery eat into the payload mass? Would SX have to use Falcon Heavy as its "go to" launcher for most payloads because of this?
« Last Edit: 03/31/2017 01:25 AM by Bubbinski »
I'll even excitedly look forward to "flags and footprints" and suborbital missions. Just fly...somewhere.

Offline Lar

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Re: F9 Second Stage Reusability
« Reply #3 on: 03/31/2017 01:30 AM »
That is big news indeed, that they will try for 2nd stage recovery.

I recall the video shown a few years back where the first stage landed, the 2nd stage deployed a Dragon, then deorbited with heat shielding material covering part of the stage, then the 2nd stage landing on a land landing pad. Would SpaceX still plan it this way, or have they come up with something else in the intervening years? If they do land the 2nd stage would they need extra barges, or try to land at a landing pad somewhere away from populated areas (like say at Vandenberg AFB)?

Edited to add: how much would second stage recovery eat into the payload mass? Would SX have to use Falcon Heavy as its "go to" launcher for most payloads because of this?

I seem to recall that
10 Kg of recovery gear on S1 costs 1 Kg of payload,
N (>1, <10) Kg of recovery gear on the fairing costs 1 Kg of payload
1Kg of recovery gear on S2 costs 1 Kg of payload

(gear is anything other than fuel, fuel is a slightly different calculation)

but I can't remember where I saw this or who did the analysis or if 10 is the right number or what N is....  And that these were rough/typical, that the numbers differ for LEO and GEO...
« Last Edit: 03/31/2017 01:31 AM by Lar »
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Offline Nomadd

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Re: F9 Second Stage Reusability
« Reply #4 on: 03/31/2017 01:46 AM »
That is big news indeed, that they will try for 2nd stage recovery.

I recall the video shown a few years back where the first stage landed, the 2nd stage deployed a Dragon, then deorbited with heat shielding material covering part of the stage, then the 2nd stage landing on a land landing pad. Would SpaceX still plan it this way, or have they come up with something else in the intervening years? If they do land the 2nd stage would they need extra barges, or try to land at a landing pad somewhere away from populated areas (like say at Vandenberg AFB)?

Edited to add: how much would second stage recovery eat into the payload mass? Would SX have to use Falcon Heavy as its "go to" launcher for most payloads because of this?

I seem to recall that
10 Kg of recovery gear on S1 costs 1 Kg of payload,
N (>1, <10) Kg of recovery gear on the fairing costs 1 Kg of payload
1Kg of recovery gear on S2 costs 1 Kg of payload

(gear is anything other than fuel, fuel is a slightly different calculation)

but I can't remember where I saw this or who did the analysis or if 10 is the right number or what N is....  And that these were rough/typical, that the numbers differ for LEO and GEO...
The fairing number should be close to the S1 number since it's jettisoned soon after staging.
« Last Edit: 03/31/2017 01:47 AM by Nomadd »

Offline ClayJar

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Re: F9 Second Stage Reusability
« Reply #5 on: 03/31/2017 02:49 AM »
Since this is a new topic on second stage reusability, I'm copying my reply from the other thread so people can easily find the a copy of the presser and the particular part at issue, which is right after he talks about the fairing (after a guy walks in and shows him the photo):



Quote from: Elon Musk from the presser at 14:23
But then the only thing left is the upper stage, which we didn't originally intend for Falcon 9 to have a reusable upper stage, but it might be fun to try like a hail mary, and you know.  What's the worst thing that can happen?  It blows up.  You know, it blows up anyway. [Martin Halliway chimes in humorously. "We need to discuss this."]


Edit/Lar: see also http://forum.nasaspaceflight.com/index.php?topic=42365.msg1661789 "Testing upper stage reusability" which is similar but different in focus.
« Last Edit: 03/31/2017 07:52 PM by Lar »

Offline meekGee

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Re: F9 Second Stage Reusability
« Reply #6 on: 03/31/2017 03:09 AM »
Since this is a new topic on second stage reusability, I'm copying my reply from the other thread so people can easily find the a copy of the presser and the particular part at issue, which is right after he talks about the fairing (after a guy walks in and shows him the photo):



Quote from: Elon Musk from the presser at 14:23
But then the only thing left is the upper stage, which we didn't originally intend for Falcon 9 to have a reusable upper stage, but it might be fun to try like a hail mary, and you know.  What's the worst thing that can happen?  It blows up.  You know, it blows up anyway. [Martin Halliway chimes in humorously. "We need to discuss this."]

Yeah, that doesn't sound like "second stage reuse is next".

Still though.

That "hail Mary" would still require (at a minimum) a heat shield, and some ways of keeping the stage stable.

But if it can be done, even with a significant penalty, then now the door is open to getting S2 back on missions that do not max-out the F9.  And that's a lot of missions.

It might even make sense to go from RTLS to Barge landing, if the extra mass means you get S2 back.

Heh - you'll get S2 back before you get S1 back...
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Offline Req

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Re: F9 Second Stage Reusability
« Reply #7 on: 03/31/2017 03:26 AM »
I should add, I think landing vertically is not the right approach for S2.

Since it can orbit multiple times, it takes negligible dV to make sure the deorbit pass is near the launch site.  Then deorbit, re-entry (possibly ballistic, since there's nobody on board), parachute deployment, and helicopter capture.

All the factors that make helicopter capture difficult for the fairing (range, duration, etc) are non-issues with S2.

I had this thought as well after the "hail mary" comment.  "Hail mary" doesn't sound like the type of commitment it would take to add legs, deal with the nozzle ratio somehow, perform a flip maneuver, and perform a propulsive landing as in the old video, and definitely rules out a raptor S2.  A natural extension of the fairing recovery system("bouncy castle" seems more likely than helicopter to me though) does seem to fit the bill in my mind.  But I remember a discussion here which concluded that the first stage is nowhere near terminal velocity by the time it performs it's landing burn, something like mach 2-3 IIRC.  I wonder what that figure would be for a second stage that only uses TPS to re-enter, and whether that would preclude parachute landings.  I know drogues can help with this, but does it work out?  As it is there is surprisingly little time between S1's entry burn and it's landing burn on these GTO missions, if that window gets even shorter for a faster S2 then I for one would be pretty impressed if they could get chutes deployed and bleed it all off.

Edit:  Having thought about this a bit more, I realized that the second stage could come in at a much more shallow angle, so perhaps there's no need to speculate about this.
« Last Edit: 03/31/2017 03:58 AM by Req »

Offline meekGee

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Re: F9 Second Stage Reusability
« Reply #8 on: 03/31/2017 05:13 AM »
I should add, I think landing vertically is not the right approach for S2.

Since it can orbit multiple times, it takes negligible dV to make sure the deorbit pass is near the launch site.  Then deorbit, re-entry (possibly ballistic, since there's nobody on board), parachute deployment, and helicopter capture.

All the factors that make helicopter capture difficult for the fairing (range, duration, etc) are non-issues with S2.

I had this thought as well after the "hail mary" comment.  "Hail mary" doesn't sound like the type of commitment it would take to add legs, deal with the nozzle ratio somehow, perform a flip maneuver, and perform a propulsive landing as in the old video, and definitely rules out a raptor S2.  A natural extension of the fairing recovery system("bouncy castle" seems more likely than helicopter to me though) does seem to fit the bill in my mind.  But I remember a discussion here which concluded that the first stage is nowhere near terminal velocity by the time it performs it's landing burn, something like mach 2-3 IIRC.  I wonder what that figure would be for a second stage that only uses TPS to re-enter, and whether that would preclude parachute landings.  I know drogues can help with this, but does it work out?  As it is there is surprisingly little time between S1's entry burn and it's landing burn on these GTO missions, if that window gets even shorter for a faster S2 then I for one would be pretty impressed if they could get chutes deployed and bleed it all off.

Edit:  Having thought about this a bit more, I realized that the second stage could come in at a much more shallow angle, so perhaps there's no need to speculate about this.

S2, if you're standing far enough away, is a lot closer to Dragon than it is to S1.

(Mass, aspect ratio, etc).

The c.g. is off, but adding a Dragon-quality heat shield, and a parachute pack, will help move it forward somewhat.

S2 already has some maneuvering capacity which might help it with reentry stabilization.

This to me sounds like a "minimum effort".

---

Also "bouncy castle" sound like a classic code word.  :)
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Offline Rocket Surgeon

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Re: F9 Second Stage Reusability
« Reply #9 on: 03/31/2017 05:23 AM »
For me, the biggest issue with second stage reuse isn't so much the hardware to get it through the atmosphere and land it, it's the fuel mass to de-orbit it, especially from GTO.

From Bob Zubrin, a rough rule of thumb for a heat shield mass is 15% of the dry mass of an object (though I think PicaX does better) and from some quick googling, the US military uses guided para-foils to land air dropped 5t payloads that weigh about 500 kg themselves with accuracies of 50m or better. Therefore, one could potentially have the hardware to get a second stage through the atmosphere and descending slowly by parachute for 1-2 tonnes. Then just grab in midair or land on a bouncy castle.

However, to deorbit a stage on a GTO with the M1D+ Vac, you would need approximately 2 tonnes of fuel based on some very basic calcs I've done. Essentially doubling the mass you need to recover it. So there goes at least 4t in total from your GTO mass....how much was SES-10? 5.5t?

Unless the ISP of the Upper Stage can be boosted magically, successful GTO reuse is, I think, off the table. LEO would work MUCH better or, in the case of their SatNet, MEO.

Does anyone have accurate numbers on what it would take to deorbit the Upper Stage from a GTO? I think that's a starting point. Then we can worry about heat shields and mid-air bouncy castles, etc.


Offline ppb

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Re: F9 Second Stage Reusability
« Reply #10 on: 03/31/2017 06:07 AM »
If they can reenter from LEO they can do it without much more weight penalty from GTO.  How?  Aerocapture.  We've been doing it already on many missions at Mars with flimsy satellites.  S2 with a nice sturdy heatshield wouldn't take many passes at perigee before the reentry would be the same as LEO. But, but, but this is much different you say than the Mars application.  I say the SpaceX engineers will find a way to make it work.
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Offline Rocket Surgeon

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Re: F9 Second Stage Reusability
« Reply #11 on: 03/31/2017 06:19 AM »
If they can reenter from LEO they can do it without much more weight penalty from GTO.  How?  Aerocapture.  We've been doing it already on many missions at Mars with flimsy satellites.  S2 with a nice sturdy heatshield wouldn't take many passes at perigee before the reentry would be the same as LEO. But, but, but this is much different you say than the Mars application.  I say the SpaceX engineers will find a way to make it work.

Oh I don't think it's that much different, I'm just wondering:

A) how much DeltaV it takes to lower the perigee of the GTO so that it's low enough to rapidly de orbit
B) How accurately you can do it?

570m/s delta V equates to roughly an extra 1t of fuel, which is 1t less of GTO payload at least, assuming the upper stage weighs about 4.5t (that's the number I'm using but I don't know how accurate that is) and your reentry/landing hardware is only 1t extra on top of that. GTO payload starts disappearing rapidly when you add mass to the second stage. Also it needs to be accurate to several hundred square kilometres at least. If you can't bring it down in a reliably accurate way, it's useless.

« Last Edit: 03/31/2017 06:20 AM by Rocket Surgeon »

Offline Kaputnik

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Re: F9 Second Stage Reusability
« Reply #12 on: 03/31/2017 06:32 AM »
If they can reenter from LEO they can do it without much more weight penalty from GTO.  How?  Aerocapture.  We've been doing it already on many missions at Mars with flimsy satellites.  S2 with a nice sturdy heatshield wouldn't take many passes at perigee before the reentry would be the same as LEO. But, but, but this is much different you say than the Mars application.  I say the SpaceX engineers will find a way to make it work.

Sorry to be a pedant but you mean aerobraking. Aerocapture refers to an object travelling higher than escape velocity, and is something that has never been done to date, strictly speaking.
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Offline ppb

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Re: F9 Second Stage Reusability
« Reply #13 on: 03/31/2017 06:42 AM »
If they can reenter from LEO they can do it without much more weight penalty from GTO.  How?  Aerocapture.  We've been doing it already on many missions at Mars with flimsy satellites.  S2 with a nice sturdy heatshield wouldn't take many passes at perigee before the reentry would be the same as LEO. But, but, but this is much different you say than the Mars application.  I say the SpaceX engineers will find a way to make it work.

Sorry to be a pedant but you mean aerobraking. Aerocapture refers to an object travelling higher than escape velocity, and is something that has never been done to date, strictly speaking.

Correction noted.  I'm not saying aerobraking would be easy, but I think the crack group of engineers at SpaceX has proven they're up to the task.
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Offline ppb

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Re: F9 Second Stage Reusability
« Reply #14 on: 03/31/2017 07:08 AM »
If they can reenter from LEO they can do it without much more weight penalty from GTO.  How?  Aerocapture.  We've been doing it already on many missions at Mars with flimsy satellites.  S2 with a nice sturdy heatshield wouldn't take many passes at perigee before the reentry would be the same as LEO. But, but, but this is much different you say than the Mars application.  I say the SpaceX engineers will find a way to make it work.

Oh I don't think it's that much different, I'm just wondering:

A) how much DeltaV it takes to lower the perigee of the GTO so that it's low enough to rapidly de orbit
B) How accurately you can do it?

570m/s delta V equates to roughly an extra 1t of fuel, which is 1t less of GTO payload at least, assuming the upper stage weighs about 4.5t (that's the number I'm using but I don't know how accurate that is) and your reentry/landing hardware is only 1t extra on top of that. GTO payload starts disappearing rapidly when you add mass to the second stage. Also it needs to be accurate to several hundred square kilometres at least. If you can't bring it down in a reliably accurate way, it's useless.



From my back of the envelope calculations, for a 150 x 35622 km GTO, I get an apogee speed of 1598 m/s.  Lowering perigee to 75 km requires an apogee speed of 1590 m/s, a deltaV of only 8 m/s.  That of course is a Hohmann transfer and current S2 design may make surviving that long difficult. Did you assume your delta V happens soon after satellite deploy when both it and S2 are going fast ?  That will definitely cost much more fuel there.
« Last Edit: 03/31/2017 07:16 AM by ppb »
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Offline Rocket Surgeon

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Re: F9 Second Stage Reusability
« Reply #15 on: 03/31/2017 07:19 AM »
Brillant!

That was my main question and I've never seen it answered as to how much it would take to deorbit. Every one always seemed to go straight to the 'how' and not think about 'how much payload is left?'

How much would it take to do a direct entry? And what mass ratio would that need? If that's small enough, second stage recovery using something similar to ULA's SMART could do the trick.

Offline biosehnsucht

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Re: F9 Second Stage Reusability
« Reply #16 on: 03/31/2017 07:36 AM »
How slow would it already have to move under parachutes for a helicopter to grab it?

Assuming you can get it that slow... how much slower can you get it?

Why not a jumbo bouncy castle?

And, of course... how much does all this equipment weigh vs a couple of SuperDracos and their fuel?

For TPS perhaps you can eject the remains of the payload adapter and have Pica-X underneath it, like Dragon jettisons the trunk? Trying to go nose first with the payload adapter still there seems rather difficult, it would probably melt/mangle and damage the TPS on the way in, leading to loss of vehicle.

Offline Rocket Surgeon

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Re: F9 Second Stage Reusability
« Reply #17 on: 03/31/2017 07:59 AM »
How slow would it already have to move under parachutes for a helicopter to grab it?

Assuming you can get it that slow... how much slower can you get it?

Why not a jumbo bouncy castle?

And, of course... how much does all this equipment weigh vs a couple of SuperDracos and their fuel?

For TPS perhaps you can eject the remains of the payload adapter and have Pica-X underneath it, like Dragon jettisons the trunk? Trying to go nose first with the payload adapter still there seems rather difficult, it would probably melt/mangle and damage the TPS on the way in, leading to loss of vehicle.

I think it would depend on how slow you could get it with just aero-drag, but given that the Dragon comes in fine, I think it could be slow enough. Only reason to do the aerocapture is to avoid landing gear mass and/or salt water from a sea landing.

As ULA has said, aero-capture of orbital returning, parachuted payloads has been done loads, just not necessarily for as heavy a payload... would be easy to test though.... are there any test Merlin1D vacs hanging around to make a dummy test stage?

Online douglas100

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Re: F9 Second Stage Reusability
« Reply #18 on: 03/31/2017 01:56 PM »
Agree that air capture is a very promising way to go.

In addition to a heat shield and other recovery aids, the stage also needs more endurance. It needs to become more "spacecraft like." If you wished for something close to RTLS, for some inclinations and launch azimuths you might have to wait up to 24 hours to recover from LEO. That restriction can be relaxed if you are prepared to recover at sea away from the launch site. It all depends on the urgency for refurbishment.

This is particularly true for recovery from GTO. The delta V to bring the perigee into the atmosphere is quite small, as already noted, but the perigee is always close to the equator. That means it will be difficult to recover the stage anywhere near the launch site. With a properly positioned ship and on board helicopter that is perfectly doable. But it will result in some delay in getting the stage back.

However, all that's a bit down the line. One step at a time...
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Re: F9 Second Stage Reusability
« Reply #19 on: 03/31/2017 03:03 PM »
Assuming the launch vehicle does no inclination change, is the perigee still at the equator? They could use supersynchronous insertion orbits to reduce the satellites work for inclination change.

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