Author Topic: F9 Second Stage Reusability  (Read 139605 times)

Offline Lars-J

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Re: F9 Second Stage Reusability
« Reply #500 on: 06/07/2017 07:56 PM »
One thing that occurred to me is a third stage would likely be necessary to be able to consider reusing the second stage on GTO missions.

LEO reentry is hard enough but a direct reentry from reentry high energy orbit would be much more difficult.

More difficulty yes, but not THAT much more.

Online IanThePineapple

Re: F9 Second Stage Reusability
« Reply #501 on: 06/07/2017 08:15 PM »
One thing that occurred to me is a third stage would likely be necessary to be able to consider reusing the second stage on GTO missions.

LEO reentry is hard enough but a direct reentry from reentry high energy orbit would be much more difficult.

More difficulty yes, but not THAT much more.

It would also be pretty easy to just burn at apogee, if of course there's enough fuel left that far into the flight (also if there's enough left to land and to account for boiloff).
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Offline hkultala

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Re: F9 Second Stage Reusability
« Reply #502 on: 06/07/2017 08:19 PM »
One thing that occurred to me is a third stage would likely be necessary to be able to consider reusing the second stage on GTO missions.

LEO reentry is hard enough but a direct reentry from reentry high energy orbit would be much more difficult.

Not really.

A good heat shield is needed anyway for LEO re-entry, and making that heat shield slightly heavier to survive the higher velocity of re-entry from GTO is not a big deal.

And for delta-v perspective: Not much more delta-v is required. The pegiree is what matters for re-entry delta-v, and that is very low also in GTO. Just very slight burn in apogee makes the pegiree drop to low enough that atmosphere does the rest.
« Last Edit: 06/07/2017 08:20 PM by hkultala »

Offline Space Ghost 1962

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Re: F9 Second Stage Reusability
« Reply #503 on: 06/07/2017 09:05 PM »
The perigee is what matters for re-entry delta-v, and that is very low also in GTO. Just very slight burn in apogee makes the perigee drop to low enough that atmosphere does the rest.

Also critically important is where that perigee will occur, for the cost/possibility of recovery/reuse and flight frequency. Which is not so easy given orbital dynamics, for efficient GTO.

Offline Lar

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Re: F9 Second Stage Reusability
« Reply #504 on: 06/07/2017 09:10 PM »
One thing that occurred to me is a third stage would likely be necessary to be able to consider reusing the second stage on GTO missions.

LEO reentry is hard enough but a direct reentry from reentry high energy orbit would be much more difficult.


Creating a third stage so that you can recover the second stage misses the point I think? The idea is to reuse everything possible (the trunk is tossed, some rings are tossed but minimize the disposables/consumables other than fluids...)
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Offline Dao Angkan

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Re: F9 Second Stage Reusability
« Reply #505 on: 06/07/2017 10:12 PM »
One thing that occurred to me is a third stage would likely be necessary to be able to consider reusing the second stage on GTO missions.

LEO reentry is hard enough but a direct reentry from reentry high energy orbit would be much more difficult.


Creating a third stage so that you can recover the second stage misses the point I think? The idea is to reuse everything possible (the trunk is tossed, some rings are tossed but minimize the disposables/consumables other than fluids...)

How much spare mass does Dragon usually come down with? I think that some of the trunk could possibly be brought back down in Dragon (maybe solar panels?). Anyway, I think that stage 2 recovery would probably be based on their Dragon recovery expertise.

Offline DOCinCT

Re: F9 Second Stage Reusability
« Reply #506 on: 06/08/2017 08:25 PM »
One thing that occurred to me is a third stage would likely be necessary to be able to consider reusing the second stage on GTO missions.

LEO reentry is hard enough but a direct reentry from reentry high energy orbit would be much more difficult.


Creating a third stage so that you can recover the second stage misses the point I think? The idea is to reuse everything possible (the trunk is tossed, some rings are tossed but minimize the disposables/consumables other than fluids...)

How much spare mass does Dragon usually come down with? I think that some of the trunk could possibly be brought back down in Dragon (maybe solar panels?). Anyway, I think that stage 2 recovery would probably be based on their Dragon recovery expertise.
Maximum of 6,600 lbs, but less due to volume considerations.  Trunk is always discarded' to return the solar panels would require a space walk(s) and opening the hatch in a vacuum. 

I would think they have to decide which end to protect from heating, PAF or Merlin.

Offline KelvinZero

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Re: F9 Second Stage Reusability
« Reply #507 on: 06/08/2017 09:26 PM »
Creating a third stage so that you can recover the second stage misses the point I think?
barely on topic note: electric third stages, and reusable tugs that stay in orbit. Obviously that is not a slam dunk. Really just saying here is a wider topic, outside this thread.

Offline mikelepage

Re: F9 Second Stage Reusability
« Reply #508 on: 06/12/2017 08:34 AM »
Suppose the second stage uses up all remaining RP-1 on the entry burn, killing as much velocity as possible just as it's reentering the atmosphere.

At this point, it would still have fully a pressurised LOX tank, right? (I'm assuming LOX is always in excess to RP-1)  Also, you still have Helium in COPVs inside the LOX tank, right?

I haven't seen it suggested anywhere: I'm wondering if this excess gas would be what is used to inflate a bouncy castle/life raft/airbag, perhaps with only minor mods to the LOX pumps?  Include an extra outlet that would be used to pump that (pre-warmed) gas into an inflatable structure once the vehicle is subsonic.  That way, the only extra mass to stage 2 would be the inflatable structure itself (plus whatever drogue chute arrangement is chosen to take the craft subsonic).

Online john smith 19

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Re: F9 Second Stage Reusability
« Reply #509 on: 06/12/2017 08:42 PM »
At this point, it would still have fully a pressurised LOX tank, right? (I'm assuming LOX is always in excess to RP-1)  Also, you still have Helium in COPVs inside the LOX tank, right?
SOP is to exhaust the heaviest propellant first, which is normally LOX, it also ensures the engine won't run LOX rich, which will normally damage the engine, unless it's been designed for it.  :(
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Offline mikelepage

Re: F9 Second Stage Reusability
« Reply #510 on: 06/13/2017 04:54 AM »
At this point, it would still have fully a pressurised LOX tank, right? (I'm assuming LOX is always in excess to RP-1)  Also, you still have Helium in COPVs inside the LOX tank, right?
SOP is to exhaust the heaviest propellant first, which is normally LOX, it also ensures the engine won't run LOX rich, which will normally damage the engine, unless it's been designed for it.  :(

Ah okay, that makes sense.  But don't you at least have the LOX tank full of pressurised Helium?  Could that be released in ordered to inflate some structure?

Online john smith 19

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Re: F9 Second Stage Reusability
« Reply #511 on: 06/14/2017 07:40 PM »
At this point, it would still have fully a pressurised LOX tank, right? (I'm assuming LOX is always in excess to RP-1)  Also, you still have Helium in COPVs inside the LOX tank, right?
SOP is to exhaust the heaviest propellant first, which is normally LOX, it also ensures the engine won't run LOX rich, which will normally damage the engine, unless it's been designed for it.  :(

Ah okay, that makes sense.  But don't you at least have the LOX tank full of pressurised Helium?  Could that be released in ordered to inflate some structure?
Well assuming the tank pressure is 2x or 3x atmospheric pressure you could use the excess to fill something provided you did not need it to resist bending and other loads (F9 is a "semi pressure stabilized structure"). The questions would be a) At this stage in the flight are the loads low enough to not need that stiffening and b)What would you fill with the "excess" Helium? and c) How would you deploy this structure?
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Offline mikelepage

Re: F9 Second Stage Reusability
« Reply #512 on: 06/18/2017 05:42 AM »
Well assuming the tank pressure is 2x or 3x atmospheric pressure you could use the excess to fill something provided you did not need it to resist bending and other loads (F9 is a "semi pressure stabilized structure"). The questions would be a) At this stage in the flight are the loads low enough to not need that stiffening and b)What would you fill with the "excess" Helium? and c) How would you deploy this structure?

There's probably not that much "excess" gas is there?  ::) and crashing from terminal velocity (even if subsonic) would need one heck of an airbag.  If you have to perform a landing burn, the heat coming off the engine would probably destroy any inflatable.  And even if it didn't, you'll get seawater corrosion.  On second thoughts, if they're going to try it at all, I imagine it would have to be some combination of what they've already done with the F9 booster and what is planned for BFR.

Better idea (?): it should be easy enough to create a "cradle" for the second stage that could be temporarily installed on a the Vandenberg ASDS for Falcon Heavy launches from the Cape.  It would catch the second stage using the same attachments that the booster stage uses.  If you then install a pair/quartet of grid fins on any second stage where you have the margin, you could then practice cradle landings on a smaller scale.

From a bit of googling, I can see the F9 landing legs (collectively) are estimated at just under 2100kg, where as the grid fins are estimated at only 41kg each.  Even if that's off by an order of magnitude, it seems far more feasible to use grid fins on the second stage than landing legs.  A grid fin plus cradle arrangement seems like a likely "hail mary" attempt that they could perform on the Falcon heavy demo.

Offline Peter.Colin

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Re: F9 Second Stage Reusability
« Reply #513 on: 07/09/2017 01:27 PM »
The main problem with re-using the second stage in a similar way as the first stage, is that it compromises the payload to much.
The only way I see you could really deal with this fundamentally is to re-fuel the second stage in orbit.

Offline meekGee

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Re: F9 Second Stage Reusability
« Reply #514 on: 07/09/2017 04:43 PM »
The main problem with re-using the second stage in a similar way as the first stage, is that it compromises the payload to much.
The only way I see you could really deal with this fundamentally is to re-fuel the second stage in orbit.

Lets put numbers to that.

1. What is F9b5's payload mass to LEO?
2. How do you think does the lighter variant (heat shield and mid-air parachute recovery) weigh?

right now:
#1 on the web site is advertised as 22 tons (so that's a minimum)

For #2, we know the dry mass of S2 is <5 tons.  Let's say a recoverable S2 is +3 tons, split between heat shield, parachute, and structural reinforcements.

So you go down from 22 tons to 19 tons.

If the recovery hardware ended up doubling the weight of S2, payload would go down from 22 to 17 tons.

Remember that SpaceX can always manufacture the expendeble S2 for missions that require it.

Also, remember that LEO missions can be volume constrained, so the mass is free.

---

Things look a lot different for GEO, where the high energy means payload is in the 6-7 ton range, and so suddenly the 3 ton payload hit it very expensive, and a 5 tons hit is basically a non-starter.

---

So my conclusion is that for GEO launches, S2 reusability is not viable, but once frequent LEO launches come into play, S2 reusability on F9 is very viable.

So what will come first?  Lots of LEO launches, or F9's retirement?
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Offline hkultala

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Re: F9 Second Stage Reusability
« Reply #515 on: 07/11/2017 08:58 AM »
The perigee is what matters for re-entry delta-v, and that is very low also in GTO. Just very slight burn in apogee makes the perigee drop to low enough that atmosphere does the rest.

Also critically important is where that perigee will occur, for the cost/possibility of recovery/reuse and flight frequency. Which is not so easy given orbital dynamics, for efficient GTO.

No need to fly the same stage 2 frequently. Use many upper stages, and wait enough orbits that pegiree is at good position at sea and park a barge there.

The pegiree will anyway be considerably southern than cape, so easiest place probably woud be at pacific, somewhere at near south coast of mexico/something like 1000-1500km south from LA. However, it would take many days for the barge to travel between port and the landing site.

"Just Read The Instructions" would normally be much less utilized than "Of Course I Still Love You" so it would get more work, and spending many days travelling betwen port and landing site would not be so bad. Though this may change when they start launching the big LEO satellite constellations to polar orbits, so they might need additional barges then.

« Last Edit: 07/11/2017 09:04 AM by hkultala »

Online envy887

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Re: F9 Second Stage Reusability
« Reply #516 on: 07/18/2017 05:33 PM »
Envy. You forgot the fact that it delays the development of the next generation vehicle. You also forgot that a reusable second stage might just not work. I would think it's bad investment. But we are again on the wrong thread for this discussion and should defer to not continuing.

That's only relevant if the new vehicle reduces operating costs, and that is what I described as the " opportunity cost of the delay".

A reuse solution specific to the constellation would be worthwhile IMO, if it minimized development costs with mostly existing hardware.

Delaying the next vehicle goes against Musk's Mars goal.  He has only so many years of life to achieve his colony dream. BIG opportunity cost.
And the opportunity cost analysis does not allow for the loss of payload mass, i.e. number of satellites per launch that a Falcon class re-useable upper stage would penalize.  So the cost of Falcon class re-useability is higher than stated.

The lost payload mass could be irrelevant if F9 is volume limited and the recovery hardware takes up volume not otherwise available for payload. And there may be additional capital cost to manufacture an additional 15 upper stages per year.

Delaying the Mars vehicle is an issue. But getting the constellation up faster, cheaper, and earning more revenue for the Mars vehicle could actually accelerate its development.

Posting from here: https://forum.nasaspaceflight.com/index.php?topic=36552.msg1703515
« Last Edit: 07/18/2017 05:50 PM by envy887 »

Offline meekGee

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Re: F9 Second Stage Reusability
« Reply #517 on: 07/18/2017 08:07 PM »
SpaceX is going through a lot of effort to catch a bloody fairing, since it's a "pallet full of cash".

With rapid reuse of S1, S2 becomes a very large pallet of cash - the largest such pallet on the rocket.

For LEO launch, even doubling the mass of S2 is perfectly acceptable.

The constellation will require tens of identical LEO launches per year.

Why would they not bother?  Especially when this constellation is what is financing the Mars drive?

The only reason is if they think that a new vehicle will take over completely, and then why bother.  But that would be good news from my perspective...
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Online wannamoonbase

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Re: F9 Second Stage Reusability
« Reply #518 on: 07/18/2017 08:57 PM »
SpaceX is going through a lot of effort to catch a bloody fairing, since it's a "pallet full of cash".

With rapid reuse of S1, S2 becomes a very large pallet of cash - the largest such pallet on the rocket.

For LEO launch, even doubling the mass of S2 is perfectly acceptable.

The constellation will require tens of identical LEO launches per year.

Why would they not bother?  Especially when this constellation is what is financing the Mars drive?

The only reason is if they think that a new vehicle will take over completely, and then why bother.  But that would be good news from my perspective...

Concur with analysis.

I think there is a better than 50/50 chance we see them attempt to develop the technology.
« Last Edit: 07/18/2017 08:58 PM by wannamoonbase »
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Offline Semmel

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Re: F9 Second Stage Reusability
« Reply #519 on: 07/18/2017 09:04 PM »
Envy, thanks for pulling it over here, I was on mobile and couldn't do it easily (and in the time I had when posting).

Envy. You forgot the fact that it delays the development of the next generation vehicle. You also forgot that a reusable second stage might just not work. I would think it's bad investment. But we are again on the wrong thread for this discussion and should defer to not continuing.

That's only relevant if the new vehicle reduces operating costs, and that is what I described as the " opportunity cost of the delay".

A reuse solution specific to the constellation would be worthwhile IMO, if it minimized development costs with mostly existing hardware.

Ohh ok, I missed that connection. Its a very hard factor to quantify but also quite a relevant one. We simply dont have the insight to know this one way or the other.


Delaying the next vehicle goes against Musk's Mars goal.  He has only so many years of life to achieve his colony dream. BIG opportunity cost.
And the opportunity cost analysis does not allow for the loss of payload mass, i.e. number of satellites per launch that a Falcon class re-useable upper stage would penalize.  So the cost of Falcon class re-useability is higher than stated.

The lost payload mass could be irrelevant if F9 is volume limited and the recovery hardware takes up volume not otherwise available for payload. And there may be additional capital cost to manufacture an additional 15 upper stages per year.

Delaying the Mars vehicle is an issue. But getting the constellation up faster, cheaper, and earning more revenue for the Mars vehicle could actually accelerate its development.

Posting from here: https://forum.nasaspaceflight.com/index.php?topic=36552.msg1703515

SpaceX is going through a lot of effort to catch a bloody fairing, since it's a "pallet full of cash".

With rapid reuse of S1, S2 becomes a very large pallet of cash - the largest such pallet on the rocket.

For LEO launch, even doubling the mass of S2 is perfectly acceptable.

The constellation will require tens of identical LEO launches per year.

Why would they not bother?  Especially when this constellation is what is financing the Mars drive?

The only reason is if they think that a new vehicle will take over completely, and then why bother.  But that would be good news from my perspective...

To both of you (which issue basically the same argument):
Delaying the (subscale) BFR/BFS is a very strong argument against any more tinkering on F9. Shotwell said on many occasions that the team, working on F9, FH and D2 are going to work on BFR once their current work is done. They are clearly manpower limited as well as cash limited. I think, without the development of BFR looming, your arguments are perfectly sensible and would make a good argument for evolving F9 to be fully reusable. With BFR though, I think its not worth doing.

The reason is, that (subscale) BFR/BFS might launch in 4 years. Lets say that tinkering with F9 brings it up to 5 years, thats one year of full steam operation of F9 (and arguably, around 100 launches per year) that are on F9 instead of (subscale) BFR. Even if in these 4 years, F9 becomes fully reusable for LEO missions, that is still a very substantial number of launches that are not reusable.

Also, S2 will not be reusable the instant they try the development. Like on S1, it may take a year or two to get right. So you have to expend quite a lot of S2s anyway. And with the added delay to BFR/BFS, this becomes not worthwile very fast.

Does a reusable S2 accelerate the deployment of the constellation? I dont think so. F9 will more likely be launch pad limited instead of production limited once the production of S1 is reduced in favour of S2. The cadence on A39 shows that the 2 weeks turnaround is required for satellite processing. To get something like 50 launches a year, things have to move towards more parallel processing of payloads. There is also the range that can not support that many launches at moment. There are many many things that need to change in order to get the launch cadence for the constellation and I fear that the least of the problem is the production of S2 (non-reusable).

It seems that a larger vehicle, that can launch more sats at once is advantageous to many smaller launches on more than just launch cost kind of ways. It can also cope much better with weather delays and technical delays.


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