Author Topic: F9 Second Stage Reusability  (Read 99514 times)

Offline uhuznaa

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Re: F9 Second Stage Reusability
« Reply #100 on: 04/02/2017 07:29 PM »
Long term we already know what SpaceX's design will be for a reusable upper stage. We have already seen it - the ITS spacecraft. Sideways reentry using body fins and movable tail flaps (much like ESA's IXV), and then vertical landing.

I would not rule out them flying some subscale version of this as an FH upper stage if they decide they need to try it at a smaller scale. But I don't consider it likely.

The ITS second stage ist not only a second stage, it is a spacecraft. Which means it has LOTS of mass forward of the tanks which means it has a very different CoG. The F9 second stage has its center of gravity very far back and will NOT reenter sideways.

To make the F9 second stage into some subscale version of the ITS craft you'd need to add quite a bit of mass in front of the tanks. Even then big thin-walled vacuum nozzle at the bottom wouldn't survive reentry and you can't land on it anyway, it has much too much thrust for that (and has the wrong expansion ratio for sea level use).

If mass isn't that important (as with a second stage for the FH which can loft much bigger payloads) you could add a heat shield, four SuperDracos, propellant tanks and legs on top of the stage. This would move the CoG far forward and you could make it reenter and land on its head. Basically make it like a stretched Dragon 2 that has the second stage propellant tanks instead of the pressure vessel and the vacuum Merlin on the top, mounted on the first stage on its head. This would probably add 2-4 tons of mass to the stage and reduce the payload by the same amount, but with FH you have quite a bit of leg room here.


Offline Ionmars

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Re: F9 Second Stage Reusability
« Reply #101 on: 04/02/2017 09:02 PM »
Interstage heat shield

Some discussions addressed aerobraking the Falcon S2 using a heatshield one side of the vehicle. Even if we can employ RCS thrusters to maintain orientation during re-entry, we would still need to protect the MD-1D engine from heat and pressure coming from the side. The most mass efficient way I could conceive for SpaceX to accomplish this is to employ the interstage ring as a heatshield. Rather than being discarded, the ring would stay with the second stage and would represent the principal mass penalty for stage reuse.  If the second stage sports a PICA-x shield on one side, then the ring would also have PICA-x on the same side. To provide protection to the engine, it would be closed off on the end like a tin can.

To employ the ring-shield, the stage would first do the re-entry burn while in space. To allow this, the ring-shield mist be hinged, like a 2-part reusable fairing (see sketch). It must open wide enough to allow the gimballed engine freedom of motion. After the burn, the "split can" would re-close during atmospheric re-entry and serve as an extended side shield as well as an end protector. Once stage velocity had dropped sufficiently to allow a second engine ignition inside the atmosphere, the split can would open again. This time it would be torn off, hopefully in a controlled manner, with no damage to the stage. Ultimately, it would be disposable, but only after serving a second purpose.
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Online RotoSequence

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Re: F9 Second Stage Reusability
« Reply #102 on: 04/02/2017 09:07 PM »
Interstage heat shield

Some discussions addressed aerobraking the Falcon S2 using a heatshield one side of the vehicle. Even if we can employ RCS thrusters to maintain orientation during re-entry, we would still need to protect the MD-1D engine from heat and pressure coming from the side. The most mass efficient way I could conceive for SpaceX to accomplish this is to employ the interstage ring as a heatshield. Rather than being discarded, the ring would stay with the second stage and would represent the principal mass penalty for stage reuse.  If the second stage sports a PICA-x shield on one side, then the ring would also have PICA-x on the same side. To provide protection to the engine, it would be closed off on the end like a tin can.

To employ the ring-shield, the stage would first do the re-entry burn while in space. To allow this, the ring-shield mist be hinged, like a 2-part reusable fairing (see sketch). It must open wide enough to allow the gimballed engine freedom of motion. After the burn, the "split can" would re-close during atmospheric re-entry and serve as an extended side shield as well as an end protector. Once stage velocity had dropped sufficiently to allow a second engine ignition inside the atmosphere, the split can would open again. This time it would be torn off, hopefully in a controlled manner, with no damage to the stage. Ultimately, it would be disposable, but only after serving a second purpose.

Might be doable... this seems like the kind of design where you want the fairing failing to close to be the primary failure mode. If that fairing doesn't open after staging, you have instant Loss of Mission unless that fairing easily breaks off by activation of the second stage engine. The whole setup is a bit risky, but it looks at least plausibly interesting.

The design would probably need to be nested within the dimensions of the existing interstage and a new pusher design would be needed. Don't mind me, I'm just "thinking out loud." Overall? I think I kind of like it.
« Last Edit: 04/02/2017 09:09 PM by RotoSequence »

Offline Ionmars

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Re: F9 Second Stage Reusability
« Reply #103 on: 04/02/2017 09:17 PM »
Interstage heat shield

Some discussions addressed aerobraking the Falcon S2 using a heatshield one side of the vehicle. Even if we can employ RCS thrusters to maintain orientation during re-entry, we would still need to protect the MD-1D engine from heat and pressure coming from the side. The most mass efficient way I could conceive for SpaceX to accomplish this is to employ the interstage ring as a heatshield. Rather than being discarded, the ring would stay with the second stage and would represent the principal mass penalty for stage reuse.  If the second stage sports a PICA-x shield on one side, then the ring would also have PICA-x on the same side. To provide protection to the engine, it would be closed off on the end like a tin can.

To employ the ring-shield, the stage would first do the re-entry burn while in space. To allow this, the ring-shield mist be hinged, like a 2-part reusable fairing (see sketch). It must open wide enough to allow the gimballed engine freedom of motion. After the burn, the "split can" would re-close during atmospheric re-entry and serve as an extended side shield as well as an end protector. Once stage velocity had dropped sufficiently to allow a second engine ignition inside the atmosphere, the split can would open again. This time it would be torn off, hopefully in a controlled manner, with no damage to the stage. Ultimately, it would be disposable, but only after serving a second purpose.

Might be doable... this seems like the kind of design where you want the fairing failing to close to be the primary failure mode. If that fairing doesn't open after staging, you have instant Loss of Mission unless that fairing easily breaks off by activation of the second stage engine. The whole setup is a bit risky, but it looks at least plausibly interesting.

The design would probably need to be nested within the dimensions of the existing interstage and a new pusher design would be needed. Don't mind me, I'm just "thinking out loud." Overall? I think I kind of like it.
Yeah. Now I'm wondering if starting the engine could blast it away safely.
Edit: Trying to think of a way to have a sloped surface inside the ring-shield so that the blast force would be unbalanced against the inside, causing it to move out and sideways.
Edit 2: Of course your breakaway hinges are needed.
« Last Edit: 04/02/2017 09:31 PM by Ionmars »
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Offline uhuznaa

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Re: F9 Second Stage Reusability
« Reply #104 on: 04/02/2017 09:33 PM »
Interstage heat shield

Some discussions addressed aerobraking the Falcon S2 using a heatshield one side of the vehicle. Even if we can employ RCS thrusters to maintain orientation during re-entry, we would still need to protect the MD-1D engine from heat and pressure coming from the side. The most mass efficient way I could conceive for SpaceX to accomplish this is to employ the interstage ring as a heatshield. Rather than being discarded, the ring would stay with the second stage and would represent the principal mass penalty for stage reuse.  If the second stage sports a PICA-x shield on one side, then the ring would also have PICA-x on the same side. To provide protection to the engine, it would be closed off on the end like a tin can.

To employ the ring-shield, the stage would first do the re-entry burn while in space. To allow this, the ring-shield mist be hinged, like a 2-part reusable fairing (see sketch). It must open wide enough to allow the gimballed engine freedom of motion. After the burn, the "split can" would re-close during atmospheric re-entry and serve as an extended side shield as well as an end protector. Once stage velocity had dropped sufficiently to allow a second engine ignition inside the atmosphere, the split can would open again. This time it would be torn off, hopefully in a controlled manner, with no damage to the stage. Ultimately, it would be disposable, but only after serving a second purpose.

But you can't employ RCS thrusters to maintain orientation during re-entry. You'd basically need to balance the mass difference between the heavy and the light end (bottom and top) by thrust. Do you want to run some 1000 lbf thrusters all the time during reentry?

And even then you would end up with a vacuum Merlin for landing, which has both much too much thrust and the wrong nozzle. And the nozzle is big which forces long (and with that wide) legs.

What you need anyway is a heat shield, propellants for landing and legs. Now put all of this on top of the stage and it will reenter head first. Now the Merlin is nicely tucked away in the plasma shadow and the only price you have to pay for that is four SuperDracos to land the thing on the head. Which is just as fine because you couldn't land it with the Merlin anyway.


Also the interstage is left with the first stage, and with good reasons. The nozzle is radiation cooled and needs to look at cold space, not at the inside of an interstage.
« Last Edit: 04/02/2017 09:35 PM by uhuznaa »

Online RotoSequence

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Re: F9 Second Stage Reusability
« Reply #105 on: 04/02/2017 09:50 PM »
But you can't employ RCS thrusters to maintain orientation during re-entry. You'd basically need to balance the mass difference between the heavy and the light end (bottom and top) by thrust. Do you want to run some 1000 lbf thrusters all the time during reentry?

And even then you would end up with a vacuum Merlin for landing, which has both much too much thrust and the wrong nozzle. And the nozzle is big which forces long (and with that wide) legs.

What you need anyway is a heat shield, propellants for landing and legs. Now put all of this on top of the stage and it will reenter head first. Now the Merlin is nicely tucked away in the plasma shadow and the only price you have to pay for that is four SuperDracos to land the thing on the head. Which is just as fine because you couldn't land it with the Merlin anyway.


Also the interstage is left with the first stage, and with good reasons. The nozzle is radiation cooled and needs to look at cold space, not at the inside of an interstage.

I suspect that engine-first re-entry would remain the dynamically stable approach, since most of the stage mass is and would remain in the engine. You are right about the rest, though; the clamshell heat shield would need to open very wide for radiative cooling, and super-dracos are the only viable engine for propulsive landing in SpaceX's inventory.

Offline Rocket Science

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Re: F9 Second Stage Reusability
« Reply #106 on: 04/02/2017 10:15 PM »
As an evolution SpaceX's concept from their video: This is my reusable S2 using a sliding rearward skirt with petals that open to a "shuttlecock" configuration for drag stability, controlability, lift (biasing) and to protect the engine bell from heat (think afterburner "turkey feathers" in video). 8 Super Dracos with hypergolic prop tanks in the nose for CoG on 4 telescopic landing legs. TPS is in tan... WIP... (Note: I'm also thinking along the lines of utilizing the stage 1-2 inter-stage adapter to create the "shuttlecock petals") since it is now discarded...

https://history.nasa.gov/SP-4220/ch7.htm
http://www.dtic.mil/dtic/tr/fulltext/u2/a231552.pdf
We have experience with "blunt body" re-entry vehicle from our experience with the Atlas warhead.
https://forum.nasaspaceflight.com/index.php?topic=39314.260
« Last Edit: 04/04/2017 09:59 PM by Rocket Science »
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Offline uhuznaa

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Re: F9 Second Stage Reusability
« Reply #107 on: 04/02/2017 10:23 PM »
But you can't employ RCS thrusters to maintain orientation during re-entry. You'd basically need to balance the mass difference between the heavy and the light end (bottom and top) by thrust. Do you want to run some 1000 lbf thrusters all the time during reentry?

And even then you would end up with a vacuum Merlin for landing, which has both much too much thrust and the wrong nozzle. And the nozzle is big which forces long (and with that wide) legs.

What you need anyway is a heat shield, propellants for landing and legs. Now put all of this on top of the stage and it will reenter head first. Now the Merlin is nicely tucked away in the plasma shadow and the only price you have to pay for that is four SuperDracos to land the thing on the head. Which is just as fine because you couldn't land it with the Merlin anyway.


Also the interstage is left with the first stage, and with good reasons. The nozzle is radiation cooled and needs to look at cold space, not at the inside of an interstage.

I suspect that engine-first re-entry would remain the dynamically stable approach, since most of the stage mass is and would remain in the engine. You are right about the rest, though; the clamshell heat shield would need to open very wide for radiative cooling, and super-dracos are the only viable engine for propulsive landing in SpaceX's inventory.

Well, I think a Merlin is about 500kg, add the nozzle and whatever, the thrust structure etc... Maybe 1500kg or 2000kg.

At the top you will add about 200kg for the heat shield, again 200kg for the SuperDracos and mounting, 1000kg propellants, then tanks for those and for helium, then the legs, more batteries for a day in orbit (deploying and retracting solar panels would be a bit over the top), some structure... I think with empty main tanks it will end up top-heavy then. I would guess you'd end up with at least 2000kg or 3000kg additional hardware on the top, maybe more. You'd basically bolt an inverted Dragon 2 minus the pressure vessel on top of the second stage. If it's still bottom-heavy, add ballast or more propellant.

The FH can spare a few tons of payload. If cutting even 4 tons into the payload gives you the second stage back and means flying for basically just the propellant costs (Ok, ideally) it would be worth it.


Of course this would only make sense for LEO launches... You're not getting your second stage back when you're throwing a Dragon to Mars.

Online RotoSequence

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Re: F9 Second Stage Reusability
« Reply #108 on: 04/02/2017 10:43 PM »
Well, I think a Merlin is about 500kg, add the nozzle and whatever, the thrust structure etc... Maybe 1500kg or 2000kg.

At the top you will add about 200kg for the heat shield, again 200kg for the SuperDracos and mounting, 1000kg propellants, then tanks for those and for helium, then the legs, more batteries for a day in orbit (deploying and retracting solar panels would be a bit over the top), some structure... I think with empty main tanks it will end up top-heavy then. I would guess you'd end up with at least 2000kg or 3000kg additional hardware on the top, maybe more. You'd basically bolt an inverted Dragon 2 minus the pressure vessel on top of the second stage. If it's still bottom-heavy, add ballast or more propellant.

The FH can spare a few tons of payload. If cutting even 4 tons into the payload gives you the second stage back and means flying for basically just the propellant costs (Ok, ideally) it would be worth it.


Of course this would only make sense for LEO launches... You're not getting your second stage back when you're throwing a Dragon to Mars.

Another 3000 kilos of hardware that comes right out of your payload capacity to every orbit, and two utterly distinct versions of S2? That's not the kind of thing that's going to make reuse pay off.

Online john smith 19

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Re: F9 Second Stage Reusability
« Reply #109 on: 04/02/2017 10:55 PM »
@JohnSmith19
The difference between now and 2014 isn't simply that the MVac thrust has changed. IMHO it's much simpler than that. They were unsure whether the first FH was going to have a payload until quite recently and having determined that it's not going to have one, Musk says they might have a go at bringing back the US. Basically it's a free opportunity to do something interesting.

I think everyone is reading way too much into this and assuming it implies some sort of future US recovery program, of which this will be a demo.
SX know that hard data is better than any amount of simulations, especially in an area like this, where there's still plenty of doubt over models for airflow and heat transfer.

Now that they know no payload is going to get damaged this allows them to do a detailed test and anchor their models. Even if US recovery is a complete failure I expect it to be returning data through the whole launch.  Maybe it will prove that US recovery is completely impossible. Maybe it will refine the uncertainties to a point where a more serious attempt can be made.

Time will tell.
yes.
The Vulcan video says 25% the weight, but 65% the cost. I don't know if it would be similar for the Falcon-9 S2.  If it were, the loading of the heat shield would become much lower than for full recovery.


It might be great in a world of 12,000-satellite constellations, if the tanks could become payload. Perhaps, cannibalized for structural components; or sent to an 'exterrestrial scrap yard' for use sometime in the future.
I thought ULA were talking about doing that for the booster stage only. IOW it's only good for the first stage. The Centaur US remains completely expendable. So it won't give SX what they already have.

The idea of reusing the tank material dates back to proposals to reuse the Shuttle ET, given it was only about 150 m/s below achieving full orbital velocity. Of course that would have meant moving away for a hand built, bespoke space station, which is what NASA wanted. 

The down side is that package becomes much denser, meaning it drops through the atmosphere fast. That means  rather than lose speed gradually in thin air it slams into thick air, giving a massive frictional heating rate.

With SX starting to work out PLF recovery and reuse US recovery and reuse is the last remaining big item that is still expendable.  I'm optimistic the FH maiden flight will teach them quite a lot, but I'm less so about wheather it will result in a full recovery.
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Offline uhuznaa

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Re: F9 Second Stage Reusability
« Reply #110 on: 04/02/2017 11:01 PM »
Well, I think a Merlin is about 500kg, add the nozzle and whatever, the thrust structure etc... Maybe 1500kg or 2000kg.

At the top you will add about 200kg for the heat shield, again 200kg for the SuperDracos and mounting, 1000kg propellants, then tanks for those and for helium, then the legs, more batteries for a day in orbit (deploying and retracting solar panels would be a bit over the top), some structure... I think with empty main tanks it will end up top-heavy then. I would guess you'd end up with at least 2000kg or 3000kg additional hardware on the top, maybe more. You'd basically bolt an inverted Dragon 2 minus the pressure vessel on top of the second stage. If it's still bottom-heavy, add ballast or more propellant.

The FH can spare a few tons of payload. If cutting even 4 tons into the payload gives you the second stage back and means flying for basically just the propellant costs (Ok, ideally) it would be worth it.


Of course this would only make sense for LEO launches... You're not getting your second stage back when you're throwing a Dragon to Mars.

Another 3000 kilos of hardware that comes right out of your payload capacity to every orbit, and two utterly distinct versions of S2? That's not the kind of thing that's going to make reuse pay off.

Yes, honestly I think that the F9 is just too small to make full reusability worthwhile. You can try to do it with the FH, but if you really want to have a common reusable design for everything you need a bigger launcher along the lines of the ITS, with the second stage being an actual spacecraft (and a payload bay for satellite launches, with a kicker stage for GTO). It doesn't need to be as big as the ITS, but the F9 is just hitting the limits of growth.

(I'm still wondering how they want to exploit the 50t of LEO payload of the FH. Isn't the F9 already fairly volume limited? As long as you don't want to launch a payload of lead, you need to go massively hammerhead or make the thing even longer as it is.)

Online AncientU

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Re: F9 Second Stage Reusability
« Reply #111 on: 04/03/2017 12:34 AM »
What about Magnetoshell Aerobraking/Aerocapture?
Did you make that trip to SpaceX, Jon?

Quote
... short answer is that using MAC for returning an upper stage may be feasible. I did some calculations a few months ago, and while I haven't had MSNW vet them, they at least suggest something like this could work. There are details--normal copper probably won't cut it, but there are at least three other ways of solving it. I haven't had the chance to pitch the idea to SpaceX, but it's definitely one I've been thinking about a lot lately.
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Offline su27k

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Re: F9 Second Stage Reusability
« Reply #112 on: 04/03/2017 03:16 AM »
Well, I think a Merlin is about 500kg, add the nozzle and whatever, the thrust structure etc... Maybe 1500kg or 2000kg.

At the top you will add about 200kg for the heat shield, again 200kg for the SuperDracos and mounting, 1000kg propellants, then tanks for those and for helium, then the legs, more batteries for a day in orbit (deploying and retracting solar panels would be a bit over the top), some structure... I think with empty main tanks it will end up top-heavy then. I would guess you'd end up with at least 2000kg or 3000kg additional hardware on the top, maybe more. You'd basically bolt an inverted Dragon 2 minus the pressure vessel on top of the second stage. If it's still bottom-heavy, add ballast or more propellant.

The FH can spare a few tons of payload. If cutting even 4 tons into the payload gives you the second stage back and means flying for basically just the propellant costs (Ok, ideally) it would be worth it.


Of course this would only make sense for LEO launches... You're not getting your second stage back when you're throwing a Dragon to Mars.

Another 3000 kilos of hardware that comes right out of your payload capacity to every orbit, and two utterly distinct versions of S2? That's not the kind of thing that's going to make reuse pay off.

It would need to be an addon kit, you could design it to be like a payload, and stack the real payload on top of it, either directly like Boeing 702SP, or using a dual launch adapter like Ariane 5

Offline Rocket Surgeon

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Re: F9 Second Stage Reusability
« Reply #113 on: 04/03/2017 06:04 AM »
So a thought about Elon's 'Hail Mary' comment and some of the numbers being batted around here.

Based on how light weight PicaX is (~600kg/m3 is a number a found recently though I’m not sure how accurate that is), you could have conceivably have a heat shield on the front of the second stage 3.7m in diameter and .078m (78mm) thick for 500kg. Alternatively, it could be thinner and partially wrap around the sides.

This would help to balance the stage, but wouldn’t do the full job, so to further stabilise it, several (4?) ‘paddles’ of reinforced picaX could extend from rear next to the engine, similar to the grid fins on the first stage, in order to act as control surfaces providing aerodynamic control and to shift the Centre of Pressure backwards behind the Centre of Gravity, shuttlecocking the stage. Potentially 125 kg each, totalling another 500 kg. 

Finally, a 250kg parachute system could also be installed near the engine. I don’t think this is an unreasonable number as systems approximately 190kg are used to air drop GPS guided payloads up to 4,500kg with parafoils that have and accuracy of less than 50 meters. This is roughly the current Stage 2 mass.

Add in roughly 250kg of propellant for 145 m/s of deltaV to deorbit from GTO via direct entry once the orbit lines itself up (assuming it does or at least comes close enough for an ASDS with a helicopter to go out and meet it…. sea water sucks…) and the total additional hardware to bring down a second stage COULD be as low as 1,500kg including fuel.

Now this is all purely hypothetical and my numbers probably miss something crucial (specifically how much deltaV/fuel it would takes to deorbit), but if it could save something worth ~$15,000,000, it could be worth it, as the Falcon 9 would still be able to deliver some worthwhile payload to GTO (maybe 4-5000kg with a first stage ASDS landing), with the Falcon Heavy filling in the gap.

Now to the point. This is why I think Elon referred to it as a ‘Hail Mary’. The big question is, is that enough hardware to bring the second stage back from GTO? If it isn’t, then there is almost no additional payload to be spent before the Falcon 9 becomes essentially useless for the GTO market. Maybe another 250-500kg, but after that, how many birds going to GTO weigh less than 3,500kg? How many weigh less than 4,000kg for that matter?
So that may be the direction they go in, if they get a chance. Slap the bare minimum that can fit within a 1,500kg mass budget, and if that doesn’t work, give up completely now on second stage reuse until ITS or some hypothetical dense methalox second stage using a mini-Raptor. And the perfect flight TO slap the bare minimum on would be Falcon Heavy Demo, assuming it can be done without holding it up further. And on top of all this, Spacex literally has all the parts now thanks to Dragon…

TL;DR – If they can return a Second Stage from GTO for less than 1,500kg of extra hardware and fuel, now you’re talking, other than that, not worth it…

Offline Ludus

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Re: F9 Second Stage Reusability
« Reply #114 on: 04/03/2017 06:34 AM »
So a thought about Elon's 'Hail Mary' comment and some of the numbers being batted around here.

Based on how light weight PicaX is (~600kg/m3 is a number a found recently though I’m not sure how accurate that is), you could have conceivably have a heat shield on the front of the second stage 3.7m in diameter and .078m (78mm) thick for 500kg. Alternatively, it could be thinner and partially wrap around the sides.

This would help to balance the stage, but wouldn’t do the full job, so to further stabilise it, several (4?) ‘paddles’ of reinforced picaX could extend from rear next to the engine, similar to the grid fins on the first stage, in order to act as control surfaces providing aerodynamic control and to shift the Centre of Pressure backwards behind the Centre of Gravity, shuttlecocking the stage. Potentially 125 kg each, totalling another 500 kg. 

Finally, a 250kg parachute system could also be installed near the engine. I don’t think this is an unreasonable number as systems approximately 190kg are used to air drop GPS guided payloads up to 4,500kg with parafoils that have and accuracy of less than 50 meters. This is roughly the current Stage 2 mass.

Add in roughly 250kg of propellant for 145 m/s of deltaV to deorbit from GTO via direct entry once the orbit lines itself up (assuming it does or at least comes close enough for an ASDS with a helicopter to go out and meet it…. sea water sucks…) and the total additional hardware to bring down a second stage COULD be as low as 1,500kg including fuel.

Now this is all purely hypothetical and my numbers probably miss something crucial (specifically how much deltaV/fuel it would takes to deorbit), but if it could save something worth ~$15,000,000, it could be worth it, as the Falcon 9 would still be able to deliver some worthwhile payload to GTO (maybe 4-5000kg with a first stage ASDS landing), with the Falcon Heavy filling in the gap.

Now to the point. This is why I think Elon referred to it as a ‘Hail Mary’. The big question is, is that enough hardware to bring the second stage back from GTO? If it isn’t, then there is almost no additional payload to be spent before the Falcon 9 becomes essentially useless for the GTO market. Maybe another 250-500kg, but after that, how many birds going to GTO weigh less than 3,500kg? How many weigh less than 4,000kg for that matter?
So that may be the direction they go in, if they get a chance. Slap the bare minimum that can fit within a 1,500kg mass budget, and if that doesn’t work, give up completely now on second stage reuse until ITS or some hypothetical dense methalox second stage using a mini-Raptor. And the perfect flight TO slap the bare minimum on would be Falcon Heavy Demo, assuming it can be done without holding it up further. And on top of all this, Spacex literally has all the parts now thanks to Dragon…

TL;DR – If they can return a Second Stage from GTO for less than 1,500kg of extra hardware and fuel, now you’re talking, other than that, not worth it…

If it's just the sort of additional mass you describe, it might be very useful even if it only allowed recovery of S2 with a class of FH flights not single core. If they're looking ahead to deploying the Constellation, that alone might be dozens of FH flights per year with precisely controllable inhouse payloads. It wouldn't matter if other S2s still had to be expendable.

With PicaX on one side and grid fins it might make it to deploying a parafoil, then it could be snagged midair by a heavy helicopter like a Skycrane.

Online john smith 19

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Re: F9 Second Stage Reusability
« Reply #115 on: 04/03/2017 06:52 AM »
For second stage reusability it all comes down to propellant and heat.

The stage is naturally bottom heavy. So the question is can the rear end and the nozzle survive the heating. If it can you don't need propellant (main or RCS) to keep the front end to bear the brunt of the heating through TPS.

2nd worst is if you have to run the engine to generate a "nose bubble" An interesting question would be could you run the engine quite fuel rich, as that's lighter. Keeping in mind that at these altitudes we're talking a few 10s of psi is 100x of (ambient) atmospheric pressure.

If that doesn't work then you have to keep the nose end on to the airstream, which suggests continuous movement of some kind of dynamic control surfaces, probably a set of grid of fins. SX are familiar with them and they operate over a wide range of Mach numbers, which will be even more important for US than booster stage recovery.

TBH doing SECO just below orbital velocity still seems like the best way to save a lot of propellant on the de-orbit burn. Just have the payload provide the last 10s of m/s delta V.  But as always the question is will it be enough?
Can side stepping needing a de-orbit burn at all, coupled with the the decelleration from control surfaces during a once around flight, coupled with thinner TPS due to the lower starting velocity lower the mass penalty for recovery enough?

This is an area where approximations don't really cut it and you're looking at fairly detailed simulation to see if something is at all possible. Slight mishaps don't add, they multiply.
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Offline Semmel

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Re: F9 Second Stage Reusability
« Reply #116 on: 04/03/2017 07:38 AM »
Maybe I am being a party pooper here, but I dont think the second stage will see any substantial design changes for the (if any) recovery attempt. And I think of things like heat shields, wings,  fold-up legs etc. that sit on the outside of the stage.
It would probably invalidate the use of the FH launch for the DOD certification process. Therefore, I suspect the recovery attempt will be something like a parachute mounted in the payload fairing adapter and have it reenter without burning up by spending fuel. How, I have no idea. But I think the most important thing is to get the DOD certification. Anything that would violate it is not going to happen.

Offline Ionmars

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Re: F9 Second Stage Reusability
« Reply #117 on: 04/03/2017 10:56 AM »
Interstage shield 2

   In Reply #101 above I suggested employing a modified interstage ring as a heat shield that would allow stage 2 to enter the atmosphere. In the following discussion the idea was found to be interesting, but the underlying assumption that RCS thrusters could hold its orientation during the maneuver wouldn't work because the CG was just too low on the rocket stage.

Now consider instead that stage 2 will enter the atmosphere tail first, allowing the heavy engine to assist in holding the correct orientation of the stage. In this case, the interstage shield would become essential for a successful return to Earth. The end cap of the interstage shield, when closed, should be constructed as a blunt capsule ablative shield in its own right. It could withstand the heat of re-entry as a capsule with a long body, but it would not have sufficient surface area to decelerate stage 2 to low velocity.  Therefore, Spx would employ the side of the rocket as the primary resistance body, as it now does with stage 1. The interstage heat shield would only serve to protect the M1D engine components during the hypersonic phase of re-entry.
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Offline MikeAtkinson

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Re: F9 Second Stage Reusability
« Reply #118 on: 04/03/2017 02:37 PM »
Maybe I am being a party pooper here, but I dont think the second stage will see any substantial design changes for the (if any) recovery attempt. And I think of things like heat shields, wings,  fold-up legs etc. that sit on the outside of the stage.
It would probably invalidate the use of the FH launch for the DOD certification process. Therefore, I suspect the recovery attempt will be something like a parachute mounted in the payload fairing adapter and have it reenter without burning up by spending fuel. How, I have no idea. But I think the most important thing is to get the DOD certification. Anything that would violate it is not going to happen.

Agree, as well as invalidating the FH certification, there does not seem to be enough time to do anything major.

Offline Ionmars

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Re: F9 Second Stage Reusability
« Reply #119 on: 04/03/2017 02:38 PM »
To make the interstage heat shield concept actually work, there are engineering questions for which I have no good answers:
1. What materials and thicknesses should comprise the different surfaces?
2. What kind of hinge system would work best in space?
3. What sequence of opening and closing of the "tin can" and engine firing would work best?
4. What would be the final disposition of the interstage shield and how to do it? 
Edit: 5. How would the heat shield, now a component of S2, attach and detach from S1?
« Last Edit: 04/03/2017 02:47 PM by Ionmars »
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