Author Topic: F9 Second Stage Reusability  (Read 88665 times)

Offline adrianwyard

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Re: F9 Second Stage Reusability
« Reply #440 on: 04/20/2017 06:20 PM »
Another thought: the S2 is coming in from orbit, so there's no penalty for RTLS. If fitting landing legs behind the plasma shield is a problem, the S2 could simply come in and land upside-down on SuperDracos in a fixed cradle with a blast trench underneath. It would reduce plume impingement/wash dramatically and really simplify the recovery hardware needed for the S2.

Have been thinking about an approach similar to this, however perhaps everything necessary for a S2 landing could be contained within one self contained 'standalone' module (minus the PICA heat shield) fixed between a standard S2 and the payload adaptor, also with grid fins fitted to this same module, first using the Merlin 1D/grid fins  for a stable re-entry similar to S1, then only once the stage has significantly slowed (perhaps after another short Merlin 1D burn) inverting the S2 for a RTLS Super Draco powered landing.
Heat shield is a non-negotiable. Hypersonic retroprop won't work at orbital speeds.

Sure, but is developing that a better plan than having an Erickson Skycrane on standby and a couple of pilots available to pick one up in mid-air? They don't come cheap, but think of the mass you save on S2 as well as the reduced complexity (risk), and also the cost of developing the SuperDraco landing hardware - and the cradle.

Offline envy887

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Re: F9 Second Stage Reusability
« Reply #441 on: 04/20/2017 07:12 PM »
Legs can go straight through the heatshield, see Dragon 2. So can the landing thrusters, they just need to blow off a tile when they start up.

Shouldn't need to add more than 1/2 a stage diameter to the height of the stage to fit everything in.
The existing S2 is more than three times the height of the Dragon 2, so those stubby little legs wouldn't provide nearly enough stability to avoid tip-over. And blowing a hole in the heat shield doesn't fit with reuse plans.

The legs can extend through the heat shield a bit sideways to make the footprint wider. If the CG is far enough forward for a forward entry, it's far enough forward for a stable nose-down landing. Especially with 6 legs.

Make it an easily replaceable tile, should be simple enough. The tiles under Dragon 2's legs will probably have to be replaced each flight.

Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #442 on: 04/20/2017 07:42 PM »
... Block 5, supposedly capable of getting an L/D ratio of 1:1 on the first stage ...

Here's Musks's comment:
Quote
the new grid fins should be capable of taking a scorching and being fine. And they'll also have significantly more control authority, so, that should improve reusability of the rocket. It will improve the payload to orbit by being able to fly at a higher angle of attack and use the aerodynamic element of the rocket to effectively glide like a big cylinder. It does have an L/D of roughly 1 if flown at the right angle of attack, but you need control authority, particularly pitch control authority, that's higher than we currently have to achieve that.
It's hard to tease this apart, but I think he's talking about increasing payload to orbit by by 'flying' the first stage which has a L/D of 1 at certain (high) speeds. Not clear if the gain comes from going uphill (engine gimbaling), or RTLS landing (less prop needed for boostback because cross range 'flying' with grid fins is increased). TL;DR I don't think this helps S2 landing scenarios.
Ah, hmm. May not. I'm fairly sure the gain doesn't come from going uphill -- engine gimbaling is by far the most dV-efficient approach -- but the "flying it back to land" thing makes sense.

That being said, autorotation on the second stage could still be viable for reducing terminal velocity.

The legs can extend through the heat shield a bit sideways to make the footprint wider. If the CG is far enough forward for a forward entry, it's far enough forward for a stable nose-down landing. Especially with 6 legs.
I'll believe it when I see it. The CG isn't great, not with that heavy Merlin up top. Note the feathered flaps for maintaining COP.

Offline envy887

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Re: F9 Second Stage Reusability
« Reply #443 on: 04/20/2017 07:58 PM »
The legs can extend through the heat shield a bit sideways to make the footprint wider. If the CG is far enough forward for a forward entry, it's far enough forward for a stable nose-down landing. Especially with 6 legs.
I'll believe it when I see it. The CG isn't great, not with that heavy Merlin up top. Note the feathered flaps for maintaining COP.

The CG appears to be slightly forward of the center of the stage once heat shield, thrusters, tanks, and legs are added to the nose. Lets say it's 6 m from the contact plane of the extended legs. If the legs extend out to the diameter of the stage , the inscribed circle will have a radius of about 1.8 m. The static tipover angle is atan(1.8 m / 6 m) = 17 degrees.

Unless landing on the side of a mountain, it will be pretty stable.

Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #444 on: 04/20/2017 08:09 PM »
Wait a minute. If it can RTLS without penalty, and if it can fly itself to a pinpoint landing site via grid fins...why not just drop into a net, suspended above the ground? Nets are cheap. Terminal velocity can't be THAT high.

« Last Edit: 04/20/2017 08:22 PM by sevenperforce »

Offline ArbitraryConstant

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Re: F9 Second Stage Reusability
« Reply #445 on: 04/20/2017 08:48 PM »
It's hard to tease this apart, but I think he's talking about increasing payload to orbit by by 'flying' the first stage which has a L/D of 1 at certain (high) speeds. Not clear if the gain comes from going uphill (engine gimbaling), or RTLS landing (less prop needed for boostback because cross range 'flying' with grid fins is increased). TL;DR I don't think this helps S2 landing scenarios.
I parse that as clearly referring to RTLS with greater atmospheric crossrange.

I don't think grid fins are out of the question for S2, just not at initial reentry velocities.

Offline adrianwyard

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Re: F9 Second Stage Reusability
« Reply #446 on: 04/20/2017 09:09 PM »
Wait a minute. If it can RTLS without penalty, and if it can fly itself to a pinpoint landing site via grid fins...why not just drop into a net, suspended above the ground? Nets are cheap. Terminal velocity can't be THAT high.



BOE says around 100 m/s, so 25 megajoules into the net. (Big error bars.)

Offline Kaputnik

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Re: F9 Second Stage Reusability
« Reply #447 on: 04/20/2017 09:36 PM »
EDIT: back of the envelope...terminal velocity for S1 is slightly subsonic...S2 masses less than 25% of S1...estimating similar drag...grid fin L/D for S2 would be 4:1...

Erm, that's not how it works. You're working off mass and drag. But you need lift and drag.

An object with a L/D of 4:1 is essentially a crude glider. Think Shuttle orbiter, or jump chute. It doesn't matter how many grid fins you stick on the back of a rocket, it's not going to turn into a glider.
Autorotation works a little differently. I don't know whether the AOA of the grid fins can be adjusted in real-time as readily as the collective and pitch on a chopper, but it's worth investigation.

It still sounds as though you are attributing a L:D to the grid fins themselves, not the stage.
How are you extrapolating from Musk's comments on the first stage to get your numbers for an autorotating second stage?
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Offline sojourner

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Re: F9 Second Stage Reusability
« Reply #448 on: 04/21/2017 12:11 AM »
I'll believe it when I see it. The CG isn't great, not with that heavy Merlin up top. Note the feathered flaps for maintaining COP.

If you're worried about tipover from wind gusts after landing, then just have it land in a walled off location. A simple, cheap solution that doesn't add weight to the stage.

Online Space Ghost 1962

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Re: F9 Second Stage Reusability
« Reply #449 on: 04/21/2017 12:48 AM »
You can't use spin stabilization with a empty stage top first, because the CG/CP are too separated and like a top heavy top, it'll tumble on its side.

You can't have the base of the fairing be anything but a PAF, because a) that's how the load paths are communicated to the F9US, and b) because when you encapsulate the payload, its horizontal, and all of the weight of the fairing as a huge offset load, which goes through the PAF along with the payload, meaning it has to be there to integrate a payload. So never draw a fairing without the adapter at the bottom.

You could integrate a portion of the payload as a recovery system - this would make business sense for a large LEO/GSO/GEO payload anyways, since the FH has extreme excess capacity, so using up some of that to recover the F9US would still allow a) unmodified FH/F9US operation, b) deployment of payload(s), and c) allow the additional resources necessary for recovery to be separate from the F9US it was to recover, thus an optional "secondary payload" whose sole purpose was to recover the F9US. Like in the place of the lower payload on Ariane's SYLDA.

The scope of such a "recovery payload vehicle" (RPV?) for development would be like that of a Dragon. You might use it to later recover a large module or spacecraft as well.

Such a vehicle would shift the CG much higher, and might also spin the combined vehicle at a high enough rate as well as a controlled entry, that one could avoid a hypersonic tumble.

It could also have permanent "legs"/other that fit within the confines of the fairing and not interfering with the primary payload(s).

This approach would follow the SX doctrine of optionally reusable or expendable - you'd not wreck the economics of the F9US (actually, you could even improve upon it, because you can get the US back and see just how much you "used" of it, how much margin still remained).

Note that it would only work with missions that could afford the decrease in payload on F9/FH. And ... as you got better at recovery, the performance of this "F9RPV" would decrease the loss incrementally, as we have seen with F9/FH performance figures marching up.

And you don't force your customers to use it, just another option.

Might even allow for BFS experiment recover too. Hows this for an idea?

Offline adrianwyard

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Re: F9 Second Stage Reusability
« Reply #450 on: 04/21/2017 06:04 AM »
You can't use spin stabilization with a empty stage top first, because the CG/CP are too separated and like a top heavy top, it'll tumble on its side.
Can you think of another way of explaining why spin stabilization won't work for S2 nose-first? A top does stay upright despite being top heavy so long as it's kept spinning fast enough...
« Last Edit: 04/21/2017 06:18 AM by adrianwyard »

Online BobHk

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Re: F9 Second Stage Reusability
« Reply #451 on: 04/21/2017 09:24 PM »
Can you make the bell of the engine the landing gear and part of the rocket body?  Say, S2 has a gimballed engine with a standard bell 'inside' the rocket body material at the level of the engine is shaped like a bell and also attaches to S1 and is reinforced in 3 areas to act as stiffened landing gear with crush zone adapters.

So two different sized, nested dixie cups at the bottom of S2 (the bigger dixie cup being the diameter of the rocket), both essentially rocket bells.  One outer bell for orbital operations, its stiffened outer area used for landing legs.  The inner bell for landings.  The need for a heat shield is largely obviated by use of the rocket plume during reentry.  Small grid fins and perhaps gas jets could be added, leveraging existing systems in S1 that are already well understood. 

You would not need any changes to the upper parts of S2, the fairings could be jettisoned and recovered as they are now. 

Would this be light enough to have enough fuel left over for landing like an F9 does now?

« Last Edit: 04/21/2017 09:24 PM by BobHk »

Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #452 on: 04/22/2017 05:02 AM »
Can you make the bell of the engine the landing gear and part of the rocket body?  Say, S2 has a gimballed engine with a standard bell 'inside' the rocket body material at the level of the engine is shaped like a bell and also attaches to S1 and is reinforced in 3 areas to act as stiffened landing gear with crush zone adapters.

So two different sized, nested dixie cups at the bottom of S2 (the bigger dixie cup being the diameter of the rocket), both essentially rocket bells.  One outer bell for orbital operations, its stiffened outer area used for landing legs.  The inner bell for landings.  The need for a heat shield is largely obviated by use of the rocket plume during reentry.  Small grid fins and perhaps gas jets could be added, leveraging existing systems in S1 that are already well understood. 

You would not need any changes to the upper parts of S2, the fairings could be jettisoned and recovered as they are now. 

Would this be light enough to have enough fuel left over for landing like an F9 does now?
A few problems here.

First of all, the engine bell is radiatively cooled, so it needs to be exposed in order to not melt. Really.

Next, the engine plume alone won't obviate the need for a heat shield. Not enough fuel, and not effective enough, either.

Finally, the MVac would undergo catastrophic flow separation at SL...and even if it didn't, its TWR is far too high to land on with an empty S2.

Offline sevenperforce

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Re: F9 Second Stage Reusability
« Reply #453 on: 04/23/2017 04:06 AM »
Dedicated thread on second-stage reuse configurations in the future: https://forum.nasaspaceflight.com/index.php?topic=42783.0

Offline rakaydos

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Re: F9 Second Stage Reusability
« Reply #454 on: 04/24/2017 01:24 AM »
So I've seen a lot of suggestions about landing legs going through the heat shield like Dragon (which has issues of balance) and engines coming out the side like dragon (which has cosign losses).

I havnt seen anyone suggest those same holes used for legs in Dragon be used as Superdraco exaust ports. A hinged cover over each protects the SDs through reentry, and as it slows through the supersonic regime the covers unlatch and the superdracos blow the covers open. (if they dont unlatch, the superdracos STILL blow them open, but additional repairs/replacement of the caps may be needed)

The benifits are twofold- first, you eliminate the cosign losses from disperced thrust plooms. Second, you get the nozzles out of the hypersonic airstream during reentry, which is more of an issue with the cylendrical S2 than the conical Dragon capsule.

Legs can still emerge from the sides, of course- protected under an aeroshell.

Online BobHk

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Re: F9 Second Stage Reusability
« Reply #455 on: 05/08/2017 11:09 PM »


Anyone thought of airbrakes?  Large deployable carbon fiber airbrakes that pop out after atmospheric re entry nose down.  The brakes deploy from the end (opposite end of the nose that has the heat shielding).  Legs deploy from the nose to land.  You can put the retropropulsive units in the base with the aerobrakes.  The fairing can be attached and deployable then retract after sat deployment (making up the 'nose' of the S2).

A heatshielded clamshell fairing nose that doesnt detach and Aerobrakes + retro propulsion just seem to be more viable than a parachute or a net.  What kind of lego model can I make to convince everyone it will work?


Offline rsdavis9

Re: F9 Second Stage Reusability
« Reply #456 on: 05/09/2017 10:46 AM »


Anyone thought of airbrakes?  Large deployable carbon fiber airbrakes that pop out after atmospheric re entry nose down.  The brakes deploy from the end (opposite end of the nose that has the heat shielding).  Legs deploy from the nose to land.  You can put the retropropulsive units in the base with the aerobrakes.  The fairing can be attached and deployable then retract after sat deployment (making up the 'nose' of the S2).

A heatshielded clamshell fairing nose that doesnt detach and Aerobrakes + retro propulsion just seem to be more viable than a parachute or a net.  What kind of lego model can I make to convince everyone it will work?

grid fins?
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Offline Ictogan

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Re: F9 Second Stage Reusability
« Reply #457 on: 05/09/2017 10:52 AM »


Anyone thought of airbrakes?  Large deployable carbon fiber airbrakes that pop out after atmospheric re entry nose down.  The brakes deploy from the end (opposite end of the nose that has the heat shielding).  Legs deploy from the nose to land.  You can put the retropropulsive units in the base with the aerobrakes.  The fairing can be attached and deployable then retract after sat deployment (making up the 'nose' of the S2).

A heatshielded clamshell fairing nose that doesnt detach and Aerobrakes + retro propulsion just seem to be more viable than a parachute or a net.  What kind of lego model can I make to convince everyone it will work?
Developing a non-detaching fairing doesn't make sense given that they are also developing reuse for detaching fairings.

Online meekGee

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Re: F9 Second Stage Reusability
« Reply #458 on: 05/12/2017 02:46 PM »


Anyone thought of airbrakes?  Large deployable carbon fiber airbrakes that pop out after atmospheric re entry nose down.  The brakes deploy from the end (opposite end of the nose that has the heat shielding).  Legs deploy from the nose to land.  You can put the retropropulsive units in the base with the aerobrakes.  The fairing can be attached and deployable then retract after sat deployment (making up the 'nose' of the S2).

A heatshielded clamshell fairing nose that doesnt detach and Aerobrakes + retro propulsion just seem to be more viable than a parachute or a net.  What kind of lego model can I make to convince everyone it will work?
Developing a non-detaching fairing doesn't make sense given that they are also developing reuse for detaching fairings.
GTO/LEO?
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Offline JasonAW3

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Re: F9 Second Stage Reusability
« Reply #459 on: 05/12/2017 03:09 PM »


Anyone thought of airbrakes?  Large deployable carbon fiber airbrakes that pop out after atmospheric re entry nose down.  The brakes deploy from the end (opposite end of the nose that has the heat shielding).  Legs deploy from the nose to land.  You can put the retropropulsive units in the base with the aerobrakes.  The fairing can be attached and deployable then retract after sat deployment (making up the 'nose' of the S2).

A heatshielded clamshell fairing nose that doesnt detach and Aerobrakes + retro propulsion just seem to be more viable than a parachute or a net.  What kind of lego model can I make to convince everyone it will work?
Developing a non-detaching fairing doesn't make sense given that they are also developing reuse for detaching fairings.
GTO/LEO?

Personally, I'm more in favor of an inflatable decelerator system.  Maximize surface area for lowest mass penalty, to decelerate the system soonest.

     Mind you, it'd be best to make a few high altitude passes prior to final deceleration and landing pass, as this could be used to bleed off velocity in the high upper atmosphere, with minimal external heating, prior to deployment of the inflatable decelerator..
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