Author Topic: SpaceX second stage secret sauce?  (Read 23398 times)

Offline LouScheffer

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SpaceX second stage secret sauce?
« on: 12/26/2016 06:58 pm »
The second stage of the Falcon 9 seems to have a PMF that's way out of line with existing vehicles.

First, the evidence for a high PMF for the SpaceX second stage:  Start by looking at SES-9.  From the webcast, at the start of second stage ignition, speed is 2280 m/s.  The second stage first has to get to parking orbit (7800 m/s), then to 40600 km apogee (2535 m/s more).  Gravity losses, at 645 m/s, should be very similar to the Saturn V, which stages at very nearly the same speed (see Orbital Mechanics: Theory and Application.  Rotation of the Earth provides about 400 m/s.  Carrying the fairing for the first 50 seconds of second stage firing costs about 15 m/s.  The net result is that the second stage must supply 8315 m/s.  At an ISP of 348, this requires a mass ratio of 11.45.  Given that the initial mass is about 121 tonnes (Elon says first stage pushes 125, but that includes the fairing) then the final mass is 10.56 tonnes.  But this includes SES-9 at 5.27 tonnes, so the empty second stage can mass at most 5.3 tonnes.  So if the initial mass of the second stage is 116 tonnes (125 quoted by Musk - fairing - payload), then the PMF is 95.4%, or 4.6% of the total mass is structure. Other methods of estimating PMF, such as comparing LEO and GTO payloads, give similar or better results.

To compare, the NASA report Propellant Mass Fraction Calculation Methodology for Launch Vehicles and Application to Ares Vehicles , Figure 3, shows the best existing upper stage is the Soyuz second stage.  In the figure it looks like 93.4%, though this table says 93.2%.  The only other upper stages that are anywhere close are the Proton second and third stages, at about 93%. All others are below 92%.

So for every kg of fueled mass, the SpaceX stage has 4.6% structure, whereas the best other upper stage has 6.6% structure.   That's 30% better than any other second stage that's ever been built.   30% better is an enormous margin in a business where folks would gladly sell their grandmother for a 10% improvement (see oxygen rich staged combustion), and upper stages are already the target of extensive mass reduction efforts. So how is this done?  Aluminum-lithium might result in a 10% mass improvement.  The stage is not pressure-stabilized.  Sub-cooled LOX is denser, but that's not  nearly enough to explain the difference.  The stage seems like a relatively simple structure (cylinder + domes) so it's hard to imagine engineering that's 20% better than the Russians.  SpaceX appears to do some mass shaving by milling (see this picture) but that seems something others could do as well.  So what's the trick?
« Last Edit: 12/26/2016 07:13 pm by gongora »

Offline yokem55

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Re: SpaceS second stage secret sauce?
« Reply #1 on: 12/26/2016 07:09 pm »
Cryo-immersed COPV's. The hassle and anguish they've caused could only be explained if they yielded a big weight savings.

Online meekGee

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Re: SpaceX second stage secret sauce?
« Reply #2 on: 12/26/2016 08:47 pm »
Especially impressive since this is an over-powered 2nd stage, and even more so if it's also going to serve (modified?) on FH.
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Offline russianhalo117

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Re: SpaceX second stage secret sauce?
« Reply #3 on: 12/26/2016 09:25 pm »
Joking, but did elon enable some Kerbal debug menu physics and mathematics cheats.

Offline baldusi

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Re: SpaceX second stage secret sauce?
« Reply #4 on: 12/26/2016 09:37 pm »
Propulsion is usually 50% of pmf. Doubling the propulsion T/W might help. And the are pseudo pressure stabilized. They need the pressure for anything beyond transport and integration. The v1.0 F9 tanks were pressurized to 50psi for this reason.

Offline ChrisC

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Re: SpaceX second stage secret sauce?
« Reply #5 on: 12/26/2016 10:14 pm »
PSA #1: EST does NOT mean "Eastern Time".  Use "Eastern" or "ET" instead, all year round, and avoid this common error.  Google "EST vs EDT".
PSA #2: It's and its: know the difference and quietly impress grammar pedants.  Google "angry flower its" .  *** See profile for two more NSF forum tips. ***

Offline LouScheffer

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Re: SpaceX second stage secret sauce?
« Reply #6 on: 12/26/2016 11:12 pm »
And the [tanks] are pseudo pressure stabilized. They need the pressure for anything beyond transport and integration. The v1.0 F9 tanks were pressurized to 50psi for this reason.
I thought this was true of all modern rockets - though they can stand up under their own weight for ease of handling, they require pressurization to handle flight loads.

Offline edkyle99

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Re: SpaceX second stage secret sauce?
« Reply #7 on: 12/26/2016 11:53 pm »
So for every kg of fueled mass, the SpaceX stage has 4.6% structure, whereas the best other upper stage has 6.6% structure.   That's 30% better than any other second stage that's ever been built.   
The Atlas D sustainer stage had a 0.97 PMF if you counted a full propellant load.  Of course much of that propellant was burned before the booster package separated, but the basic structural idea could still have worked as a stand-alone stage - and this was 1958-ish.  For Atlas, it was about the stainless steel common bulkhead balloon tank and, especially, the relatively lightweight sustainer engine dry mass.  I expect the same is true for the Falcon 9 second stage.  Merlin 1D has a superior thrust to weight ratio, and the stage structure (also common bulkhead) is ultra-light.  (I find it interesting that today's Falcon 9 second stage happens to gross about the same as that old Atlas D sustainer stage.)

That two of these Falcon 9 upper stages have failed shows that this is where SpaceX is pushing hardest against the envelope of possible.  The design may have to back off a bit to succeed.

 - Ed Kyle
« Last Edit: 12/26/2016 11:59 pm by edkyle99 »

Offline sdsds

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Re: SpaceX second stage secret sauce?
« Reply #8 on: 12/27/2016 12:04 am »
Propulsion is usually 50% of pmf. Doubling the propulsion T/W might help.

Hmm. Extreme case: suppose you started with a Soyuz-like PMF: 93.2%. Your dry mass fraction is then 6.8%. Now imagine your propulsion suddenly had zero mass. Your dry mass is cut roughly in half; so too your dry mass fraction: 3.4%. Yielding a prop mass frac of 96.6%. Or more carefully: 93.2/(93.2+3.4) = 96.5%. So how close to zero mass can they have made that engine?

I thought this was true of all modern rockets - though they can stand up under their own weight for ease of handling, they require pressurization to handle flight loads.

Centaur tanks still need pressurization for handling?
« Last Edit: 12/27/2016 12:05 am by sdsds »
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Offline Jim

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Re: SpaceX second stage secret sauce?
« Reply #9 on: 12/27/2016 12:37 am »

I thought this was true of all modern rockets - though they can stand up under their own weight for ease of handling, they require pressurization to handle flight loads.

no, pressurization is for inlet conditions.

Offline Jim

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Re: SpaceX second stage secret sauce?
« Reply #10 on: 12/27/2016 12:43 am »
.

  Given that the initial mass is about 121 tonnes (Elon says first stage pushes 125, but that includes the fairing) then the final mass is 10.56 tonnes.  But this includes SES-9 at 5.27 tonnes, so the empty second stage can mass at most 5.3 tonnes.  So if the initial mass of the second stage is 116 tonnes (125 quoted by Musk - fairing - payload), then the PMF is 95.4%, or 4.6% of the total mass is structure.

 So what's the trick?


Bad mass assumptions.

Offline LouScheffer

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Re: SpaceX second stage secret sauce?
« Reply #11 on: 12/27/2016 12:55 am »
So for every kg of fueled mass, the SpaceX stage has 4.6% structure, whereas the best other upper stage has 6.6% structure.   That's 30% better than any other second stage that's ever been built.   
The Atlas D sustainer stage had a 0.97 PMF if you counted a full propellant load. 
The main trick they used (but SpaceX does not) was keeping the stage pressurized at all times.  This keeps everything in tension and avoids the need to support buckling loads.  This is the same trick that lets suspension bridges (which are purely in tension) be about twice as long as arch bridges, which are in compression and hence need to worry about buckling.

Perhaps careful consideration of buckling loads is the secret?  If the limiting factor was tension loads, an aluminum tank should be even lighter than an Atlas balloon tank (if you could weld thin aluminum perfectly, as least).  Aluminum is half the strength (500 MPa vs 1000 MPa) but a third the density.  So it needs to be twice as thick, but that's still lighter.  And Al-Li is stiffer as well as lighter, which should help with buckling.  Plus this might explain why SpaceX uses a heavy structural payload fairing.  It makes the fairing heavier, but puts less stress on the second stage, making the stage structure lighter.  That looks like a good tradeoff.

There are lots of first stages with high PMFs.  The Titan II first stage was around 0.96.  Second stages with high PMF seem much less common.

Offline Jim

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Re: SpaceX second stage secret sauce?
« Reply #12 on: 12/27/2016 01:02 am »

The main trick they used (but SpaceX does not) was keeping the stage pressurized at all times.


No, the stage could be (and was) placed in mechanical stretch so that it could be worked on.


Plus this might explain why SpaceX uses a heavy structural payload fairing.  It makes the fairing heavier, but puts less stress on the second stage, making the stage structure lighter.  That looks like a good tradeoff.


That is a wrong assumption.

A.  heavy structural payload fairing puts more stress on the second stage.
b.  Ground handling determined the need for a heavy structural payload fairing.

Offline LouScheffer

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Re: SpaceX second stage secret sauce?
« Reply #13 on: 12/27/2016 01:04 am »
.

  Given that the initial mass is about 121 tonnes (Elon says first stage pushes 125, but that includes the fairing) then the final mass is 10.56 tonnes.  But this includes SES-9 at 5.27 tonnes, so the empty second stage can mass at most 5.3 tonnes.  So if the initial mass of the second stage is 116 tonnes (125 quoted by Musk - fairing - payload), then the PMF is 95.4%, or 4.6% of the total mass is structure.

 So what's the trick?
Bad mass assumptions.
And which one is wrong?  The satellite seems consistent with other SES satellites.  The fuel in the second stage was 90 tonnes in the 1.0 version (called out on the audio).  Add in the stretch and the sub-cooled LOX and 111 tonnes of fuel seems quite reasonable.  The stack mass of 125 tonnes was an explicit Elon statement.   The mass of the fairing is a guess, but even if you set it to 0 you still have a mass fraction that is much higher than any other second stage.  So which assumption is bad?

Online meekGee

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Re: SpaceX second stage secret sauce?
« Reply #14 on: 12/27/2016 01:21 am »
50 PSI?   If so, then pressurization goes much beyond "preventing buckling".

50 PSI x F9 area = 0.75 E6 lbf. (350 tons)  That's a big fraction of total rocket thrust.

That's a 35 ton payload at 10 g.

This means that the stage needs barely hold its own in 1 g, and then once pressurized, not care anymore about compressive loads, static or dynamic.

Did I get my multiplication wrong?
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Offline LouScheffer

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Re: SpaceX second stage secret sauce?
« Reply #15 on: 12/27/2016 01:22 am »
Plus this might explain why SpaceX uses a heavy structural payload fairing.  It makes the fairing heavier, but puts less stress on the second stage, making the stage structure lighter.  That looks like a good tradeoff.
That is a wrong assumption.

A.  heavy structural payload fairing puts more stress on the second stage.
b.  Ground handling determined the need for a heavy structural payload fairing.
This is not clear to me.   Yes, the structural fairing is needed for ground handling.  But if the fairing alone can support the payload when horizontal, then it adds no buckling stress to the second stage when it's attached (provided the fairing is supported, as the SpaceX TEL seems to do).

Then they tilt it upright.  The heavier fairing now adds additional axial stress, but that's not the limiting factor.

Then the second stage is pressurized for flight.  From this point on,  buckling is not a problem, since the stage in now in tension.

So i agree with (b), the heavy fairing is required for ground handling.  But once they did that, they can then use its strength to remove handling loads from the second stage.  This suffices until the rocket is pressurized, at which point it is capable of handling the increases stress of the heavier fairing.

Online meekGee

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Re: SpaceX second stage secret sauce?
« Reply #16 on: 12/27/2016 01:28 am »
Plus this might explain why SpaceX uses a heavy structural payload fairing.  It makes the fairing heavier, but puts less stress on the second stage, making the stage structure lighter.  That looks like a good tradeoff.
That is a wrong assumption.

A.  heavy structural payload fairing puts more stress on the second stage.
b.  Ground handling determined the need for a heavy structural payload fairing.
This is not clear to me.   Yes, the structural fairing is needed for ground handling.  But if the fairing alone can support the payload when horizontal, then it adds no buckling stress to the second stage when it's attached (provided the fairing is supported, as the SpaceX TEL seems to do).

Then they tilt it upright.  The heavier fairing now adds additional axial stress, but that's not the limiting factor.

Then the second stage is pressurized for flight.  From this point on,  buckling is not a problem, since the stage in now in tension.

So i agree with (b), the heavy fairing is required for ground handling.  But once they did that, they can then use its strength to remove handling loads from the second stage.  This suffices until the rocket is pressurized, at which point it is capable of handling the increases stress of the heavier fairing.

Both fairing and payload attach at the top ring of the second stage, so the fairing can't help.

If the fairing were to encompass the second stage, then yes.  But that's not the case with the F9.
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Offline LouScheffer

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Re: SpaceX second stage secret sauce?
« Reply #17 on: 12/27/2016 01:29 am »
50 PSI?   If so, then pressurization goes much beyond "preventing buckling".

50 PSI x F9 area = 0.75 E6 lbf. (350 tons)  That's a big fraction of total rocket thrust.

That's a 35 ton payload at 10 g.

This means that the stage needs barely hold its own in 1 g, and then once pressurized, not care anymore about compressive loads, static or dynamic.

Did I get my multiplication wrong?
This is my understanding as well.  This is only for the second stage, though.  The first stage will still see some compressive loads, though they are much reduced by pressurization.

Offline LouScheffer

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Re: SpaceX second stage secret sauce?
« Reply #18 on: 12/27/2016 01:39 am »
Plus this might explain why SpaceX uses a heavy structural payload fairing.  It makes the fairing heavier, but puts less stress on the second stage, making the stage structure lighter.  That looks like a good tradeoff.
That is a wrong assumption.

A.  heavy structural payload fairing puts more stress on the second stage.
b.  Ground handling determined the need for a heavy structural payload fairing.
This is not clear to me.   Yes, the structural fairing is needed for ground handling.  But if the fairing alone can support the payload when horizontal, then it adds no buckling stress to the second stage when it's attached (provided the fairing is supported, as the SpaceX TEL seems to do).

Then they tilt it upright.  The heavier fairing now adds additional axial stress, but that's not the limiting factor.

Then the second stage is pressurized for flight.  From this point on,  buckling is not a problem, since the stage in now in tension.

So i agree with (b), the heavy fairing is required for ground handling.  But once they did that, they can then use its strength to remove handling loads from the second stage.  This suffices until the rocket is pressurized, at which point it is capable of handling the increases stress of the heavier fairing.

Both fairing and payload attach at the top ring of the second stage, so the fairing can't help.

If the fairing were to encompass the second stage, then yes.  But that's not the case with the F9.
You don't need to enclose the second stage for the fairing to help.  My understanding is that SpaceX can turn the fairing sideways with the payload inside, without attachment to the second stage.  Then clearly the fairing is taking all the torque.  Now you bolt the combination to the top of the second stage.  The fairing is still taking all the torque.  You keep supporting the fairing until you turn the stage upright.    So the second stage is never called upon to support the payload + fairing while horizontal.

If you need to remove the fairing support for some reason, you would need to at least partially pressurize the stage.  But it looks to me like the TEL is designed to support the fairing until the rocket is upright.

Offline Jim

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Re: SpaceX second stage secret sauce?
« Reply #19 on: 12/27/2016 01:58 am »
  But if the fairing alone can support the payload when horizontal, then it adds no buckling stress to the second stage when it's attached (provided the fairing is supported, as the SpaceX TEL seems to do).

Fairing doesn't support the payload when attached to the second stage.

The fairing is unsupported on the TEL

http://www.spacex.com/media-gallery/detail/129166/5246
« Last Edit: 12/27/2016 01:59 am by Jim »

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