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.
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.
Propulsion is usually 50% of pmf. Doubling the propulsion T/W might help.
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.
. 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?
Quote from: LouScheffer on 12/26/2016 06:58 pmSo 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.
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.
Quote from: LouScheffer on 12/26/2016 06:58 pm. 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.
Quote from: LouScheffer on 12/27/2016 12:55 amPlus 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.
Quote from: Jim on 12/27/2016 01:02 amQuote from: LouScheffer on 12/27/2016 12:55 amPlus 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.
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?
Quote from: LouScheffer on 12/27/2016 01:22 amQuote from: Jim on 12/27/2016 01:02 amQuote from: LouScheffer on 12/27/2016 12:55 amPlus 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.
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).