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#1 2015-08-17 15:49:44
- RobertDyck
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- From: Winnipeg, Canada
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Liquid stage propellant tank design
I noticed the second and third stages of a Saturn V use a common bulkhead between cryogenic tanks. Modern stages use seprate tanks. Why?
Common bulkhead reduces mass for several reasons. This design placed the LOX tank on the bottom, LH2 on top. The top of the LOX tank was domed, like any other LOX tank. But that means the LH2 tank has no bottom, instead the LOX tank is the bottom of the LH2 tank. Eliminating the bottom of the LH2 tank reduces mass. It also means the inter-tank support ring does not exist. Propellant feed from the LOX tank is the usual location: a sump at bottom centre. And the LOX tank has the usual anti-vortex baffle in the sump. The LOX tank also has anti-slosh baffles at the widest part of the tank. But the LH2 tank has suction lines on the bottom of the tank, which is the outer edge since the LOX tank is domed into it. This reduces propellant feed line length as well. The Shuttle ET placed the LOX tank on top, but that meant a propellant feed line from the LOX sump to the outside of the tank. The LOX line had to be longer, because it had to reach from the centre to the outer edge. And the LH2 tank is longer, so running a LOX feed line the entire length of the LH2 tank is long. Shuttle had a short LH2 line because it was on bottom, but the LOX line was very long. Saturn V stages had a short LOX line because the sump was on bottom near the engines, but he LH2 line was also short because the LOX tank was short. And of course the LH2 tank had no anti-vortex baffle, because a vortex would be helpful, forcing propellant into the feed lines.
So weight savings of a common bulkhead: no LH2 bottom bulkhead, no inter-tank, shorter feed lines, no LH2 anti-vortex baffle.
Because LOX and LH2 have different temperatures, the common bulkhead had insulation. This document from MSFC NASA shows the bulkhead for S-II had a "forward facing sheet, honeycomb insulation, aft facing sheet".
The S-1C stage used RP1 and LOX, so had separate tanks. You don't want to chill RP1 to cryogenic temperature. But why do modern designs keep LOX and LH2 tanks separate? The only reason I can think of is extended time sitting fuelled on the launch pad. A common bulkhead may cause more rapid LH2 boil-off. During launch, propellant will siphon through the feed line, and reducing volume results in reducing pressure, which will help keep propellants cold. So the only reason I can think of is reducing continuous LH2 feed to replace boil-off while sitting on the launch pad. And placing the colder tank on bottom reduces heat transfer. Again, reducing boil-off, but while on the launch pad that boil-off is replaced by continuous feed from the gantry. Is that it? Have modern launch vehicles incurred greater dry mass just to reduce LH2 replacement while sitting on the pad?
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#2 2015-08-17 16:04:56
- GW Johnson
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Re: Liquid stage propellant tank design
Would not surprise me if reducing on-pad makeup LH2 were exactly the reason you state.
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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#3 2015-08-17 18:43:21
- SpaceNut
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Re: Liquid stage propellant tank design
There appears to be several patents relating to Fuel tank system Common bulkhead cryogenic propellant tank of which we still have an active useage Centaur is a rocket stage designed for use as the upper stage of space launch vehicles and is currently used on the Atlas V
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#4 2015-08-17 18:48:36
Re: Liquid stage propellant tank design
The Falcon 9 and Ariane 5 core also use common bukhead design.
Bob Clark
Old Space rule of acquisition (with a nod to Star Trek - the Next Generation):
“Anything worth doing is worth doing for a billion dollars.”
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#5 2015-08-17 20:39:58
- RobertDyck
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Re: Liquid stage propellant tank design
Ok, so why is Boeing afraid of common bulkhead for their Delta IV upper stage? Which will be re-purposed as the upper stage for SLS block 1.
And the Orion 606 Service Module (now replaced by the European ATV-based SM), was designed to require a fairing. A separate fairing instead of integrated with the tank is heavier, but can be discarded. The integrated fairing doubles as micrometeor shield. And an integrated stage skin can allow pressurized propellant tanks to provide structural support. A separate fairing must provide all structural support during launch. Change from the previous Orion service module made several changes: from LCH4/LOX to MMH/N2O4, and change from integrated stage skin to discardable fairing. I see both as negative. However, using Apollo as the baseline, the Apollo SM did have separate fuel and oxidizer tanks (I believe 2 of each), not integrated with stage skin like Saturn V stages. So asking for that is asking for a major improvement.
Yea, I guess a service module has a lot of stuff. For the Apollo SM, each RCS thruster quad had its own propellant tanks. And there were oxygen and hydrogen tanks for fuel cells. And sections of skin were actually radiator panels for the electrical system, or environmental system. The SM for CST-100 is built similar to the Apollo SM, just smaller. It has propellant for rendezvous with ISS, not the Moon. Is expecting that from Orion asking too much?
The ATV-based service module has multiple tanks. Just like Apollo or CST-100. But Apollo and CST-100 have integrated radiators, and the stage skin acts as aerodynamic fairing, stage adapter for structural support during launch, micrometeor shield, and sun shade for thermal control. Why does Orion need a separate fairing? If one goal was to go to a single fuel tank and single oxidizer tank, for mass reduction, then do so within the stage skin. And just as Saturn V stages integrated the propellant tank with stage skin for structural support, do the same with the SM. I noticed the Orion 606 SM has different diameter propellant and oxidizer tanks, so I thought "common bulkhead" was an issue. Perhaps common bulkhead for the SM would allow integration with stage skin.
Looking at SLS and Orion has left me frustrated.
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#6 2018-12-28 12:29:03
- SpaceNut
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Re: Liquid stage propellant tank design
This is as important as the fuel types and engines used to support why the choices are made for the materials and construction technique.It is something that BFR is in the process with for its designing of its reuseable second stage.
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