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Liquid fuelled engine structure Q&A by DSA on 01 Jul, 2014 22:15
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This question is about liquid fuelled engine structure. I can see there are many types of rocket fuels like RP-1, LH2, liquid methane, etc. So how do LH2 engines and RP-1 engines are different as in terms of structural design? I've tried to search all around the web but I couldn't find about it.
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#1 by Burninate on 01 Jul, 2014 23:18
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Welcome to NSF!
Treat this as a research project, rather than asking us to write a whole guide for you on rocket science on the spot. I'll help you start off by giving you some keywords to google. Things to summarize in your notes that distinguish one vehicle from another:
Distinctions between solid, liquid, hybrid, nuclear thermal, and 'ion thruster' rocket engines
Distinctions between liquid rocket engines come down mostly to what sort of pumping & combustion goes on, and in what order at what pressure, as well as restart and throttle implications
Distinctions between fuel thermal conditions, performance ('specific impulse'), cost, convenience of ground operations, volatility for orbital storage, margins for tank dry mass, densities, types of tank pressurization and feed systems
Distinctions between expansion ratios of different bell nozzles used for different altitudes
Separately: A few hundred hours playing Kerbal Space Program learning orbital dynamics are always a good idea -
#2 by Burninate on 01 Jul, 2014 23:43
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To answer your specific question: Hydrogen - LOX engined launch vehicles get about 1.5x as much velocity every time they burn half their fuel, as RP-1 / kerosene - LOX engines get. We say 'The specific impulse ("Isp") is 450 seconds instead of 300 seconds'. Over many halvings of mass, this translates to a huge advantage. Downside #1 is hydrogen is an extreme cryogen - a very low temperature liquid - and it's boiling off rapidly in all circumstances unless you put it in an extremely high pressure, actively cooled, large tank on the ground. Downside #2 is hydrogen is very low-density relative to hydrocarbons - and tank mass tends to scale as the surface area, so the bigger the better. A hydrogen tank big enough for a first stage will tend to be enormous, a major engineering challenge just to get it to the pad, and very expensive to build. (While it would also have greater drag in the lower atmosphere, this is almost negligible for very large rockets). Hydrogen engines also tend to be somewhat harder to build because hydrogen embrittles metal both chemically and thermally, and creeps through seals much better than larger molecules.
This combination means most launch vehicles tend to use hydrogen for upper stages in very lightweight pressurization-dependent tanks, as a way to reduce initial mass, but can't use it for either the first stage, or (because of boil-off) for in-space propulsion more than a few days after launch. Kerosene tends to be a lower-to-the-ground option, and solid rocket boosters are mostly used for enough short-term thrust to get a big hydrogen/kerosene vehicle off the pad, in a mixed vehicle, and then dropped; This is because they're denser / cheaper / easier to handle than hydrogen, and it doesn't hurt the size of the vehicle as much to add a bunch of low-efficiency thrust at the beginning and drop it when it's exhausted.
Here's some launch vehicles that come to mind for comparison, with fuel for stages -
Atlas V _01- kerosene, hydrogen
Atlas V other configs - solid boosters, kerosene, hydrogen
Delta II - solid boosters, kerosene, hydrazine
Delta IV M - solid boosters, hydrogen, hydrogen
Delta IV H - hydrogen boosters, hydrogen, hydrogen
Falcon 9 - kerosene, kerosene
Falcon Heavy - kerosene boosters, kerosene, kerosene
Ariane V ECA - solid boosters, hydrogen, hydrogen
Ariane V other configs - solid boosters, hydrogen, hydrazine
Proton-M - hydrazine, hydrazine, hydrazine, hydrazine
Shuttle - solid boosters, hydrogen, hydrogen
Saturn V - kerosene, hydrogen, hydrogen
Soyuz - kerosene, kerosene, kerosene
H-II_ - solid boosters, hydrogen, hydrogen
Long March 3_ - hydrazine boosters, hydrazine, hydrazine, hydrogen
PSLV - solid boosters, solid, hydrazine, solid, hydrazine -
#3 by Jim on 01 Jul, 2014 23:54
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This question is about liquid fuelled engine structure. I can see there are many types of rocket fuels like RP-1, LH2, liquid methane, etc. So how do LH2 engines and RP-1 engines are different as in terms of structural design? I've tried to search all around the web but I couldn't find about it.
There isn't much visible difference between liquid rocket engines using different propellants. In some cases, the same engine has been able to operate on different propellants. The differences are in relative duct sizes, turbine blades and pump impellers. This is due to the different fluid properties of the various propellants -
#4 by john smith 19 on 02 Jul, 2014 06:58
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This question is about liquid fuelled engine structure. I can see there are many types of rocket fuels like RP-1, LH2, liquid methane, etc. So how do LH2 engines and RP-1 engines are different as in terms of structural design? I've tried to search all around the web but I couldn't find about it.
Welcome to the site.
With reference to RP1 to LH2 the there are several. LH2 is roughly 1/10 to 1/12 the density of liquid fuels like aviation kerosine or RP1. So you have to pump more of it to get the same mass flow rate (bigger ducts). Pumps are normally described in "head rise." If the pump generates 100 atm of pressure in metric units you need to work out how high a fluid column would be to get that. LH2 density, being about a SG of 0.07 relative to water is over 16x that of say LO2. That means driving both pumps off a single turbine with direct drive is normally impossible (you need some kind of gearing) and you need more stages. A 2 stage LO2 pump can get you 2000psi, you need maybe 4 to get LH2 up that high.
LH2 is also the only common(ish) liquid that can be compressed at these (relatively) low pressures (about 4% at these sorts of pressure IIRC), which absorbs pump power without raising system pressure. LH2 pumps are normally "axial flow" IE like a set of propeller blades on a shaft, rather than the in at the front, out at the side types, which tend to be easier to make and design (pretty much every rocket pump has used this style of impeller)
As others have noted it is an extreme cryogen. The LO2 tanks on the original Atlas (steel has 1/10 the thermal conductivity of Aluminium alloys) had no insulation and just let the water vapour freeze out. An uninsulated LH2 tank would freeze out Oxygen with consequent massive fire hazards.
In use LH2 needs to be kept liquid either by high pressure or high insulation. This means either verythick foam layers or "Vacuum jacketed line" piping, a 2 layer pipe with a vacuum in the walls, which is expensive, a PITA to reduce heat leaks at joins and of course is destroyed by any pinholes breaking the vacuum.
Or of course you could just wrap the whole engine in an evacuated box.
This is all much less of an issue for upper stage engines, that virtually start in a vacuum to begin with. However the residual heat in the engine ("heat" in this case means anything warmer than -253c) will flash boil the first fuel unless it's been pre chilled with circulating LH2 first.
The other thing is the mixture ratio (oxidizer mass divided by fuel mass) is much different to LO2/HC fuels so the injectors are different too.
Interestingly the Aerojet LR87 has the unique distinction of running LO2/RP1 (for the Titan 1 missile) hypergols (NTO/Aerozine 50) and LH2. The first 2 used basically the same hardware structure, but the LH2 needed a new design turbopump. IOW sizes and flows for LO2/RP1 were easily adaptable to hypergols, but LH2 needed a whole new piece of hardware.
Generally I suggest people read the NASA SP8000 series for all rocket engineering type stuff, available through NTRS. They may not be the most up to date papers, but they are driven by what has been done (more than you might realized) and what has been shown to work.
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#5 by DMeader on 02 Jul, 2014 18:36
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This question is about liquid fuelled engine structure. I can see there are many types of rocket fuels like RP-1, LH2, liquid methane, etc. So how do LH2 engines and RP-1 engines are different as in terms of structural design? I've tried to search all around the web but I couldn't find about it.
There isn't much visible difference between liquid rocket engines using different propellants. In some cases, the same engine has been able to operate on different propellants. The differences are in relative duct sizes, turbine blades and pump impellers. This is due to the different fluid properties of the various propellants
Any differences in the injectors? -
#6 by Jim on 02 Jul, 2014 21:14
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them too
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#7 by R7 on 03 Jul, 2014 14:58
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Any differences in the injectors?
Hollow post and sleeve injector elements are common in hydrolox engines. See for instance page 6 in this.
