So ULA and Blue Origin have announced they are designing a LOX/LNG rocket engine (link). From what I understand, LNG is just a less pure form of methane (CH4). Is this correct? Why are they announcing a LNG engine, not a methane engine?
In theory, Liquid Natural Gas (LNG) is less pure and may contain smaller amounts of higher saturated Hydrocarbons (Alkanes) like Ethane, Propane, Butane, and also Nitrogen, Carbon Oxides, and so on.
In practice, even Liquid Methane propellants may be mixed with other compounds intentionally for combustion stability, performance or to control exhaust temperature, and ullage gases (Nitrogen or Helium) providing propellant pressure and preventing sloshing may mix with it in smaller amounts by design.
From performance standpoint (specific impulse), there ought not be much advantage in favor of Liquid Methane. LNG with its higher Hydrocarbons introduces slightly more Carbon to the combustion so the exhaust temperature might also slightly increase with more Carbon Dioxide combustion products, demanding throttling the engine back a bit. But since rockets usually fly with a fuel rich mixture, and fuel rich burning of LNG produces more free Hydrogen (that's how most of it is actually produced), you'd technically also get slightly higher exhaust products velocity as the secondary burn, as the exhaust products mix with atmospheric Oxygen. This should even things out, as long as you design for somewhat more pronounced combustion instabilities of LNG.
So not much of a difference there, and perhaps the most notable one is that LNG uses natural in its name, which might go a mile or two farther on fumes alone in the increasingly more important realm of public relations.
Edit to add: This is related so I wanted to add it to this thread:
- 17 Sep 2014, United Launch Alliance and Blue Origin Announce Partnership To Develop New American Rocket Engine (YouTube video)
During the press conference announcing United Launch Alliance (ULA) and Blue Origin partnership in developing BE-4 (Blue Engine 4), a US built successor (but not a 1:1 replacement) to Russian built RD-180 engine, Jeff Bezos (entrepreneur and founder of Blue Origin) and Tory Bruno (ULA President) discuss a few details on the announced new engine. A few takeaways are:
- BE-4 will use Oxygen enriched stage combustion cycle
- Using Liquified Natural Gas (LNG) as fuel and Liquid Oxygen (LOX) as oxidizer
- Booster engine capable of ~ 550,000 lbf of thrust
- Single turbopump design
It is yet unclear which vehicles it is planned to be used on, pending engine's further validation. It is however probably not too bold of an assertion that the first family of launch vehicles it will be used on will be the same one that currently depends on supply and availability of RD-180 engines and the vehicles that ULA uses to provide assured access to space to United States Air Force (USAF) and its Evolved Expendable Launch Vehicle (EELV) program - so Atlas V.
Considering RD-180's thrust of 860,568 lbf (3.83 MN) with its dual nozzle design, it's also probably fair to assume that BE-4 powered booster stages will initially use two of these engines for a total thrust of roughly 1,100,000 lbf (sea-level) on new EELV workhorses delivering heavier payloads to higher orbits and a single engine for lighter payload applications, possibly as a successor to Delta II (more than double the thrust of Rocketdyne RS-27A could simplify its dependence on solid rocket boosters).
Liquid Natural Gas is primarily CH4 (Methane). However since it is derived from natural sources, not synthesized, it has other components in low quantities.
Low enough that a power plant, stove, dryer, or even car engine would probably not care.
But a rocket engine is much more sensitive, using TONS of fuel a second and anything less than purity can be a real issue.
RP1 is similarly basically kerosene. But RP1 is really just a refined form of kerosene, minimizing other non-desired components.
Technically CH4 or Methane is different than LNG, but in the common parlance, LNG is becoming cheap and more available due to fracking and horizontal drilling, people are more commonly aware of LNG.
Thus it is likely a marketing decision. LOX/LNG people will comprehend as cheap fuel, where as Methane of CH4 looks expensive and exotic. But by designing for LNG, they save costs on fuel, which in a reusable world, might become a major factor. Interestingly SpaceX, with its Raptor engine is still sticking with Methane instead of LNG.
It's purely economics. Price is key for the space launch business, and one of the most important components of launch price is fuel. The price of kerosene is about 20USD per million btu, and LNG can be bought in the US for around 6USD per million mBtu.
Back before fracking became mainstream it was thought that the future was in liquidizing natural gas from fields where pipelines were not possible and transporting it cryogenically. There was a big market in the US and Japan, with China looking to become bigger then both in the future. The economics seemed to be there so hugely expensive liquification plants started to be built all over the world. Then fracking came on the scene and the US gas price went through the floor and LNG lost its biggest market. This has caused the LNG pricing to drop. A substantial portion of LNG receiving terminals in the US are in the gulf coast, convenient for the proposed launch sites of both companies. So what you have is loads of LNG to be bought at competitive prices with a delivery infrastructure which is already in place.
Something that kerosene does have going for it is energy density, kerosene has somewhere around 50% more energy per unit of volume than LNG, so you need bigger fuel tanks with corresponding weight penalties. This means you need more fuel to lift more weight. However at more than 3 times the cost LNG must be still worth it or these companies wouldn't be doing it.