Will natural gas aviation become a viable transportation option? The answer has been an emphatic “yes” since the early 1980’s, when a Soviet aircraft company called Tupolev conducted over 100 natural gas powered flight tests of a modified commercial aircraft.
In this blog, I’ll de-mystify the idea of using natural gas as an aviation fuel, and explain the remaining challenges.
Why switch away from jet fuel?
I like to say that natural gas is the gift that keeps on giving. It’s both cleaner, cheaper and lighter than diesel or petroleum. No other alternative aviation fuel can make two of these claims, let alone all three.
Natural gas is cheaper because of newfound supply: We now have approximately 150 years of natural gas reserves vs. 50 years for jet fuel. The spot price of natural gas is about 1/3 that of jet fuel and the final “pump” price becomes about ½ that of jet fuel once infrastructure and delivery are accounted for.
Since natural gas is cleaner, the turbines last longer and the impact on air quality is substantially less: LNG contains roughly 80% less sulfur and 80% less particulate matter than oil and diesel which helps to reduce respiratory illness and global warming. Think of it this way: would you rather walk behind a diesel transit bus or a natural gas transit bus?
Conventional jet fuel is a very close cousin to diesel in terms of its molecular makeup, pollutant content and yes, its vulnerability to crude oil prices. However, because natural gas is not tied to the crude market, the use of this clean fuel will likely result in lower airfares and increased economic stability for airlines.
How does a natural gas airplane work?
- First, natural gas must be cooled into liquefied natural gas (LNG) at -260df. LNG is about half the density of jet fuel but is also ~20% lighter for the same amount of energy.
- The LNG tanks are vacuum insulated to keep the temperature low and are pressurized at a lower level than CNG tanks: roughly 70-150 psi versus 3600 psi. The LNG fuel and its onboard fuel system together can be lighter or heavier than the equivalent jet fueled system depending on how much fuel is stored and how the system is integrated into the aircraft.
- Next, the LNG must be pumped to the engine and vaporized back into a gas before entering the engine combustion chamber.
Will an entirely new aircraft engine be required?
The answer is no! We can use the very same aircraft engines with a few minor modifications. In fact, we have been using aircraft engines with natural gas for years – as turbines in natural gas power plants. Instead of turning a turbofan or a propeller, the engines are turning a generator. The modifications to jet engines required for natural gas aviation mostly have to do with the combustor to account for the differences in combustion temperatures and fuel injection. To clarify, you can use an existing engine but you need a new airframe in order to get the earth-shattering cost and emissions savings that will change the industry.
The challenge of density
Density is an issue for natural gas aircraft, and many people have researched solutions of where to put the LNG fuel on board the aircraft:
- The second place entry in this year’s Airbus Fly Your Ideas competition from RMIT in Australia showcased innovative external tanks with laminar flow.
- As a graduate student at MIT, I participated in a team that proposed an internal cargo bay tank in the 2011 Airbus Fly Your Ideas competition, which we later published.
- Not long after Savion was founded as General Cryo Corp during MIT’s Clean Energy Prize competition, we relented and accepted that a hybrid LNG-jet fueled aircraft just didn’t cut the mustard compared to new forecast aircraft or when compared to a new airframe designed with LNG in mind.
But what about Tupolev? Why didn’t they continue their natural gas aviation program?
There are no official answers to that question, but I believe the main reason is that a viable, disruptive adoption pathway was not identified:
- LNG was not currently stored at sufficient scale on airport grounds
- Hybrid jet-LNG designs did not offer far superior performance.
- The collapse of the Soviet Union likely hindered R&D funding for the project.
Today, we know a lot more about what it takes for technology adoption to occur and we can use the lessons learned from previous attempts. At Savion, we think we have found the right adoption path and are developing an aircraft and a system level solution around it.
If we are successful, get ready for cleaner skies, lower fares, and an aviation revolution.
Jonathan M. Gibbs is an aerospace engineer and the founder and president of Savion Aerospace Corporation