Right now, several methane-fueled rockets are in a race to orbit. With Starship from SpaceX, Vulcan from United Launch Alliance (ULA), and Neutron from Rocket Lab, all of the most active US launch providers are committed to using methalox-methane and oxygen.
Upcoming launchers such as New Glenn from Blue Origin and the Terran family from Relativity Space are also on the way toward flight, while the Chinese ZhuQue-2 rocket from Landspace may even be a favorite to fly before any of the American vehicles.
The answer to why methane-fueled rockets have not flown before is a matter of chemistry and engineering complexity. But as new designs prioritize reusability as well as in-site resource utilization (ISRU) for missions to Mars, the combination of methane and oxygen has become the standard for next-generation launch vehicles.
Combustion stability is especially problematic in comparison to the two most common liquid propellant combinations: kerolox (kerosene and oxygen) and hydrolox (hydrogen and oxygen). The boiling points of hydrogen and Rocket Propellant-1 (RP-1) kerosene are very different from that of liquid oxygen (LOX). However, the boiling point of methane is very close to its oxidizer.
Raptor 2 generates more than 230 tons of thrust at sea level, up from Raptor 1’s 185 tons pic.twitter.com/o1Rqjwx6Ql
— SpaceX (@SpaceX) February 11, 2022
For a hydrogen engine, the combustion occurs in a state where oxygen droplets are surrounded by hydrogen gas molecules during ignition, and the reverse occurs for RP-1. For methane, the boiling points are similar, which means there is no obvious state in which both molecules will be during vaporization and combustion. This can lead to combustion instability and makes methane harder to work with as rocket fuel.
While the development of the engines that will power these next-generation vehicles has not been without setbacks and challenges, recent advancements in rocket propulsion technology have made methane engines feasible. These new development efforts have been driven by new reusability goals and new space destinations, such as Mars.
Methane is the best possible propellant to use in a situation requiring refueling on the Red Planet. Producing methane rocket fuel is a possibility on Mars with the help of the “Sabatier reaction,” which can produce water and methane from hydrogen and carbon dioxide. This would allow ISRU of Mars’ natural resources to enable new missions by not needing to bring all of the fuel required from Earth.
Another reason for using methane is cost. Almost all of the next-generation launchers that will use methane are pursuing an idea of reusability in some shape or form. Neutron and New Glenn are both, at least initially, aiming for partially reusable vehicles, using propulsively landed first stages and expendable upper stages. Starship and Terran R, on the other hand, are planned for full reuse with no expendable stages. Even Vulcan may still have engine recovery in its future evolution plans.
In addition to reusability, manufacturing improvements have also decreased the cost of building and operating launch vehicles. And as these factors decrease, the factor that increases in importance is fuel economy. If a rocket launch costs $250 million, it does not matter if the fuel is two or four million dollars per launch. But if the total is $25 million per launch, fuel becomes a much larger percentage of overall launch costs. And methane is the cheapest of the three liquid fuels, beating both hydrogen and RP-1 by far.
Another factor, compared specifically to RP-1 engines, is coking. RP-1 does not burn clean like hydrogen or methane, but leaves other substances behind, comparable to gas in a car. These remains can get stuck in the engine and nozzle and cover it over multiple uses. This effect is visible on used Falcon 9 stages, as the rocket is flying through its exhaust during the reentry and landing burns, leaving the remains of the combustion on the outside of the rocket.
Before the age of reusability, these kerolox engines were only ever used once, so coking did not become an issue as new engines were built for every flight. Coking is not a show-stopper for reusability; after all, SpaceX’s kerosene-fueled Falcon 9 continues to break reusability records. But as designs add rapid and full reusability, reducing coking will reduce the time and effort required to prepare recovered vehicles for re-flight.
While hydrogen is a much cleaner-burning fuel, it has its own problems for reusability, specifically density. Hydrolox is the least energy-dense propellant of the three, meaning a reusable hydrolox stage must be much larger than those fueled by kerolox or methalox. It is here that another advantage of methalox becomes apparent: it is a clean-burning, dense, and efficient propellant. Not only does methane offer a similar density to kerosene, but it also offers a specific impulse (efficiency) that is closer to those of hydrogen rocket engines.
Since the temperature of liquid oxygen and liquid methane are so similar, the application of a common bulkhead between the two tanks within a stage also becomes easier. With hydrogen and LOX and their very different boiling temperatures, the shared tank area can cause thermal issues. With methane this is not the case, meaning the common bulkhead design is a feasible way of reducing the mass of vehicles.
These new methalox launch vehicles are slated to begin making their orbital debuts this year. While some of them have a significant amount of development work remaining, others are already near flight readiness, though it’s not yet clear which one will be the first methalox powered vehicle to achieve orbit.
Perhaps the most prominent one is Starship, built by SpaceX. With its 33 methane-fueled Raptor engines, it is a prime example of the advantages of methalox. Not only is it designed to bring payloads to Mars and utilize the Sabatier reaction to bring humans and cargo back, but it also is designed to fly many times without major refurbishment. Currently, the entire Starship system is planned to attempt its first flight in 2022 and is one candidate for the first methane-fueled rocket to achieve orbit.
Another candidate is Terran 1 of Relativity Space. The smallsat launch vehicle is powered by the Aeon 1 engine, which will inform the design of the larger and reusable Aeon R engine. This larger version will power Relativity’s second rocket, Terran R, which will be fully reusable and fly no earlier than 2024. The smaller, expendable Terran 1 vehicle, is still planned to fly in 2022.
The last American contender for the first orbital methalox rocket is ULA’s Vulcan rocket, powered by Blue Origin’s BE-4 engine: the same powerplant which will power New Glenn. The expendable launch vehicle will use a hydrogen-fueled upper stage, but the methane-fueled first stage will be a critical part of the orbital launch system. Vulcan’s first flight is still currently slated for this year.
While Blue Origin is also developing a methalox-powered rocket in New Glenn, this vehicle will not be ready this year, and Blue Origin must supply ULA with BE-4 engines for Vulcan before New Glenn.
Meanwhile, Rocket Lab’s Neutron rocket will be powered by the methalox Archimedes engine, which is to begin testing this year for a debut on Neutron in the mid-decade period.
Pleiades-1B captured this image of a newer launch site at the Jiuquan Satellite Launch Center on 2022-01-15 at 04:26:24 UTC.
It appears there may be a stage (or possible mockup) of LandSpace's ZQ-2 rocket at the pad. pic.twitter.com/plJctAP72E
— Harry Stranger (@Harry__Stranger) January 17, 2022
Outside of the US, there is one more contender for first methalox rocket to orbit: the Zhuque-2 from China. Powered by the TQ-12 methalox engine, gas-generator engine is planned to debut this year. Recently, hardware arrived at the pad that is related to a pathfinder for checkouts, and the ZQ-2 rocket could have a very real chance to be the first methane-based rocket in orbit, racing against Starship, Vulcan, and Terran 1.
(Lead photo: Ship 20 and Booster 4 stacked on the orbital launch site next to the tank farm which will provide orbital Starships with methane and oxygen before launch. Credit: Mary (@bocachicagal) for NSF)