Currently, several methane-fueled rockets are in a race for orbit. With SpaceX’s Starship, United Launch Alliance’s (ULA) Vulcan, and Rocket Lab’s Neutron, all of the most active US launch vendors have committed to using methalox-methane and oxygen.
Upcoming launch vehicles such as Blue Origin’s New Glenn and Relativity Space’s Terran family are also on their way to flight, while Landspace’s Chinese ZhuQue-2 rocket might even be a favorite to fly before any of the US vehicles. .
The reason methane-fueled rockets haven’t flown before is a matter of chemistry and technical complexity. But while new designs prioritize reuse as well as on-site resource utilization (ISRU) for missions to Mars, the combination of methane and oxygen has become the norm for next-generation launch vehicles. .
Combustion stability is particularly problematic with respect to the two most common liquid propellant combinations: kerolox (kerosene and oxygen) and hydrolox (hydrogen and oxygen). The boiling points of hydrogen and kerosene Rocket Propellant-1 (RP-1) are very different from those of liquid oxygen (LOX). However, the boiling point of methane is very close to its oxidant.
Raptor 2 generates more than 230 tons of thrust at sea level, compared to 185 tons for Raptor 1 pic.twitter.com/o1Rqjwx6Ql
—SpaceX (@SpaceX) February 11, 2022
For a hydrogen engine, combustion occurs in a state where oxygen droplets are surrounded by hydrogen gas molecules upon ignition, and the reverse occurs for RP-1. For methane, the boiling points are similar, meaning there is no obvious state the two molecules will be in during vaporization and combustion. This can lead to combustion instability and make methane more difficult to use as rocket fuel.
While developing the engines that will power these next-generation vehicles has not been without setbacks and challenges, recent advances in rocket propulsion technology have made methane engines feasible. These new development efforts have been driven by new reuse 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. Methane rocket fuel production is a possibility on Mars using the “Sabatier reaction”, which can produce water and methane from hydrogen and carbon dioxide. This would allow the Mars Natural Resources ISRU to enable new missions by not needing to bring all the required fuel from Earth.
Another reason to use methane is cost. Almost every next-generation launch vehicle that will use methane is pursuing some form of reuse idea. Both Neutron and New Glenn are aiming, at least initially, for partially reusable vehicles, using propellant first stages and non-reusable upper stages. Starship and Terran R, on the other hand, are intended for full reuse without consumable steps. Even Vulcan may still have engine recovery in its future evolution plans.
In addition to reusability, manufacturing improvements have also reduced the cost of building and operating launchers. And as these factors decrease, the factor that becomes more important is fuel economy. If a rocket launch costs $250 million, it doesn’t matter if the fuel costs two or four million dollars per launch. But while the total is $25 million per launch, fuel represents a much larger percentage of overall launch costs. And methane is the cheapest of the three liquid fuels, beating hydrogen and RP-1 by far.
Another factor, compared specifically to RP-1 engines, is coking. RP-1 does not burn cleanly like hydrogen or methane, but leaves behind other substances, comparable to gas in a car. These remnants can get stuck in the motor and nozzle and coat it over multiple uses. This effect is visible on used Falcon 9 stages, as the rocket flies through its exhaust during reentry and landing burns, leaving the remnants of the combustion outside the rocket.
Prior to the reuse era, these Kerolox engines were used only once, so coking did not become an issue as new engines were built for each flight. Coking is not an obstacle to reuse; after all, SpaceX’s kerosene-fueled Falcon 9 continues to break reuse records. But since the designs add rapid and complete reuse, the reduction in coking will reduce the time and effort needed to prepare salvaged vehicles for re-flight.
Although hydrogen is a much cleaner fuel, it has its own reuse issues, particularly density. Hydrolox is the least energy dense propellant of the three, which means a reusable hydrolox stage must be much larger than those powered by kerolox or methalox. This is where another advantage of methalox appears: it is a clean-burning, dense and efficient propellant. Not only does methane provide a similar density to kerosene, it also offers a specific impulse (yield) closer to that of hydrogen rocket engines.
Since the temperatures of liquid oxygen and liquid methane are so similar, applying a common bulkhead between the two tanks of 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, which means that common bulkhead design is a feasible way to reduce vehicle mass.
These new methalox launchers should begin making their orbital debut this year. While some of them still have a lot of development work to do, others are already ready to fly, although it’s not yet clear which one will be the first metalox-powered vehicle to reach orbit.
Perhaps the most important is Starship, built by SpaceX. With its 33 methane-fueled Raptor engines, it is an excellent example of the advantages of methalox. Not only is it designed to bring payloads to Mars and use the Sabatier reaction to bring humans and cargo back, it’s also designed to fly multiple times without major retrofit. Currently, the entire Starship system is set to attempt its first flight in 2022 and is one of the candidates for the first methane-fueled rocket to reach orbit.
Another candidate is Relativity Space’s Terran 1. The smallsat launcher is powered by the Aeon 1 engine, which will inform the larger, reusable Aeon R engine design. 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 vehicle Terran 1 is still slated to fly in 2022.
The latest US contender for the first methalox orbital rocket is ULA’s Vulcan rocket, powered by Blue Origin’s BE-4 engine: the same powerplant that will power New Glenn. The expendable launcher will use a hydrogen-powered upper stage, but the methane-powered first stage will be an essential part of the orbital launch system. Vulcan’s first flight is still scheduled for this year.
While Blue Origin is also developing a metholox-powered rocket at New Glenn, that 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 Archimedes methox engine, which is due to begin testing this year for a mid-decade debut on Neutron.
Pleiades-1B captured this image from a more recent launch site at the Jiuquan Satellite Launch Center on 2022-01-15 04:26:24 UTC.
It looks like there may be a stage (or possible mockup) of LandSpace’s ZQ-2 rocket on the pad. pic.twitter.com/plJctAP72E
— Harry Stranger (@Harry__Stranger) January 17, 2022
Outside of the United States, there is another contender for the first Metholx rocket in orbit: China’s Zhuque-2. Powered by the TQ-12 methox engine, the gas generator engine is expected to debut this year. Recently, hardware has arrived on the pad that is linked to a scout for crates, and the ZQ-2 rocket could have a very real chance of being the first methane-based rocket in orbit, racing against Starship, Vulcan and Terran 1.
(Main photo: Ship 20 and Booster 4 stacked at the orbital launch site next to the tank farm that will supply the orbiting ships with methane and oxygen before launch. Credit: Mary (@bocachicagal) for NSF)