While SpaceX, Blue Origin and other rocket providers target reusability, there’s a part of rockets that hasn’t changed very much in recent decades – the fuel.
Rocket fuels don’t change much
Many rockets today use liquid oxygen and/or liquid hydrogen – the same main fuels used in NASA’s space shuttle program, for example, which ran for 30 years between 1981 and 2011.
They are reliable, a known commodity, and properly handled, safe to work with. Accidents are rare.
Yet environmental regulations in Europe, and possibly in the United States, are causing change. The European Union might ban hydrazine, a toxic yet effective satellite propellant, in the early 2020s. NASA is also working on the hydrazine problem with the Green Propellant Infusion Mission, of which one aspect involves replacing hydrazine fuels with safer, low toxic, hydroxyl ammonium nitrate salt.
Toxins are on Stephen Matier’s mind as president and chief executive of Maritime Launch Services (MLS), a Halifax-based startup that plans to build a spaceport in Nova Scotia. The company submitted its mandated Nova Scotia Environment Focus Report last week and hopes to launch a Ukrainian CM4 derivative in 2021 or 2022.
The report will in part discuss rocket fuel, spacecraft fuel, spaceports, safety, and aims to alleviate any misconceptions held by those who don’t know the facts.
Matier used to examine rocket fuel alternatives as a former engineering manager at NASA’s White Sands Test Facility, where he worked for 16 years during the space shuttle era. “The drawback, the holdback, the thing from stopping that development is there is nothing [green] that is hypergolic – it won’t burn upon mixing,” he told SpaceQ. “That’s the key … they require spark plugs. Those are not reliable and add weight, complexity and risk.”
While he applauds the initiative to reduce hydrazine use, he points out a large drawback – it is a fuel that can be used precisely, in small bursts, for matters such as docking a spacecraft with the International Space Station or adjusting a satellite’s position in space. “You have to have those fine motor skills, if you will, to accomplish that docking and not smash into stuff,” Matier pointed out.
He added that it’s also a well-known fuel used in human spaceflight ever since the Gemini era – the mid-1960s program that prepared astronauts for docking and spacewalks for the moon landings of Apollo. Since engineers know what to expect with hydrazine now, it’s more difficult to replace – especially since it also requires redesign of spacecraft components.
Yet for many decades now, Canadians, Americans and Europeans have been concerned about the effect of toxins in the environment. Hydrazine isn’t the only fuel of concern. While China, India and Russia commonly use toxic unsymmetrical dimethylhydrazine or other hydrazine derivatives – which are stable and easy to store, but more toxic – first and second stages that fly in North America and Europe often use kerosene as the fuel and liquid oxygen as the oxidizer, although there are variant designs.
Matier points out that his company is an infrastructure company that is looking to build a spaceport, not rockets. The Ukranian CM4 launch vehicle which would be the first to use the spaceport, uses liquid oxygen and kerosene in the first stage like SpaceX and many others. The second stage uses nitrogen tetroxide and unsymmetrical dimethylhydrazine. Customer satellites on the CM4 are likely more often than not, use hydrazine as their fuel.
The MLS spaceport could also be used by other rockets in the future, including companies from the U.S., and possibly Canada.
“The good thing about [kerosene and hydrogen] is they are a hypergolic mixture. They are able to achieve ignition by themselves, without having to arrange some kind of spark,” said Tomislav Friščić, a chemistry professor at McGill who studies rocket propulsion. He added that ionic liquid (just developed in the past decade or so) is seen as less corrosive and toxic than traditional propellants, although it’s not commonly used.
New, more efficient fuels coming online could cut down the costs of rocket launches. Montreal based Reaction Dynamics, for example, has a secret and proprietary (patent pending) propulsion system that features a novel high-density impulse fuel. The fuel is about 35% more dense than kerosene (RP-1) and liquid oxygen that companies today use in their rocket engines.
While kerosene and liquid oxygen are effective in getting rockets off the ground, the drawback is their complexity. Reaction Dynamics promises that its fuel formulation will cut down vehicle components to less than 1,000 pieces, from the industry standard of nearly 100,000. Better yet, the fuel is safer to handle and easy to produce, all of which will reduce the cost of going to orbit.
“Being non-toxic, the propulsion is really interesting for some of our partners,” said Bachar Elzein, the chief executive and chief technology officer of Reaction Dynamics. “It’s really cheap and cost-effective, and we are still building up the performance.”
Less commonly, rockets may use solid fuels. A common type of solid is based on ammonia perchlorate as the oxidizer, with different kinds of fuels (such as aluminum.) Binders are required to help the fuel and oxidizer bind together, with one example being hydroxyl-terminated polybutadiene (HTPB).
Solid propellants are most notorious for being difficult to stop, once the propellant is ignited. But solids have been used even in human exploration, such as the boosters on the space shuttle. The boosters experienced a fatal failure in 1986 after the O-ring joint failed in cold temperatures during the shuttle Challenger’s launch. The joint was redesigned, temperature ranges for the shuttle were restricted to warmer climes, and the booster worked for the remainder of the program.
Hybrid engines can use hydrazine derivatives and dinitrogen tetroxide, which are powerful but are also highly toxic and volatile. A safer alternative could be nitrous oxide and propane, such as what is used for Sierra Nevada’s Dream Chaser. “The drawbacks of such systems is often you don’t use all of your propellant,” Friščić said. But there are numerous advantages to hybrids, such as being easier to store, simpler to manufacture and easier to transport.
Rocket propulsion may evolve quickly as more commercial companies enter the market, but there is that fine line they must walk between innovation to impress investors, and using proven technology with known costs. This means it is difficult to predict the pace of change in rocket fuels in the coming years, even with commercial incentive to reduce the cost to orbit.
There is also the ongoing concern about what would happen in the event of an accident. Friščić pointed out that although accidents are rare, we cannot relax our vigilance – it takes “a lot of investment and effort” to ensure that rockets lift off safely. That’s also true of other industries with dangerous chemicals or products.
NASA’s Green Propellant Infusion Mission is scheduled to launch on a SpaceX Falcon Heavy in the summer. The results of this demonstration mission could eventually lead to greener fuels being used. In the meantime, launch providers and their customers will continue to use proven fuels, including hydrazine.
Update April 2:
We have corrected the following sentence to replace the word “Oxygen” with “Hydrogen”. The good thing about [kerosene and oxygen] is they are a hypergolic mixture. They are able to achieve ignition by themselves, without having to arrange some kind of spark,” said Tomislav Friščić.
“Many rockets today use liquid oxygen and/or liquid hydrogen”
You might want to rephrase that. There is no choice between oxygen or hydrogen. A rocket can not run on oxygen alone. You always need oxygen and a fuel.