Rocket Fuel: Is it rocket science?

The History

Fuel can be defined as a substance that burns when exposed to oxygen to create heat or power. In this case, a rocket fuel produces enough energy to allow a launch vehicle to reach space.

On the 16th of March 1926, Dr. Robert H. Goddard launched the first ever liquid-fuelled launch vehicle from Auburn, Massachusetts. Goddard began his work years before at Worcester Polytechnic Institute where he found a passion for space technology, and by 1914 he had acquired two US patents for both solid and liquid-fuelled rockets. The privacy of his research left it up to predecessors to rediscover these technologies, but his work shaped our thinking about space flight and cleared the way for the development of launch vehicles and their propellants.

Rocket engines contain a combustion chamber and nozzle. To launch a vehicle into space, we require fuel and an oxidiser to be mixed and pumped into the combustion chamber. Here a change in pressure occurs creating a hot exhaust, and when this passes into the engine nozzle a downward thrust is produced.

Newton’s Second Law states that every action has an equal and opposite reaction. Therefore, when the thrust produced by the burning fuel creates a force downwards into the surroundings the rocket, in turn, accelerates upwards. This space technology has held its place in rocket science for years, but many specifications during launch vehicle design can affect how efficiently our rockets are propelled into space. One of these main varying factors is the type of fuel used.

Solid and Liquid Rocket Fuels

Any bipropellant rocket requires the combination of a fuel and an oxidiser, where the fuel can be found in either a liquid or solid-state. There are various characteristics assigned to each fuel type, and so potential fuels are tested, analysed, and selected based on which best meets the launch vehicle and mission requirements.

Fuels and oxidisers are stored inside the engine and the state of the propellants determines how this is done. Liquid fuel and a liquid oxidiser will be stored separately in tanks, before being pumped into a combustion chamber where they are burned to produce thrust. In a simpler solid fuel motor, the solids can be found compressed in a solid casing with an oxidiser and a binder, which is also consumed as fuel, and this will burn when ignited.

Solid fuels appear in the form of a powder. Various solid fuels have been tested in combination with different oxidisers, but only a few are still in use while others continue to be investigated. The most common and desirable fuel source tends to be powdered aluminium. During combustion, the fuel is oxidised into aluminium oxide, and the aluminium increases combustion temperature and density of the propellant, thus the specific impulse of the vehicle (how much thrust is produced for a given amount of propellant over time) is increased. Other solid materials such as boron and beryllium have beneficial characteristics however features such as toxicity make them unlikely choices.

For fuels in a liquid state, most petroleum-derived hydrocarbons have the potential to be used as rocket fuels- the most common of these being kerosene. The properties of a liquid fuel depend massively on the crude oil from which they were obtained, and the processes used to transform the fuel into its desired state. The refined kerosene mixture RP-1 has a narrow range of densities and vapour pressure and has been regularly used with liquid oxygen as the propellant for rocket engines.

Liquid hydrogen fuel is another source that gives a high performance when mixed with a good liquid oxidiser. It was used in the Centaur upper stage, Space Shuttle main engine, and could be found in the upper stages of engines produced in Japan, Russia, Europe, and China.

Advantages of Solid and Liquid Fuels

Liquid fuels can be seen as the most advantageous fuel choice, however, both present numerous benefits.

Solid rocket fuels traditionally allow for a far simpler design with fewer moving parts. They are storable (non-cryogenic) and can be ready to operate quickly. Solid materials also have higher densities, creating room for a more compact engine design and hence a smaller vehicle overall with less drag during flight.

Liquid fuels offer the highest specific impulse for a fixed propellant mass, which in turn increases the maximum velocity of the vehicle. The liquid substance allows more flexibility- a big selling point in launch vehicle development is the ability to test at full thrust on the ground before flight, and take control where the engine can be throttled, stopped, and restarted as required. Some liquid fuels can also be stored for a very long time and operated quickly. Most liquid fuel and oxidiser combinations are clean-burning and have a nontoxic exhaust which, compared to the burning of solid powdered metal fuels, is advantageous from an environmental perspective.

Disadvantages of Solid and Liquid Fuels

Despite the benefits of these fuels as a contribution to our space innovations, there are disadvantages that can often become the deciding factors of a chosen rocket fuel.

Solid rocket fuels require an ignition system, where liquid fuels do not. Once ignited this process cannot be stopped and the fuel must burn until exhausted entirely, which also means the system cannot be tested before use. Solid fuels stand to increase the risk of explosion and are more likely to have a smoky exhaust which can be toxic.

Whilst liquid fuels present disadvantages such as the potential for hazardous spills or leaks, one of the biggest issues discovered with such fuels is the relatively complex design, with an increased likelihood of things going wrong. If the liquid substance is cryogenic the fuel cannot be stored for long, and so the foundations for cryogenic storage facilities must be set up at the launch site. This is an area where Skyrora stands out from market competitors, with the propellants of our Skyrora XL vehicle designed to be stored for a longer launch window which is crucial for UK launches where weather conditions make go-for-launch difficult.

Skyrora puts significant effort into making our end-to-end launch vehicle service as environmentally friendly as possible, but an industry such as this would appear to be contradictory after reviewing the negative environmental impacts of large quantities of fossil fuel consumption. This is not the case for Skyrora, however, as we introduce our alternative fuel called Ecosene.

Introducing Ecosene

At Skyrora our mission is to secure the future of Earth by enabling environmentally conscious access to space. Ecosene is a proprietary IP-owned product of Skyrora, with which plastic waste is converted into high-grade kerosene for use by the aviation and aerospace industries. It provides several environmental benefits as a replacement for traditional oil-derived fuels, whilst maintaining the standards required for a Jet A-1 fuel.

Skyrora’s Ecosene technology uses catalytic pyrolysis and hydrotreatment to convert plastics including Polypropylene (PP), Polyester (PE), and Polystyrene (PS) with reduced greenhouse gas emissions in the conversion process. Plastics have become a great threat to life on Earth and this is a problem we must come together to change. This innovation meets the essential need for a global change in attitude towards fossil fuel consumption and plastic waste but also serves the increasing demand for fuel in the launch vehicle industry. By offering a remedy for single-use plastic waste we also consequently reduce secondary microplastics production. The opportunity for this circular economy is significant for the crucial launch vehicle technology required to place satellites in orbit, where they will tell us what we need to know to create a sustainable world.

Currently our Ecosene technology is at Technology Readiness Level (TRL) 5 as the team have proven the feasibility of the pyrolysis stage of Ecosene production. Skyrora have managed to successfully recycle plastic waste and obtain kerosene, petrol and diesel fractions, and this fuel was validated in its relevant environment during testing of Skyrora’s 3.5kN engine tests. Here we found that not only is the resultant fuel 1-3% more efficient than kerosene but it also reduces CO2 emissions during launch by 45%, compared to the common LOX (liquid oxygen)/RP-1 (kerosene) propellant combination.

We are now ready to progress commercially and create foundations for a viable business on Scottish soil. Find out more about our Ecosene rocket fuel here.

Skyrora’s Fuel Solution

Whilst continuing the research and development phases of our Ecosene fuel, Skyrora’s current launch vehicles use a bi-propellant liquid fuel-oxidiser combination of HTP (high-test peroxide) and kerosene. This choice is inspired by UK pioneered solutions from times of the Black Arrow programme, with which Skyrora are proud to follow in its legacy. Hydrogen peroxide is an ideal oxidiser for our Skyrora launch vehicles as the absence of cryogenics allows the propellants to be stored for long periods of time, reassuring our customers of their imminent space flight whilst waiting for an approved launch window. If a mission is delayed, our versatile launch service means the vehicle can remain fully fuelled on the launch pad for an almost unlimited window until the next opportunity for launch arises.

Our combination of HTP and kerosene provides reliability and security, with flexibility in transportation and timings. Not only will the onboard payloads complete their journey despite holdbacks with UK weather conditions, but it allows the lowest stress possible during flight at just 5G instead of 10G. This choice means significantly reduced costs for our customers, and infrastructure and launch management logistics are greatly simplified.

References

1. NASA. (2017). Dr. Robert H. Goddard, American Rocketry Pioneer. Available: https://www.nasa.gov/centers/goddard/about/history/dr_goddard.html. Last accessed 11th Aug 2021.

2. Tom Benson. (2021). Specific Impulse. Available: https://www.grc.nasa.gov/www/k-12/rocket/specimp.html. Last accessed 30th Aug 2021.

3. MasterClass Staff. (2020). What Are the Different Types of Rocket Fuel?. Available: https://www.masterclass.com/articles/what-are-the-different-types-of-rocket-fuel-learn-about-solid-and-liquid-rocket-fuel-and-how-rocket-fuel-has-changed-over-time#quiz-0. Last accessed 30th Aug 2021.

4. George P. Sutton, Oscar Biblarz (2000). Rocket Propulsion Elements. 7th ed. United States: Wiley-Interscience