What material innovations are advancing space and satellite technology?
What are the material innovations helping to advance space and satellite technology? Robin Hague, Lead Engineer at Skyrora Ltd, Scotland, takes a look.
Scottish space engineering company Skyrora, founded in 2017, is developing a new satellite launch vehicle to serve the soaring quantities of small satellites that currently have limited and unsatisfactory routes to space. The vehicles will fly from one of the prospective UK spaceports presently in development. While smaller missions are expected to command an additional premium over hitch-hiking on a bulk launch, minimising the cost of producing and operating vehicles is critical. At Skyrora, three material innovations are helping achieve this.
Orbital launch requires so much energy that rockets have a far higher propellant mass fraction than any other type of transport. Where airliners are typically around 30–40% fuel at take off, and fighter aircraft might approach 50%, orbital rockets push towards 90% consumable mass at launch. As a result, the design and operation of rockets is dominated by the characteristics of their propellants. One option is a have a neglected propellant combination, which offers several keys to cost-effective launch – hydrogen peroxide and kerosene.
Building on the past
These propellants were a characteristic of the historic rocket programme that culminated in the UK's only orbital launch to date – that of the Royal Aircraft Establishment’s Prospero satellite by the Saunders Roe/Rolls-Royce Black Arrow rocket on 28 October 1971. Black Arrow, built as a technology demonstrator and too small to lift the first generations of commercial satellites, would be an ideal vehicle for today’s small spacecraft. Firstly, hydrogen peroxide is non-cryogenic, whereas liquid oxygen must be kept below -186ºC, so in combination with kerosene the entire vehicle operates at ambient temperature. It is also non-toxic, and while it should not be released into the environment undiluted, an accidental spill would have no long-lasting effects. Other storable oxidisers, such as hydrazine, nitrogen tetra-oxide and nitric acid are much more challenging to deal with.
Hydrogen peroxide systems also operate with very high optimum mixture ratios, typically six to eight oxidiser-to-fuel. This leads to vehicles that are mostly composed of the peroxide tank, with only a small reserve of fuel needed. In addition, because peroxide is 35% denser than liquid nitrogen and the vehicle design is prioritised towards the densest propellant, this also minimises the size of the vehicle. A further benefit of the peroxide-kerosene combination is having copious quantities of a dense and effective coolant – rocket engines are almost always cooled by the fuel supply to minimise the risk of setting light to the cooling channels. But, the sheer quantity of peroxide, and its water-like characteristics (H2O2), means that the thermal stresses on the engine can be reduced compared with other propellant combinations, leading to simpler, cheaper and more reliable engines.
But beyond all the advantages the physical characteristics of peroxide brings, it has yet another asset, the reason it is used in such high oxidiser-to-fuel ratios. Hydrogen peroxide brings additional energy to the reaction in its extra oxygen bond, and can even be used as a rocket monopropellant. If forced through a silver-coated mesh, peroxide catalytically decomposes into 600oC steam and free oxygen. This reaction is how the iconic Bell Rocket Belt jet pack flies using peroxide by itself, but this characteristic is tremendously useful in bi-liquid rocket engines too.
Skyrora’s engines, like those of the Black Arrow programme, feature a catalytic injection system, where the peroxide is decomposed prior to entering the main combustion chamber. This means that starting the engine simply requires the propellant valves to be opened in the correct sequence, as the decomposed peroxide efflux is more than hot enough to ignite the kerosene fuel. This process removes the risk of hard starts, where propellants pool in an engine and detonate, and it vastly simplifies injector design – a typically challenging part of engine development. Finally, the ability to produce superheated steam from the oxidiser alone also dramatically simplifies the heart of the engine, and the subsystem that has traditionally been responsible for the most programme delays from the V2 to the Space Shuttle, the turbopump.
Turbopumps vary in design, but are all in some form twin centrifugal impellers – one for each propellant and a turbine to drive them. They are the key enabling technology for efficient orbital launch, as they force the propellants into the engine against the high pressure inside – without them the propellant tanks, which make up the majority of the vehicle, would have to withstand pressures higher than the engine for the propellants to flow into the chamber. This would make the vehicle too heavy to be practical, whereas with pumped propulsion, tanks typically operate at only two to four bar. The Blue Streak and Atlas missiles were famously so thin walled that they needed to be continually pressurised to support their own weight.
Various combustion cycles are used to operate the turbopumps, but all involve burning some percentage of the propellants to drive the turbine. This requires a separate dedicated combustor, mixture control and a turbine able to survive an operating temperature of thousands of degrees Celsius. In Skyrora’s orbital Skyforce Turbo engines, however, the pumps will be driven by separately decomposed peroxide flow, again like the Rolls-Royce Gamma engines that propelled Black Arrow. This removes the need to mange a second combustion process, simplifies the propellant flow systems reducing mass and enables the use of a steam turbine operating at a comparatively benign 600ºC. The reduced temperature means that these turbopumps can be manufactured cost-effectively from more conventional metals, and significant portions can be 3D printed.
Using metal additive manufacture
Despite the historic inspiration of these unusual engines, the second key process to streamlining production and reducing cost is metal additive manufacture. Bi-liquid rocket engine combustion chambers have typically been labour-intensive items to construct. They operate at such high temperatures and pressures that they must be actively cooled, which is usually accomplished by passing one of the propellants through cooling channels in the wall of the chamber. Yet to ensure sufficient heat transfer, these channels are generally only millimetres thick, and often need to wind helically around the chamber. These chambers tend to be produced from high-temperature alloys that are not easy to machine, yet at the same time should ideally be flowing, aerodynamic complex curves. Added to this, the forms of manifolds, impellers and turbines in the pump make for slow and intricate machining.
However, a range of powder bed and direct deposition printing systems are now making it convenient to create complex parts in aluminium, stainless steel and Inconel. For additive manufacture, any shape is as practical as any other, so combustion chambers can be conveniently printed with idealised curving, cooling channels integrated directly into the wall. This removes a number of traditional manufacturing processes and reduces the total parts count, though it does present its own challenges. Unfused powder must be extracted from the cooling channels, especially as these engines are oxidiser cooled, and consideration must be given to gripping these complex shapes for post printing machining.
The first examples of Skyrora’s printed engines have been successfully tested – in this case the 350kg thrust Leo upper stage engine, which will push customers payloads on the last leg into orbit, with the latest version produced in cooperation with Frazer Nash due for testing at Spaceport Cornwall in early 2019. Next up, the first metal parts for the turbopump will be printed, post machined and tested in Scotland during the year. Being able to produce these key components in far fewer steps and parts than with traditional processes is expected to dramatically reduce the cost of engines, and therefore the whole launching system.
For the a new launch vehicle system, Skyrora is aiming to minimise its environmental footprint from the start, working towards the principle of a circular economy, recovering and reducing wasted resource. Although initial engine testing is being carried out using commercially available Jet A1 kerosene, launch vehicles typically use more energetic versions, such as RP1 used by the likes of Saturn 5 and Falcon 9 in the USA, or Syntin employed by the Soviet Union, and now Russia in the Soyuz launch vehicle. An equivalent is not conveniently available in the UK, which is not such a loss as fossil fuel use should be minimised where possible. As a result, Skyrora has invested in a process called Ecosene.
A more sustainable alternative
The Ecosene project is working on producing kerosene, including rocket-grade kerosene, from currently non-recyclable plastic waste streams. This is a two-step process where the plastic is converted to basic fuel by pyrolysis then further enhanced through hydrotreatment. Development is proceeding well, with standard-grade already being produced and the second stage in testing. It has also already provoked great interest from a number of potential industrial partners with compatible waste streams.
Although still ultimately a fossil resource, the process recovers an energy source that would otherwise be lost, and that would present a greater hazard to the environment in its unreformed state. In addition, the high temperature and oxygen-rich combustion in the peroxide rocket engines leads to efficient combustion – Black Arrow’s near invisible exhaust is a marked contrast to the bright yellow plumes of liquid nitrogen-kerosene engines. This is almost exclusively water and CO2, which can be effectively offset due to the relatively small quantities involved in launch operations.
Despite the fact that UK launch vehicle development has been interrupted for 45 years, there is now a compelling business and sovereign capability case for UK launch. The UK government has set a target to capture 10% of the global space market by 2030, and a key part of that is to enable the vibrant UK satellite constructors to quickly and cost-effectively launch payloads with crossing borders. Those working on UK orbital capability still face many challenges, but now there is a determined will across industry and government to see satellites fly to orbit from the UK.