There are lots of weird and wonderful schemes for powering spacecraft, but in practice only three classes of engine have been built today that could realistically power a spacecraft in the next few decades. (aside: I exclude cold gas thrusters used for small manoeuvres such as docking).
Before we look at engines, there are two measures that are particularly relevant in assessing rocket engines – thrust, measured in Newtons, and efficiency in turning fuel into thrust or roughly the rocket equivalent of miles per gallon, which is a measure called Specific Impulse (Isp). Space travellers care a lot about “mpg” as in space there are no filling stations – yet! If you can’t casually mention Specific Impulse when drinking down the pub – you will never be a steely eyed missile man.
Isp is an odd measure – it relates directly to the exhaust velocity (Ve). If I is the total impulse of a rocket – then Isp is that quantum of Impulse created for each unit weight (on earth) of propellant.
Isp has the slightly surprising unit of seconds – as the force units cancel to leave only seconds (and the bigger the better).
Whilst how effectively the engine turns the fuel into thrust is very important, there are many other considerations when looking at the whole vehicle, including weight, and size which are consequences of fuel, oxidiser and other choices.
I have made a table of some of existing engines – all have been built; not all have been flown. An Olympus Concorde engine included for comparison.
There are three basic types of engine in this list:- Chemical, Nuclear and Electric. Chemical and Electric have been flown on real spacecraft, nuclear hasn’t although a working prototype has been built and tested.
Let’s start with chemical rocket engines the ones we are most used to seeing. As you can see from the table a single shuttle side booster produces a massive amount of thrust, but it isn’t very efficient. Solid rockets are what the chinese first used nearly a thousand years ago and are what your firework rockets you set off on Brexit independence day will be.
They burn solid material, the burning happens where the fuel sits rather than moving the fuel to a nozzle, and once started they can literally not be stopped, or even throttled up or down. As they contain burning material the sidewalls of solid rockets tend to be made of thick metal which makes them heavy. The features of lack of control and weight of vehicle means that probably no-one will design a future manned spacecraft that uses them for propulsion (see also STS51 Challenger Disaster).
Solid rockets are commonly used for missiles, as their long term stability means they are much easier to have the vehicle on ready standby –an example of this standby readiness is the charge that sets off the airbag in your car is probably a charge of the same sort of material used as a fuel in solid rockets.
All the other chemical rockets in the list are liquid propellant, and that is the primary choice for most spacecraft today. Liquid fuelled rocket engines are potentially controllable (both throttle up and down and switch off and relight) and relatively efficient (higher Isp).
The F1 used in the Saturn V was huge and had a good thrust to weight ratio but not a brilliant Isp i.e. not that efficient in turning its fuel into thrust. The RD180 is the russian built engine that the US made Atlas V uses. The RS-25 shuttle main engine is probably the most efficient chemical rocket engine ever flown – it has an Isp of 452 – to get an idea how good this is, you can calculate theoretical maximum Isp of an LOX/LH2 engine from the energy contained in the chemical reaction and it would be an Isp of just over 500; that assumes no losses from anything and that all the energy is directed perfectly along the axis of thrust – engineering that manages to get around 90% of the effective work out of the messy bitch that is combustion, is really good engineering.
The downside of the RS-25 is that it uses liquid hydrogen which as mentioned in the last installment means huge fuel tanks and really tricky storage and pumping.
The Merlin 1D from spaceX is not very powerful (but 9 of them are used together) but is fairly efficient for kerosene fuel, and just incredibly light.
Two brand new engine designs in development use liquid methane with LOX, this can give a higher Isp than kerosene (RP1) but lower than LH2. The advantages, though – particularly for interplanatery travel is that it is much easier to carry than LH2; it is denser, doesn’t require active cooling, and it is simpler to pump (and as we will see in the next part the pump in a liquid rocket engine is the most critical and amazing component), and finally Methane can be synthesised on Mars! Those engines are the Blue Origin BE-4 and the SpaceX Raptor, both have got as far as some test firing.
The second form of propulsion is electric – many satellites use electric thrusters to maintain position and three long distance space craft have used electric thrusters to make their long journeys from Orbit to their destination. The advantage is that they can make fantastically efficient use of the propellant they carry (very high Isp) but as you can see the thrust they produce is miniscule – The Dawn engine produces 90 milliNewtons – about enough to lift a newspaper off the ground.
There are various forms of electric propulsion (electrostatic, thermal and magnetic) but in each case the plan is take a stream of atomic particles and eject from the back very very fast (high Ve = high Isp), so if you are patient and only have a tiny spacecraft you can eventually get to very high speed. The dawn thruster operated for more than 600 days in one continual “burn”. The Dawn spacecraft arrived at Ceres in 2015, trajectory and timeline can be seen here https://dawn.jpl.nasa.gov/mission/timeline_trajectory.html.
To date all craft using electric propulsion gain their electric power from solar arrays – A big problem with this is that as you move away from sun that radiation becomes much less effective., if you leave earth orbit with a massive 100KW solar array – enough to power the X3 thruster in the table above – then by the time you get two planets along to Jupiter you are down to 5KW.
Adding a nuclear power plant is a credible way of overcoming this problem. Without such an addition, electric propulsion with solar power is never going to be the propulsion for a manned mission as the journey times are just too long.
Which brings us to the third type of rocket engine – Nuclear Thermal.
Nuclear Thermal rocket engines are not sci-fi, they have been built and tested by both the US and Russia.
A nuclear thermal rocket does not involve the use of atomic bombs! It uses a nuclear reactor to heat a gas (typically Hydrogen) to such a temperature that it will provide thrust when expelled through a nozzle, in some ways it is very simple, hydrogen is pumped through pipes inside a cylindrical reactor (very reminscent of a steam engine boiler design) and thus heated it is expelled.
The American test programme, carried out at the wonderfully named “Jackass Flats” , achieved some incredible things during the 1960s when the engine was under serious consideration for inclusion in the space programme – the most powerful engine they built a Phoebus 2 produced just shy of 5GW of power in something the size of a shipping container. Just 7 of these objects would provide enough power for all of the modern UK’s daytime electricity demand. The picture below gives a good idea of how compact the reactors were – the whole engine is the cylinder on top of the test building.
They built a number of different engines, some tested with all the components for a real flight, for flight certification. If the development hadn’t been cancelled it seems they could have pushed up performance and longevity a lot further.
As it is the engines they built were heavy (they did have a lot of Uranium in them after all) and didn’t work brilliantly at sea level but were excellent in Vacuum, giving good thrust and very good Isp – about twice what the best chemical rockets are capable of.
If the prospect of travelling to Mars sitting on top of a nuclear reactor doesn’t take your fancy – then pervesely you may be interested to know you are might well get less radiation that way than any other – the much quicker transit time – perhaps a couple of months that such an engine would allow, would mean you were exposed to much less natural radiation – which is fierce this close to the Sun when outside the protection of the Earth’s magnetic field.
Probably my favourite of all the tests the Nerva project did was named “Kiwi-TNT”. Imagine waking up in the morning and deciding to strap a load of TNT onto a nuclear reactor and blow it up to test the consequence of a launch failure….
Although Nerva was cancelled in 1973, there are many enthusiasts for this technology, and NASA started funding a new programme in 2017 with external contractor BXWT, albeit with a relatively small sum of $19M.
If you are interested a complete spec for one of the Nerva engines can be found here https://fas.org/nuke/space/nerva-spec.pdf
Next –(Turbo)Pumping Gas
© Ross 2018