Posted by Pete Hague on 08 Jun 2012
Reaction Engines Skylon is a proposed spacecraft that can reach Low Earth Orbit (LEO) in a single stage, taking off and landing like a plane. This has long been a dream of rocket engineers, and has thus far been unrealised. However, Skylon utilising some potentially game-changing technology that might make the 'spaceplane' an idea whose time has finally arrived.
All vehicles currently used to access space use staging. They discard empty fuel tanks and engines in order to shed weight and improve their performance. The reason for this can be found in the Tsiolkovsky rocket equation:
this is a fairly simple equation, which can be derived from Newtonian mechanics (not hard, but fairly tedious to do). The change in velocity (ΔV) is equal the velocity of the rocket's exhaust products (Ve) times the natural log of the ratio between the mass at the start of the burn (M0) and the mass at the end of the burn (M1). The best exhaust velocity you can get from chemical propellants is about 4km/s, using liquid hydrogen and liquid oxygen. Liquid hydrogen is not an easy material to handle - it must be stored under pressure at around 20K (-253°C) - but its good performance has made the effort worth it in rocketry.
You need to change your velocity by (quite roughly) 10km/s to achieve orbit. This doesn't include gravity losses (the ΔV you need to counter the acceleration due to gravity whilst you achieve orbital velocity) which come to about 500m/s - but by happy coincidence that is approximately equal to the delta V kick you get from the Earth's rotation if you launch near the equator.
So, with the best chemical propellant around, I estimate you need a mass ratio of 12, i.e. 11 parts fuel to 1 part spacecraft (probably an underestimate, for technical reasons beyond the scope of this post). This is not impossible; but its hard to do, and by the time you've finished you might not have much room left for payload, or for equipment you need to re-enter the atmosphere.
The key technology behind Skylon, and the part currently being developed and tested by Reaction Engines, is the air-breathing engines. The Tsiolkovsky rocket equation assumes the vehicle carries all its own propellant, and contradicting this assumption opens up the possibility of substantially higher performance.
The SABRE engine has an air intake, and a cooling unit to liquify the air (it must also remove sources of ice, water and carbon dioxide, but Reaction Engines are tight-lipped about how they do this). The liquid air then obtained is used in place of liquid oxygen for the early part of the flight to orbit.
Harvesting oxygen from atmospheric air provides a number of advantages. Hydrogen and oxygen mixed in the ratio to produce water (2 molecules of hydrogen to 1 molecule oxygen) have a mass ratio of 1:8, so the component of the propellant you are getting from the air is by far the heaviest. This means that even though you can only use atmospheric air for part of the flight (Skylon must switch to on board liquid oxygen after having achieved about 20% of its orbital velocity) you save substantial mass.
Carrying less propellant at launch also improves the thrust/weight ratio of the spacecraft. This is a continuing problem for liquid hydrogen/liquid oxygen rockets, and if you take more time getting to orbit, you lose more velocity to gravity. This is why many hydrogen powered rockets have solid rocket boosters - which have poor exhaust velocity, but can produce a lot of thrust for early in the flight, when the rocket is heaviest.
The actual exhaust velocity of the engine does not change much between modes, but you can calculate an effective exhaust velocity (i.e. the one you would have to stick into the rocket equation to get the right ΔV) of the air breathing mode in order to compare performance - and it comes out as at about 35km/s compared to a about 4km/s for a conventional rocket. Engineers would do this comparison using a measure called "specific impulse" or Isp, but I think that is conceptually a little harder to understand.
This potent performance relieves some of the pressure to keep the mass of the spacecraft down, and means it can carry the thermal protection it needs to re-enter the atmosphere, and the control surfaces and undercarriage it needs to take off and land like a plane - which reduces costs compared to a conventionally launched/recovered spacecraft.
The Role of Skylon
Putting stuff into orbit is already possible, so how does Skylon change things? Well, firstly Reaction Engines project a substantial reduction in launch costs, based only on current demand. However, similar costs are also projected by SpaceX, using more conventional rocket technology which has already been proven.
Skylon appears to me to be aimed at an increased demand for launch services. One of the interesting aspects of Skylon is that it can use containerisation. Rather than integrating payloads with the launch vehicle one at a time, payloads are integrated into standardised payload containers, which are then lowered into the payload bay of Skylon.
This is borne out by the case studies for the application of Skylon; an orbiting station for assembling spacecraft, and a small fleet of Mars spacecraft to be assembled in it. There is also this video, wonderfully narrated by Brian Blessed, showing the delivery of a passenger module that could deliver 8 crew to the International Space Station, even though the ISS in its current configuration only supports a total crew of 6 (not all of which need to be changed in one visit.) In some configurations, the same passenger module can carry 24 people to orbit.
All this points to a spacecraft designed for missions of larger scope that are currently performed, or considered for the near future. The only exception I can think of is the recently proposed asteroid mining venture. The ISS can be supported by a 3 man Soyuz capsule, so a 24 man Skylon passenger module would logically support a station 8 times the size (with a rather impressive mass of about 3,600 metric tonnes.) Containerisation of payloads makes most sense when there are a great many of them, and many transport providers who need to be able to pick up whatever payload is ready to fly when they have a launch available.
What happens next?
Currently, Reaction Engines are conducting tests of the pre-cooler module. They have told the media that they will be ready to make an announcement by the time of the Farnborough Air Show in July.
Reaction Engines do not intend to produce the spacecraft themselves, only the engines. They need a manufacturing consortium, and there probably aren't many candidates. They have already stated, that because of the restrictions of ITAR they cannot work with any US companies, so the most likely candidate would be EADS, who already manufacture Airbus aircraft and the Ariane 5 rocket. This may make Skylon vulnerable to European politics. The French lead European launcher development, and might not be willing to play nicely with a British-led program to replace their flagship rocket.
There is also the possibility that Skylon might not be realised in its presently envisioned form, even if the engine technology proves its worth. It may be that spacecraft manufacturers have differing ideas about how to use SABRE (perhaps forgetting about SSTO in favour of having a powerful, flyback first stage similar to the mothership of SpaceShipOne, but faster and higher altitude.) There might be a collapse of the launch market that would make the economics of Skylon unfavourable. On a purely aesthetic level, it would be a shame if Skylon were radically redesigned before it got to fly, as it is a very distinctive looking vehicle. If it is realised, I think it may become a British design classic; a sort of space Mini.
That is about the sum of what I know about Skylon at this time. I'll probably post another update once they've talked about the engine test next month. Perhaps then there will be more information on who is going to build it.
Posted by Matt Cro on 09 Jun 2012
As I've said in the past, it is promising and its good that after several years of research as Skylon/SABRE, development has continued positively and they haven't had too many (major) setbacks.
It will be interesting to see the results at Farnborough (9th - 15th July). Just need more people knowing about it, perhaps it would be our second Concorde?
You're right about the French though, as the primary contributors and financiers behind EADS and Airbus (often referred to as the French Aerospace company), they won't want to have their primary launcher made redundant by a British design. But at a cost of ~£12 billion over say 10 years, we could possibly fund it. Considering High Speed 2 is costing between £15-17 billion to build, its not out of the realms of possibility of a potential investment.