On the single stage to orbit (SSTO) development front, British scientist Alan Bond, formerly of the HOTOL concept and now classified RB545 air-breathing rocket engine that was to be mated to the HOTOL, has gotten go-ahead funding from the European Space Agency (ESA) and the government of the United Kingdom to the tune of a million euros.
Bond's company - Reaction Engines LTD - designed the Skylon SSTO as the spiritual successor to the British HOTOL, which was to be Great Britain's answer to the Space Shuttle. The cancellation of HOTOL was due to design issues stemming from the placement of the the RB545 engines.
With its massive engines placed at the rear of the craft, HOTOL's center of gravity was further back than normal airplane designs and it introduced some stability issues. A redesign of the spacecraft reduced the cargo capability of the HOTOL and affected the economics of the launch system. Skylon is the answer to the design issues of the HOTOL.
Placing the new SABRE engines outboard on wings moves the the center of gravity back to the middle of the spacecraft, giving ideal stability and cargo room, as the engines are no longer taking up space in the the main body of the spacecraft. Skylon is big - 82 m long, 6.3 m in diameter and a 25 m wingspan. Its dry weight is estimated at 41,000 kg and can carry 12,000 kg of payload. The heart of the system are the SABRE engines.
On Skylon, the oxidizer used is liquid oxygen (LOX). LOX is very heavy and for a standard hydrogen oxygen reaction, you need two oxygen atoms at 8 times the weight each of hydrogen for every hydrogen atom burned. For comparison, the Space Shuttle carries 106,000 kg of hydrogen at liftoff and 630,000 kg of LOX. 85% percent of the Space Shuttle's fuel weight is oxidizer. The good people at Reaction Engines LTD decided that carrying their own LOX was crazy when Skylon would be flying through it on its way up. The SABRE engine harvests oxidizer from the atmosphere during flight.
The SABRE engine is essentially a closed cycle rocket engine with an additional precooled turbo-compressor to provide a high pressure air supply to the combustion chamber. This allows operation from zero forward speed on the runway and up to Mach 5.5 in air breathing mode during ascent. As the air density falls with altitude the engine eventually switches to a pure rocket propelling Skylon to orbital velocity (around Mach 25).
style="font-weight: bold;">The Precooleran>
As the air enters the engine at supersonic/hypersonic speeds, it becomes very hot due to compression effects. The high temperatures are traditionally dealt with in jet engines by using heavy copper or nickel based materials, and by throttling back the engine at the higher airspeeds to avoid melting. However, for an lass="blsp-spelling-error" id="SPELLING_ERROR_20">SSTO craft, such heavy materials are unusable, and maximum thrust is necessary for orbital insertion at the earliest time to minimise gravity losses. Instead, using a gaseous helium coolant loop, SABRE dramatically cools the air from 1000 °C down to -140 °C in a heat exchanger while avoiding liquefaction of the air or blockage from freezing water vapour.
Previous versions of precoolers such as HOTOL put the hydrogen fuel directly through the precooler, but inserting a helium cooling loop between the air and the cold fuel avoids problems with hydrogen embrittlement in the air precooler.
Avoiding liquification improves the efficiency of the engine since less liquid hydrogen is boiled off; even simply cooling the air needs more liquid hydrogen than can be burnt in the engine core, the excess is dumped overboard (through a ramjet.)
However, the dramatic cooling of the air raised a potential problem: it is necessary to prevent blocking the precooler from frozen water vapour and other fractions. A suitable precooler, which rejects condensed water before it freezes has now been experimentally demonstrated.
The cooled air is then passed into a reasonably conventional turbo-compressor, similar in design to those used on a jet engine, but in this case powered by a gas turbine running on the helium loop, rather than off combustion gases as in a conventional jet engine. Thus, the turbo-compressor is powered by waste heat collected by the helium loop.
After being launched and brought to speed by a short burst of the rockets, the jets are started, fed by air bled from the shock cone. At this point the precooler/turbo-compressor is not being used. As the craft ascends and the outside air pressure drops, more and more air is passed into the compressor as the effectiveness of the ram compression alone drops. In this fashion the jets are able to operate to a much higher altitude than would normally be possible.
At Mach 5.5 the jets become inefficient and are powered down, and stored liquid oxygen/liquid hydrogen is used for the rest of the ascent in the separate rocket engines; the turbopumps are powered by the helium loop from the heat produced by cooling the engine.
The Helium Loop
The 'hot' helium from the air precooler, and cooling the combustion chambers is recycled by cooling it in a heat exchanger with the liquid hydrogen fuel.
The loop forms a self starting Brayton cycle engine, and is used to both cool critical parts of the engine, but also to power turbines and numerous miscellaneous parts of the engine.
The heat passes from the air into the helium. This heat energy is not entirely wasted, it is in fact used to power the various parts of the engine, and the remainder is used to vaporise hydrogen, which is burnt in ramjets.
The designed thrust/weight ratio of SABRE ends up several times higher—up to 14, compared to about 5 for conventional jet engines, and just 2 for scramjets. This high performance is a combination of the cooled air being denser and hence requiring less compression, but more importantly, of the low air temperatures permitting lighter alloy to be used in much of the engine. Overall performance is much better than the RB545 engine or scramjets.
The engine gives good fuel efficiency peaking at about 2800 seconds within the atmosphere. Typical all-rocket systems are around 450 at best, and even "typical" nuclear thermal rockets only about 900 seconds.
The combination of high fuel efficiency and low mass engines means that a single stage to orbit approach for Skylon can be employed, with air breathing to mach 5.5+ at 26 km altitude, and with the vehicle reaching orbit with more payload mass per take-off mass than just about any non-nuclear launch vehicle ever proposed.
Like the RB545, the precooler idea adds mass and complexity to the system, normally the antithesis of rocket design. The precooler is also the most aggressive and difficult part of the whole SABRE design. The mass of this heat exchanger is an order of magnitude better than has been achieved previously; however, experimental work has proved that this can be achieved. The experimental heat exchanger has achieved heat exchange of almost 1 GW/m³, believed to be a world record. Small sections of a real precooler now exist.
The losses from carrying around a number of engines that will be turned off for some portion of the flight would appear to be heavy, yet the gains in overall efficiency more than make up for this. These losses are greatly offset by the different flight plan. Conventional launch vehicles such as the Space Shuttle usually start a launch by spending around a minute climbing almost vertically at relatively low speeds; this is inefficient, but optimal for pure-rocket vehicles. In contrast, the SABRE engine permits a much slower, shallower climb, air breathing and using wings to support the vehicle, giving far lower fuel usage before lighting the rockets to do the orbital insertion.
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