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Aviation History
1997
1997 - 3258.PDF
The advantage of air-breathing propulsion 8,000 r- 7,000 - 6,000 - 5,000 - / tt Turbojets • 4,000 K/ / —- / Ramjets FLIGHT o Q. to Hydrogen (spaceplanes) Hydrocarbon (missiles) 10 12 14 Mach number 20 22 24 Bushnell still believes that the current approach is flawed, however. "We are taking lots of small steps towards a big goal, to reduce affordability [of space access] from $10,000 per pound [$22,000/kg] to $1,000, when what we need is $ 100", he says. In what he describes as his "own prejudice", Bushnell prescribes a back-to- the-drawing-board approach, reselecting the best technology from what is available in the late 1990s, and collaborating on an international basis to develop the technologies to maturity. In particular he urges "evaluation" of pulsed-det- onation wave (PD W) rocket engines and solid- hydrogen fuel which, he claims, could dramatically increase the specific impulse of propulsion systems. Aerodynamic and active control of inherently unstable configurations, based on technology recently developed for fighter aircraft, needs to be investigated for space vehicles, he says, with the added consider ation of controlling the entrainment of the exhaust flow in the ambient atmosphere, prob ably using a skirt ejector, being revisited. PDW engines are applicable to all sizes of propulsion systems at all speeds, from launch to upper hypersonic and for orbital manoeuvring, although they would require oxidisers above 50km, says Bushnell. During a detonation, a flame front travels through the propellant so rapidly that little expansion can occur, and det onation approximates to constant volume com bustion which is far more efficient then the conventional constant-pressure combustion in a rocket engine, says NASA. A cylinder is filled with fuel and oxidiser at low pressure, without the need for expensive HyTech hypersonic missile trajectory Source: Boeing Not to scale 100,— 80 1" L 20 — Burn-out and staging FLIGHT 11min 30s Flameout Climb-acceleration to Mach 8 / 40s Scramjet ignition 36s Thrust tail-off Terminal dive J 0s Launch |_ Mach 0.8 Rocket boost to Mach 8 5s Booster ignition 48 Range km 794 12min \| Impact 1,362 1,426 high-pressure turbopumps, and ignited with a spark plug similar to that used in automotive technology. The resulting blow-down impulse, rapidly repeated, provides a pulsed thrust at 100-l20Hz, says Bushnell. NASA has a "flight experiment" planned for 2001, and has designed a hybrid pulse-detona tion air-breather/rocket in conjunction with US company Adroit Systems, which has tested an experimental propulsion tube. Bushnell says that such compressorless engines produce toroidal vortices, but will not comment on strange contrails associated with the "Aurora" - the much-rumoured US military replacement aircraft for the Lockheed SR-71 Blackbird. Scramjets may be the next step, but Bushnell believes that PDWs could bypass them and even replace gas-turbine engines. "You don't have any parts," he says. Pulsed detonation is already being applied in combustion-augmen- tors in the proposed scramjets for the HyTech programme to help reduce ignition-delay. RUSSIAN SUCCESS Mercier acknowledges that only the Russians have ever flown a scramjet, known as "Kholod", in the early 1990s on an SA-5 missile Hypersonic Flying Laboratory, in conjunction with French research agency ONERA. NASA came close as long ago as 1967 before its Scramjet Incremental Flight Test programme was cancelled, and is now working closely with the Russians to use their engine for validation. The head of Russian TsAGI test-centre, Vladimir Neiland, says that the hydrogen- fuelled Kholod engine is due to be tested again "soon" on the second-generation "Igla" Hypersonic Flying Laboratory, which is part of Russia's ongoing ORYOL-2-1 research pro gramme. Under ORYOL-2-1, the Central Institute of Aviation Motors in Moscow has developed two possible Igla designs, and is lead ing an SSTO spaceplane effort and a two-stage- to-orbit design conceived to build a "Mir-2" space station. Neiland, who's organisation is an Igla participant, says that"...transonic is really the problem", echoing American views. Russian researchers say that estimates made "with the help of NASA" show the feasibility with current technology of building a craft which carries twice the payload weight-fraction of the Space Shuttle, at 3-4% of launch weight. Various "non-traditional" technologies are being researched, however, as studies have shown such a design is still not "commercially profitable". These include projecting plasma or "ball-lightning" discharges upstream, into the wake and along the wings, which has been shown to reduce shock-wave intensity, reduc ing drag and increasing lift, and dramatically cutting sonic-boom intensity; microballoons for hydrogen storage to halve the weight penal ty of cryogenic hydrogen systems and give a fur ther 10% energy output through stored elastic energy; and a thermal memory device where the engine cuts in and out to avoid over-heating. • 46 FLIGHT INTERNATIONAL 26 November - 2 December 1997
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