FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1961
1961 - 0735.PDF
fLIGJT, 1 June 1961 745 Mi. sites and Space flight . . . FIVE YEARS WITH HTP By E. G. O. ANDREWS Chief Designer, Rockets, Bristol Siddeley Engines Ltd This week Bristol Siddeley have a Stentor rocket engine on show at the Paris Salon. Stentors are pictured on the right in final assembly at the company's Coventry factory THE Rocket Department of Bristol Siddeley Engines began tostudy rocket engines in 1946, when it was part of ArmstrongSiddeley Motors in Coventry. By 1948 the programme of basic development was well advanced which led to the flights ofthe Snarler in the Hawker P. 1072 in 1950 and 1951. Development of the Screamer, described in Flight for July 27, 1956, followed almostimmediately; this engine was originally intended for the main pro- pulsion of the Avro 720 intercepter, but was cancelled at the end of1955 on completion of its flight clearance test. The cancellation was to us a disappointment, because it marked the close of anera of experience with liquid oxygen used in conjunction with kerosine as a rocket propellant. Immediately, however, contracts were won for two new rocketengines, both larger than the Screamer, but both using hydrogen peroxide (HTP) instead of liquid oxygen. Of these, one culminatedin the Gamma Mk 201 and the other in the Stentor, some details of which have now been released. Alone of British rocket-enginemanufacturers, Bristol Siddeley thus have a close association with both liquid oxygen and hydrogen peroxide, and it is interesting tocompare the design of these two recent engines in the light of this experience.There is no perfect oxidant for rocket engines. From considera- tions of performance alone, the most attractive are fluorine, oxygenand others consisting entirely of light elements which will combine exothermally with a fuel; these are mainly cryogenic. Less ener-getic, but liquid, oxidants are also used: in these, reagent elements are combined with elements which do not react, or are alreadysaturated, and which merely dilute the gaseous combustion pro- ducts. Hydrogen peroxide represents an extreme example,because less than half of the oxygen present is available for com- bustion. Thus 85 per cent HTP decomposes into a gaseous mixturewhich contains only 40 per cent free oxygen by mass, the remainder being steam. It qualifies as a rocket propellant only because heatis released during its decomposition—sufficient, in fact, to raise the decomposition products to a temperature of 900°K. In both theGamma and the Stentor, kerosine is then burnt in these decomposi- tion products. At the very high temperatures encountered in the combustion ofrocket propellants, the products are not completely reacted. Thus, in addition to carbon dioxide and steam, carbon monoxide andfree oxygen may also be present, along with many unstable radicals. This tendency for the combustion products to dissociate increasesas the temperature is raised: at 2,000 K. there is little dissociation, but at 2,700 K approximately one-tenth of the available energy islost through incomplete reaction, while at 3,400 K nearly a third of the energy is lost in this way. During the expansion process it ispossible that some of this heat may be regained by reassociation, but the benefit is not usually very striking. The heat of reaction of HTP with kerosine is only two-thirds ofthat of liquid oxygen with kerosine. However, because the com- bustion temperature of the former is 2,700 K, compared with3,400 K for liquid oxygen/kerosine, the useful energy is only about one-fifth less, corresponding to a difference in specific impulse ofonly one-tenth. There is also a greater amount of carbon dioxide in the combustion products of liquid oxygen and kerosine, whichtends to increase the mean molecular weight of the exhaust gases and hence to reduce the specific impulse. Of course, if the fuel ishydrogen, this does not apply, and liquid oxygen then shows up to greater advantage. As an indication of merit, specific impulse is relevant to vehicleswith lifting surfaces. Here, thrust is mainly required to overcome aerodynamic drag, and the range of the vehicle is a function ofburning time. This type of vehicle can weigh more than the thrust, and a propellant load one-tenth greater than that required for aliquid oxygen/kerosine engine may be acceptable in order to gain the advantage of having a storable oxidant like HTP. As this seemsto have been one of the arguments which resulted in the cancellation of the Avro 720 it is worth considering in detail. A bucket of liquid oxygen will evaporate in about an hour. Inbulk storage in large quantities it is possible to provide sufficient insulation to keep evaporation to a few per cent per year, but in thesize of tank required for a fighter, with 2in of foamed plastic for lagging, the evaporation rate might be 1 per cent per hour. [Continued overleaf An exceptional record of reliability has been established by the Black Knight research vehicle, one of which is portrayed at left on its pad at Woomera. Powerplant is a Bristol Siddeley Gamma Mk 201
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
