The rise and fall of the rocket interceptor

“The only time you have too much fuel is when you’re on fire.” – Anonymous

The world had entered the Second World War with piston-engine propeller aircraft technology, and these were sufficient for the duration of the war. However, shortly after Germany’s surrender the former Allies suddenly found themselves in the jet age, as well as kicking off a conflict between the US and Western Europe on the one hand and the Soviet Union and Eastern Europe on the other. It was clear that any new air war would be fought by planes equipped with turbojets and/or rocket engines: no piston-engine aircraft could ever hope to keep up with the new jet fighters that were already flying at the end of the war, particularly the German Me 262 and Domier 234 and the British Gloster Meteor turbojet.

Because of this paradigm shift in aircraft propulsion, advanced but conventional propeller fighters developed at the end of the war became obsolete almost overnight, just as a new generation of aircraft became necessary in order to maintain the uneven military balance of the Cold War and to fight in the limited-scope conflicts that this spawned (such as in Korea and the Middle East). It was soon recognized that major development efforts were required for the new generation of high-speed fighters, but whether these would primarily be propelled by rocket or jet engines remained to be determined.

The need for rapid interceptors quickly became very urgent in Europe because of the proximity of the countries of the Warsaw Pact and NATO: their bombers could attack one another’s cities and military facihties within minutes of crossing the Iron Curtain. Moreover, an incoming bomber could spend a lot of time getting up to high altitude and speed in friendly airspace prior to crossing the border, but a defending interceptor would have little time to react. In fact, the situation was very similar to that which had faced the German Me 163 pilots in 1944 and 1945. Interceptors which could achieve high speeds and high altitudes in little time were therefore a priority in post-war Europe.

As we have already seen, the push for fast developments in aeronautics during the 1940s meant that new aircraft designs followed one another in rapid succession. This situation would continue into the 1950s and 1960s, with concepts sometimes already being obsolete prior to their first flight. Amidst this design fury, rocket aircraft had to compete with turbojet aircraft for development funding.

By 1945 several rocket planes had been successfully flown, starting with simple gliders fitted with solid rocket boosters, through modified piston-powered planes to dedicated rocket propelled aircraft. However, only the Me 163B had seen operational service. Overall, this experience did not bode well for the rocket powered interceptor concept. There were many accidents due to the poorly understood aerodynamics of transonic flight. Also the propellants tended to be difficult to handle and downright dangerous (notably the corrosive hydrogen peroxide of the German Walter engines and the almost equally nasty nitric acid used by the Russian engines), especially in combination with the rather low reliability of the rocket engines used. Moreover, the very short range of the rocket aircraft restricted it to quick attacks on enemy aircraft flying in the vicinity of the interceptor’s base. Protecting an entire country such as Russia would require vast numbers of rocket interceptors and airfields. Furthermore, Germany’s investments in often rather fanciful ‘wonder aircraft’ and rockets were of little help during the war: if the Luftwaffe and the Wehrmacht (Army) had bought conventional weapons for the money they invested in highly novel technology, they might have been able to extend the war considerably. In other words, the value of rocket aircraft remained to be proven. However, near the end of the war the problem of how to efficiently operate rocket airplanes did not seem to be so much intrinsic to the type of technology than to its immaturity.

The aerodynamic problems associated with transonic speeds, which were an issue for new jet aircraft as well, could only be solved using the proper wind tunnel tests, theoretical modeling and innovative wing design. Of course, the route to supersonic flight had already been partly explored by Lippisch and other aerodynamicists. The German, Japanese and Russian high-speed interceptors of that time looked distinctly futuristic, but their swept-back or delta wings, tail exhaust nozzles, cockpits placed in front of the wings and lack of propellers would soon become the norm for fighter aircraft. Some of the early jets and rocket aircraft still look rather modern today! As regards the unpleasant rocket propellants, fortunately less vicious alternatives were able to be produced.

The only true disadvantage inherent in rocket power was short range, limiting the role of rocket-propelled fighter planes to point defense: an interceptor used to defend a specific target by taking off and climbing to altitude as rapidly as possible in order to counter an approaching threat, and then land and prepare for another mission. But it had already become apparent during the war that the range issue could be partly resolved by a combination of rocket and jet engines, with the relatively simple rocket providing the thrust for a rapid ascent and the more complicated jet engine enabling the plane to cruise for relatively long periods.

Because of the rapid developments during the war, jet engines had become more competitive, in terms of thrust per engine weight, in comparison to rocket engines. The early Heinkel HeS 3 jet engine that powered the world’s first jet aircraft in 1939, the He 178, weighed 420 kg (930 pounds) and produced a maximum thrust of 4,400 Newton (equivalent to about 440 kg). This meager thrust to weight ratio meant the

engine could vertically lift just 1.05 times its own weight. In 1941 the ‘cold’ Walter RII-203 rocket of the Me 163A was far better. It had a thrust of 7,400 Newton and weighed only 76 kg (170 pounds), which is a thrust to weight ratio of around 10. The early rocket engine could thus produce some ten times more thrust per unit of engine weight than the early jet engine and was simpler to manufacture and maintain. The jet engine was however more fuel-efficient, requiring less propellant than the rocket engine for the same amount of thrust. Near the end of the war the Junkers Jumo 004D turbojet engine, weighing 745 kg (1,640 pounds), could already deliver a thrust of 10,300 Newton at low altitudes and had a thrust to weight ratio of about 1.4. The Me 163B’s Walter HWK 109-509A2 of 1944 still had a higher maximum thrust of

17,0 Newton, weighed only 160 kg (350 pounds) and so had a thrust to weight ratio of about 11. Then again, at least in terms of thrust, the difference was shrinking. Not only were jets more economical in propellant consumption than aircraft rocket engines (translating into longer flights and greater ranges), they were now producing similar amounts of thrust while their thrust to weight ratio was rapidly improving.

A disadvantage early jet engines had with respect to rockets was that they required very careful throttle movements: changing power rapidly would often extinguish the combustion in the turbojets (so-called flameouts). The start procedures for early jet engines were also very sensitive, as there was no automatic sequencing of the various events built in. An error could cause the engine to quit or possibly overheat. Even worse for the pilots was that everyone on the airfield would know about his mistake because the engine would produce a loud rumble and shoot a huge flame out of the tailpipe. Rocket engines were in general much less fidgety. However, the sensitivity issues of turbojets were soon solved by more advanced fuel controls.

In other words many advantages rocket engines still had over jet engines for use in fighter planes were being eliminated one after another. One strong inherent plus was the lack of the need for an inflow of air and associated air intakes, which gave rocket aircraft an advantage in terms of aerodynamic drag. Air intakes also cause technical complexity because the air going into a conventional jet turbine engine needs to flow at subsonic speed even when the plane itself is flying supersonically. Hence using rocket engines also meant that the propulsion was independent of the vehicle’s flight speed. A more important remaining advantage of rocket engines was that they were much more effective than jet engines at high altitudes. Rockets have their own oxidizer whereas jets need to scoop up air to use its oxygen. The density of the atmosphere drops with altitude, so for each jet engine there is an altitude at which it can no longer be fed sufficient air to work properly. Rocket engines can even work in vacuum, and in principle actually work better there since their exhaust flow is no longer hindered by air blocking the nozzle’s exit. As a result, the thrust of a jet engine decreases with altitude whilst that of a rocket engine increases (by up to 25%, at least for a rocket engine with a nozzle that is optimized for use at high altitudes).