Black Knight and the Re-entry Experiments

Of the 22 Black Knight launches, two were proving flights and one was for ELDO, testing the range instrumentation. The remaining 19 were all for re-entry experiments. Initially, these were to test out the design of the re-entry, but soon they broadened out into a more general study of re-entry phenomena.

The first two flights were the proving flights; it was the third launch, BK04, which was the first to carry a separating re-entry head. This had thermocouples on the head to measure the temperatures at re-entry, the data being radioed back to the ground. Later re-entry vehicles would have a tape recorder to store the data – which was another good reason to ensure that the re-entry head was found after the flight. The data showed that the peak heating was similar to that predicted, and thus the design was now proved experimentally. There were other issues which could be explored in later flights.

After the early flights had verified the re-entry body design, the direction of the programme began to shift. Defence against ballistic missile attack seemed almost impossible, but there was now an opportunity to investigate whether such a defence might be possible. There was also another objective – to discover how best to make Britain’s missiles safe from an anti-ballistic missile defence.

The first few flights had shown some interesting phenomena. Firstly, that the exhaust plume from an ascending rocket gave a very strong radar response.1 There was the possibility of using this to detect enemy launches, although this would mean some form of over-the-horizon radar. Secondly, that the re-entry vehicle gave a very weak radar response – what today would be called ‘stealthy’. This tied in with work being done at RAE by the mathematician Grant Dawson, who was studying the radar response of the V bombers.

Ballistic missile defences have been divided into exo-atmospheric and endo – atmospheric – or, to put it more simply, intercepting the re-entry vehicle outside the atmosphere or once it had re-entered. In order to intercept the vehicle, it first had to be tracked. Radar was the only way of tracking the vehicle outside the atmosphere – and, as mentioned, the re-entry vehicle shape had a low radar cross section – particularly viewed from head on. It was also relatively easy to hide the re-entry vehicle within a host of decoys which gave similar radar responses.

Interception within the atmosphere has to be done within a very short space of time – certainly less than a minute. Again, one problem is how to discriminate between the re-entry vehicle and decoys. Thus further flights were planned, using optical instruments to observe the re-entry. These were the ‘Gaslight’ series of experiments. The results were sufficiently promising to lead on to a further set of experiments, Dazzle, with American participation. For these flights, the range at Woomera would be much more heavily instrumented.

Roy Dommett, who was involved in the Dazzle experiments, describes them thus:

The DAZZLE programme was sold to the Governments on the basis of exploiting the Blue Streak technology of a low radar observable profile, which was then an advanced concept for the west.

Chosen was the simple conical GW20 shape for which the UK already had derived an extensive experimental aerodynamic data base. The intention was to observe the re-entries of bodies with heat shields made in simple, reasonably well understood materials. Our agreed choices were fused silica, copper, PTFE (Teflon) and loaded durestos (an asbestos-phenolic composite).

For comparison there were to be two reference copper spheres flown. Their manufacture proved surprisingly difficult. The copper shapes were turned to shape by hand held tools in a workshop behind a garage in North London by men wearing armoured vests, and then had to be kept spotlessly clean to avoid sodium contamination. The PTFE was moulded from powder in large sections under pressure, which bulked down about 30% more than expected. ICI, the supplier, was very helpful as no one had ever made such big pieces before, and its final profile varied noticeably with the room temperature. Being PTFE, it was very difficult to machine. The silica glass sections were made in rough by a glass blowing firm in the north near Newcastle using sand moulds, and we had change from £100. The crud had to be machined off with diamond tools in F1E workshop at RAE where we discovered that glass stress relieved itself hours after it was touched. It could only be assembled by having layers of asbestos felt mat between every glass-glass and glass – metal interface.

The conical copper bodies were expected to fail at some point during re-entry by softening and distortion of the nose, but up to that time they would be clean and the observables would be entirely due to the interactions with the atmosphere unaffected by contamination from ablation products. It was thought that the PTFE would massively sublime at a much lower surface temperature than the silica and the products probably suppressing the flow observables, and that the durestos would ablate messily, enhancing them.

He also has this to say about the sabot system described in the previous chapter:

To be absolutely sure of the quality of the data, it was decided that the re-entry experiments should be pushed into the atmosphere ahead of the upper boost stage using a sabot that was firmly restrained by a lanyard to the upper boost stage, so that the experiment should have been several thousands of feet ahead of it during re­entry. The sabot was driven by four Imp solid propellent motors. In vacuum the plumes spread enormously, and the section of the tether near the sabot had to be in steel. Playing out the tether could exceed the critical speed for the undrawn nylon rope, chosen to avoid elastic bouncing around, and a very careful packing technique had to be found. Finally the rope still broke in flight quite early, despite extensive ground based testing, then it was eventually realised that nylon type plastics have “attached water molecules” which boil off in vacuum, cooling the rope and making it brittle. We also found it near impossible to get the re-entry vehicles to come in without a coning motion of the order of 20 degrees generated by the separation disturbances.