V0.2 - 17-04-2020
Understanding supersonic and hypersonic flight
When discussing the first rockets, one often comes to the founding fathers of rocketry: Gottard, Esnault-Pelterie, Tsiolkovsky, and Oberth. But when discussing the first operational rockets, von Braun is the first name that comes to mind. There is a lot that has been written on the history of rocketry and the early days of space flight. This page aims to discuss not these firsts, but the history of atmospheric entry and faster-than-sound flight.
Where Ludwig Prandtl already presented work on supersonic flows before the wright brothers officially made their first flight, the first examples of aerothermal heating and atmospheric entry were noticed much earlier. One classic example is the entry of a meteor. One could say that Yale professor Benjamin Silliman was the first one to link meteors to outer space and thus became the first to "discover" atmospheric entry.
It was known in WW2 that vehicles moving near the speed of sound encounter “weird” and interesting effects. Effects such as high drag lead to increased stresses on the structure and control reversal. This was not even new in the second world war, as it was already accidentally observed during the first world war. When pilots took high-speed dives, their planes would mishandle, sometimes leading to crashes. Now we know that an object racing the speed of sound encounteres a high drag peak, but back then it was thought that this increased drag would form an actual barrier, the sound barrier.
To aid in the design, several wind tunnels were made and used throughout Germany including tunnels going up to Mach 5 and even Mach 10. This extensive flight testing and ground testing lead to the development of an operational rocket which in 1944 reached about 4345 km/h, a record that would only be broken by the X-15's in 1961.
That the V2 was a potent system was proven when both the US and the Soviet Union captured both hardware and engineers to develop their own V2 clones. The US launched on October 24th 1946, made a photograph at an altitude of 105 km making it the first photograph from space, even though there was no universal definition of the boundary of space yet.
The first operational rocket to be launched was the German A4 or V2 rocket, in hindsight. During the early test flights, rockets would often break up in flight due to unknown reasons. It was only after Werner von Braun and several of his colleagues would stand in the region where the rocket would fail they noticed buckling. Buckling is a deformation of the skin due to the stresses of flight. As the V2 was the first ballistic type missile - meaning it flies upwards, leaves the atmosphere and comes back into the atmosphere - the re-entry element was not properly known. The V2's thus became the first objects to reach space and re-entry. But also the first manmade objects to fly supersonically and even hypersonically.
The first effect of aerothermal heating, heating due to high-speed flight, was observed when meteors entered the earth’s atmosphere and would heat up. The first human-made object to experience this effect is somewhat unknown but most likely would be the German V2 rocket was well during its final dive. During a flight in the first half of 1944, the rocket disintegrated. after analysis of the wreckage, it was seen that aerothermal heating was not a significant problem for these flights.
During the development of medium-range ballistic missiles, thermal protection became more of a problem. The Russian R-5 rockets used ceremic heat shields to insulate the "payload". Later, long-range ICBMs such as the R-7 were only possible due to blunt entry vehicles and ablative heat shields. Often the military funded these projects enthusiastically and the hardware was used for later civilian usage. Examples are not only the US-launched V2 rockets but also the R-7's launch of Sputnik and Gagarin.
The effects of ablative thermal protection were also already observed before the age of ICBMs again with the aid of meteors. IN 1920 Goddard stated:
"In the case of meteors, which enter the atmosphere with speeds as high as 30 miles (48km) per second, the interior of the meteors remains cold, and the erosion is due, to a large extent, to chipping or cracking of the suddenly heated surface. For this reason, if the outer surface of the apparatus were to consist of layers of a very infusible hard substance with layers of a poor heat conductor between, the surface would not be eroded to any considerable extent, especially as the velocity of the apparatus would not be nearly so great as that of the average meteor."
the shape of the vehicle itself also changed quite a bit, where the V2 rockets were sharp and pointy, and the ICBMs became blunt. In 1951 H. Julian Allen and A.J. Eggers made the discovery that the heat load experienced by a vehicle is inversely proportional to the drag coefficient. in another word, the higher the drag the lower the thermal load. The bluntness of an object makes sure air gets trapped in front of the vehicle as it cannot get out of the way fast enough This air then creates a layer of insulation thus the hottest gas does not get in contact with the entry vehicle.
Where ICBMs tend to fly at very high Mach numbers for a short amount of time, supersonic aircraft fly at lower supersonic velocities for a longer amount of time. The famous X-1 piloted by Captain Charles "Chuck" Yeager was the first confirmed aircraft to break the sound barrier. Later the Bell X-2 was the first aircraft to fly above 100.000 feet. Nine years after the first supersonic flight the Mach 3 barrier was broken. After this it went very fast during the X-15 program. This program took the record from Mach 3 to Mach 6+ within three months!