Scott Crossfield’s engine explodes

The US government’s research program into rocket‑propelled aircraft made its tests at Edwards Air Force Base (formerly Muroc Army Air Force Base). Between 1950 and 1955 Scott Crossfield flew nearly all of the experimental aircraft under test at Edwards. These included the X‑1, XF‑92, X‑4, X‑5, D‑558‑I and the Douglas D‑558‑II Skyrocket. On 20 November 1953, he became the first man to fly at twice the speed of sound.

NACA was primarily concerned with research and did not usually try to break records, although in the case of Mach 2 it was allowed to make an exception to gather the research data.

In 1988 Crossfield described many of his experiences, including the value of the experimental aircraft program:

The research airplane program was probably the most successful government research program on record. It involved about 30 airplanes for 30 years, running from 1945 to 1975, and probably produced almost all of the information that has been essential to our transonic and supersonic flights, our transonic transports, and our space program.

Crossfield described the purpose of the X‑1 program:

The X‑1 was the first of the research airplane series – post‑war research series. Its primary purpose – or its sole purpose – was to see if we could, in fact, exceed the speed of sound with a manned aircraft. There were a lot of people who said that we could not. And a lot of reputable opinions that said that we could.

It was very simply designed. It was an airplane that incidentally was patented by Bob Wood in 1945. It used an RMILL4 engine which was the beginning of our successful rocket era [and was] developed by the Navy and Bob Truax. The all‑point simplicity and design – and the objectivity and design – made it very successful. It did accomplish its end of flying supersonically in 1947, of course, [we all know] with Captain Charlie Yeager at the controls.

The design of the D‑558‑II Skyrocket became the standard model for swept‑wing aircraft. Crossfield commented:

The D‑558‑II was one of the research airplanes funded by the Navy. That is the reason that it did not have the “X” designation.

It was primarily the review to look at what the transonic effects of the swept‑wing would be. With it we flew some several hundred flights and wrote the book on how we could design and build modern swept‑wing airplanes. It proved many of the things that we have learned since then.

The D‑558‑II was a very productive airplane. Almost every airplane in the air today has a little bit of the D558‑II basic information – or what we learned from it – in it.

Crossfield gave a specific example of this:

It has been well known for many years that a characteristic of swept‑wing aircraft is instability at high angles of attack. With the D‑558‑II, we learned many many ways to relieve that instability so that we could get the handling qualities that pilots need to fly airplanes in a commercial environment. Handling qualities… engineering ease for controllability… [are all] desirable characteristics.

Test pilots, like Crossfield, often had to fly several different experimental aircraft during a single day. Crossfield described such a day:

In the days of the research airplane program, things were somewhat different than the bureaucracy that we find ourselves in today. For instance, there could be a day where I would do an X‑1 launch early in the morning, fly the X‑4 over lunch hour, and do a D‑558‑II launch in the afternoon. That was not a typical day, but there were days of that type. We were very versatile in our operation in those days.

Crossfield gave his impression of the different handling characteristics of the X‑1 and the D‑558‑II:

Well, I flew both the X‑1 and the D‑558‑II. They were quite different in their flying characteristics even though they were both pretty good flying airplanes. [They were not]… necessarily as good as we would like to see because they were experimental.

I am often asked what goes through your mind when you are flying these airplanes? The answer has to be that you do not have time to [ponder] philosophical considerations of what is in your mind. You are concentrating all of your capabilities on the job at hand.

The basic shape of the X‑1 came from the .50 calibre bullet. Crossfield:

When they were designing the X‑1, we did not have the capability to do wind tunnel testing transonically. So they made a very good… decision. They made the forebody of the X‑1 shaped like a 50 caliber bullet which was a well‑known supersonic projector at the time.

It was [that kind of] judgmental design characteristic that was essential at that time; but we had no way to test [it]. And that is the sole reason for the research airplane program. We had the capabilities with engines to speeds and altitudes; [but] we had no capability to test. We did not know how to analyze, so flight test was the only way.

Crossfield described the development of the X‑15, including discussions with Walt Williams:

Well, as I remember the genesis of the X‑15, one time coming home from a fishing trip with Walt Williams (who was my boss at NACA)… We heard on the radio that a 75,000 pound thrust Viking rocket engine was successfully fired at Santa Suzanna. Of course nothing would do but I got a piece of paper out of his glove compartment and we decided what we could do to man a plane with a 75,000 thrust rocket. That became the X‑15. We gave that idea to Hilbert Drake who developed it in 1955. In that year the X‑15 went under contract.

Crossfield described what they were trying to achieve at that stage of X‑plane development and how it contributed to the drive toward space and eventually hypersonic travel:

The research airplane program’s primary goal was to develop technology that we could put to useful purpose – supersonic high‑speed aerodynamics… We had plans to take us [all the way] into space. That was part of the long‑range goal for the research airplane program. Unfortunately, that got diverted by many other circumstances.

The productivity of an airplane is gauged by its speed times its payload, divided by its fuel consumption. The way to get that productivity is to go fast. And the way to go fast is to go high. With the engines that we were developing in those days, we were trying to find out what it took to go high so that we could go fast and get the productivity that we needed for the air transport as we saw it at that time – and the way we see it today.

Unfortunately, we took a moratorium on that development for some years; but we are back on track today with the National Aerospace Plane, which is nothing more than an extension of the very successful research airplane program.

On his third flight on the X‑15 Crossfield ran into longitudinal instability with pitch oscillation. Crossfield described his actions in the cockpit and how he responded to the problems with the aircraft:

Well, the checkout in the X‑15 was rather abrupt in that, on our first flight, we flew it as a glider alone. That gave me three minutes and fifty‑eight seconds to learn how to fly the airplane and bring it in for a landing.

On the approach and landing, I had a control problem that really turned out to have a very simple solution. But the airplane, for all intents and purposes, appeared to be unstable and pitched to me, which meant that it was very difficult to control it. The pitching oscillations got very high and I had to figure out a way to get the airplane on the ground at the bottom of the pitching oscillation so that it would not wrap up in a ball of metal.

As it turned out, I succeeded. However, I landed at 140 knots instead of my anticipated 174 knots.

Crossfield described how they adapted the controls to enable the pilot to handle the aircraft when it was in violent motion:

With the X‑15 we anticipated that there could be some rather violent motions on re‑entry or in some of its maneuvers. One of the difficulties with flying the high speed jet aircraft with the powerful flight control systems is that the man’s arm gets into the thing. The weight of his arm feeds the action of the airplane. That is the so‑called “JC Maneuver.”

For the X‑15 to preclude that possibility, we made a control system such that [the pilot] could put his arm into a rest that would resist all of these external forces and control the airplane only with the movement of his wrist. With the axis of control being in this manner – with the roll control – we made it so that it could roll on the armrest… So we took all of this spring and mass system – [the pilot’s] arm out of the control system – to make it a very precise control system. It was part of the design of that system that [caused] the problem I had on the first landing. And it was easily correctable.

Crossfield commented on the distinction between pilots and “test pilots”:

Well, we keep talking about test pilots but there is no such thing as a “test pilot.” There are all kinds of people. There are tall people, small people. Some of them are functionally illiterate and some are intellectual. Some are moral. Some are immoral. They are all just people who incidentally do flight tests. It is a profession just like anything else. There is not, to my mind, any common thing called a test pilot.

The opportunity to be a test pilot… is there for all – and probably within the grasp of most. In my mind, we should divest ourselves of this idea of special people [being] heroes, if you please, because really they do not exist.

Crossfield described how his experience has been applied to contemporary aircraft:

At one end of the jet airplane era – with very powerful control systems – we found that the [pilot’s] arm began to become an important part of the inertia. As we got into the jet era and high‑speed flight – the very powerful control system that we had – we began to see the effects of a man’s arm on the control stick having an effect on the airplane as the forces on his arm varied.

With the X‑15, we anticipated… some of the violent maneuvers [on re‑entry] that could probably cause us some problems. So what we did was design a sidearm controller to preclude any input from his arms. It consisted of an armrest that you could hold your arm down on so that the only thing that was involved in the control of the airplane was the rotation of the wrist, with the pitch axis being… through that point and rolling your arm on the armrest. As a consequence then we could take all of that spring and mass system out of the control system. This proved to be a very useful development that we now find very often in our current‑day fighters which would have had the same problems if they didn’t go to something of this nature.

Also, that sidearm control was a contributor to the problem that I had on the first landing and we corrected that quite easily.

Crossfield described what happened with the ground test on the first XLR99 engine. He explained why he was in the cockpit when the engine exploded and what happened to him:

When we installed the large engine on the X‑15, [because of] our flight test plan we were going to demonstrate that the engine could be started. It could be throttled from 50 to 100 percent as designed on the first flight. The way that we were flying, I was limited to the speeds that I could allow the airplane to get so it took a very precise engine‑on‑off and thrust program to stay within that flight plan. To make sure that all of the systems would respond to this plan we made the last test of the engine on the airplane on the ground.

This is kind of humorous because the pilot gets into the airplane to run the engine. Everybody else gets into the block house. That is called “developing the confidence of the aviator.” In doing that run, we had a propulsion system failure that was born of something unique to the ground run that caused the airplane to blow up. About 1,000 gallons of liquid oxygen and 1,200 gallons of… and 800 pounds of 98% hydrogen peroxide got together and did their chemical thing. It was a pretty violent activity for a moment or two. It was like being inside the sun. It was such a fire outside that it was a very brilliant orange. The fore part of the airplane, which was all that was left, was blown about 30 feet forward – and I was in it. Of course I was pretty safe because I was in a structure that was designed to resist very high temperatures of re‑entry flight.

Crossfield explained how he became a test pilot:

Well, I am an aeronautical engineer, an aerodynamicist, and a designer. My flying was only primarily because I felt that it was essential to designing and building better airplanes for pilots to fly. My professional endeavor really was more in that line than being a pilot per se. It was part of the whole circumstance of designing and building airplanes.