Early Ablation Tests
In 1954 I was working at North American Aviation in Preliminary Design. The primary project at the Downey facility was the Navajo ram jet intercontinental missile. Preliminary Design was given the task of preparing a proposal for an Intermediate Range Ballistic Missile (IRBM). Management called it Comanche. They liked Indian names. It was recognized that the nose cone was a difficult new problem. There were a number of German engineers working at North American who had been part of the V-2 program in World War Two. They pointed out that the Germans used graphite jet vanes in their rocket exhaust region. Although the jet vanes lost some material they successfully survived this extreme environment. North American engineers thought some form of Fiberglas might also work. I was asked to investigate using such materials for the Comanche nose cone. I would have preferred a metal nose cone because it would be easier to analyze.

It was decided to fabricate several nose cones with various binder materials and test them in a rocket exhaust in North American Rocketdyne’s Santa Susana facility. In the tests some of the test cones blew up and others ablated but remained intact. Several years later ablative heat shields which dissipate reentry heat by allowing its outer layers to vaporize and/or combust in a controlled manner became the preferred type of heat shield.

I used the data from the tests to predict ablation of the Comanche nose cone by assuming that the ablation was proportional to the aerodynamic heating. This was not that far from the method of ablation analysis developed a few years later. Unfortunately the Comanche proposal did not result in a contract.

Air Force X-17 Reentry Heating Flights
In December 1955 I left North American to join Lockheed Missiles in Van Nuys. My office mate was Daniel Tellep who had recently obtained his MS degree at Berkeley. As you may know Dan eventually became CEO of Lockheed and arranged the merger with Martin. About two weeks after I arrived at Lockheed about 20 of the scientists that ran our organization left, and the majority of them formed Aeronutronic. Dan and I remained behind to continue work on the X-17 reentry test vehicle. The Air Force X-17 project was sold to the Air Force by the departing scientists. It was decided that all of the three stage vehicle except the nose cone would be the responsibility of the project engineers Dan and I, under the direction of the remaining scientists, were responsible for the nose cone. The object of the project was to learn how to penetrate the "heat barrier" during reentry of ballistic missiles. The first stage of the X-17 was fired upward and the second and third stages were fired down toward the earth to simulate high speed reentry.

The full-scale X-17 employed three solid-fueled stages. The first stage was powered by a Thiokol Sergeant rocket which could produce a thrust of 48,000 pounds. The second stage employed three Thiokol Recruit rockets which could each produce a thrust of 33,900 pounds. The third stage was powered by a single Thiokol Recruit rocket which could produce a 35,950-pound thrust by utilizing an enlarged exhaust nozzle.

The early test flights failed a few seconds after second stage ignition. One day Dan and I took a look at the inter-stage hardware at the base of the second stage. The second stage nozzles were buried deep inside the skirt. At high altitude the expanded plume could burn off the skirt. Dan and I were able to convince the chief engineer to modify the inter-stage structure. After this change there were about 20 successful flights.

Ramo Wooldridge was responsible for technical direction of the Lockheed X-17 project and the reentry vehicle work at AVCO and GE. The scientists at Ramo Wooldridge and ours that had left originally thought that it would be impossible for a vehicle to safely reenter the atmosphere unless it experienced only laminar flow. The X-17 nose cone was designed as a nickel plated copper spherical shape with a 2 micro-inch finish to prevent transition to turbulence. The wall thickness was thinned out away from the stagnation point in accordance with the predicted drop off in heating rate in laminar flow. I thought what if it becomes turbulent can we handle that? I developed a turbulent analysis¹ based on what I had learned from Dr. Ed Van Driest² at North American and some ideas of Professor Lester Lees³ on the effective Reynolds Number on a three dimensional body with dissociated variable flow outside the boundary layer. The results showed that unlike laminar flow where maximum heat transfer was at the stagnation point, in turbulent flow the maximum heat transfer would be near the sonic point of the flow outside the boundary layer analogous to the throat conditions in a nozzle. On a sphere the sonic point would be about 40 degrees from the stagnation point. Therefore we redesigned the nose cone with a constant wall thickness. This made it heavier than the original design and led to a somewhat degraded performance of the vehicle.

The data that we obtained from the flights consisted of temperature measurements from an array of about ten thermocouples. We located the thermocouples very close to the surface of the nose cone. To interpret the results we really wanted to obtain the heat transfer rate rather than just the temperature. We solved the transient heat transfer problem backwards to get the instantaneous heat transfer rate. Of course this problem is inherently unstable so that we got wiggley bands of heat transfer rates. Nevertheless, it was obvious that the largest heating rate was near the sonic point. To interpret flight data we used three IBM 650 computers and about a dozen women running Friden machines.

Although we were allowing for turbulent flow the Ramo Wooldridge people were still hoping for laminar flow. One day Dr. John Sellars came over to inspect a nose cone that was about to fly. It was impossible to manufacture these nose cones without some flaws. John worked out an elaborate analysis for allowable length width and depth of a pit. The nose cone flunked. In order not to delay the flight someone got the brilliant idea of taking the nose cone to a dentist to fill the pit with gold. The nose cone flew and performed as usual.

Another fun episode had to do with trying to reduce stagnation point heat transfer. It was well known that stagnation point heat transfer on a sphere is inversely proportional to the square root of the sphere radius⁴. Therefore if we make a nose cone with a flat stagnation point region (infinite radius of curvature) there would be no heat transfer. Professor Wallace Hayes was one of our consultants with whom we discussed this idea. He came back the next day and said, "I was thinking of your problem while taking a shower. As the water hit my nose I realized that it is not the body radius but the shock shape that governs the flow." He worked out a simple theory, based on his previous work⁵, that Dan Tellep implemented for analyzing various shapes. More elaborate numerical methods for the shock layer were later created at AVCO and GE. We designed some pretty weird shapes including a 5^th degree paraboloid. This shape had a very flat nose and no discontinuity in radius of curvature, but the peak turbulent heating was still near the sonic point.

Blunt Body Metal Heat Shields
At about this time H. Julian Allen and A. J Eggers, Jr wrote their famous NACA Report 1381 "A study of the motion and aerodynamic heating of ballistic missiles entering the earth’s atmosphere at high supersonic speeds". The report is available on line at NASA Site. They showed that the higher the drag the higher the altitude of peak reentry heating and the lower its magnitude. This led to the era of blunt body metal heat sink reentry vehicles like the GE Mark 2.

Graphite and Other Ablative Heat Shields
I continued my interest in reentry at Aeronutronic. Don Dooley who I knew at North American showed up in an office down the hall. He was just finishing up his PhD with Professor Frank Marble at Caltech. His thesis was in the new field of Aerothermochemistry promoted by Theodore von Karman. Dooley got Frank Marble to come over as a consultant. This began a relationship for me that lasted virtually the rest of my career. Frank is a brilliant scientist-engineer with all sorts of insights and perspectives on technical problems and people. Dooley introduced me to the ideas of Aerothermochemistry, and we thought maybe we could explain the behavior of a graphite nose cone during reentry which might be better than the metal heat shields that were used on all the US ballistic missile programs. We did an analysis for a laminar boundary layer and wrote it up⁶. Some people at Ramo-Wooldridge were thinking along similar lines⁷.

At around this time Dr.George W. Sutton, who was at GE, reported on some ablation experiments⁸ in a rocket exhaust that started a flurry of interest in ablative materials for heat shields. Dr. Sutton fabricated thermosetting plastics with refractory fibers such that the orientation of the fibers were randomly distributed to prevent delamination. Moreover, the materials were selected very methodically, and the testing method was to test them in the supersonic flow of a rocket exhaust before the first barrel shock. In the same paper Dr. Sutton defined the effective heat of ablation. The first complete analysis with melting and blowing was done at GE by Dr.Sinclair Scala and Dr. Sutton ⁹. Later Professor Lees wrote his own version¹⁰.

Frank Marble and I had been talking to Dr. Robert Bromberg and Dr. C. B. (Budd) Cohen at Ramo Wooldridge about the idea of using graphite for nose cones. Professor Marble came up with the idea of testing graphite combustion in pure oxygen to simulate the greater heating rate in reentry that we could not reproduce on the ground. We had sent in a proposal to do such testing and briefed them on our boundary layer theory. We got the job.

George Carver designed and supervised construction of the blow down facility that was required to supply the oxygen. Chuck Hallum and Dale Jefferies were the technicians. In the process of checking out the facility we had a bad accident. Chuck was turning the valve manually to allow oxygen to flow through the test section. He said later that it seemed a bit harder to turn than usual. There was a flash fire. Chuck was burned on 70% of his body but he survived.

The test results were in pretty good agreement with our predictions. We wrote up a company report¹¹ and later sent the results in for publication¹². The theory was published in a company report¹³ and later published in the IAS Journal¹⁴. The theory was based on an extension of a Van Driest like model to include the chemical and ablation ideas that were influenced by Don Dooley and Frank Marble.

When low drag ablative vehicles became the preferred design the concept of effective heat of ablation was employed to compare materials. The effective heat of ablation was defined as the heat flux to a non ablating zero temperature surface divided by the rate of mass loss. The value could be obtained by experiment, but it could also be obtained from theory including heat of vaporization, combustion and the heat blocking effect of blowing into the boundary layer.

Near Wake Studies
After Aeronutronic I moved on to Electro Optical Systems. The company was founded and run by Dr. Abraham Zarem. Abe was a unique personality. He was once voted the most outstanding young Electrical Engineer after he got his PhD at Caltech, but it was his personality and insight that really made him special. With help from Abe I got a contract from Cliff McLain at ARPA to study the near wake of reentry vehicles. This was aimed at the problem of predicting wake observables during reentry. In a paper written with Dr Eric Baum¹⁵ we calculated a shear layer which began with a laminar Blasius distribution from the boundary layer on the body. For large distances downstream the solution approached the Chapman distribution. This paper became pretty popular.

The work continued on the near wake. Dr.Artur Mager held sessions at the old IAS building in Hollywood where a particular investigator would give a talk on his work before publishing it. At this and other meetings we had discussions with Professor Lester Lees of Caltech, Professor Marty Bloom of Brooklyn Polytechnic Institute, and Dr.Andrew Hammit and Dr. Les Hromas of Space Technology Labs.(which later became TRW) A summary of our additional work was published in the AIAA Journal¹⁶.

Reentry Observables
I continued working on reentry at TRW. Les Hromas was in this organization. He had weekly meetings with Lester Lees to go over aspects of the reentry wake problem. The wake was important to ballistic missile defense because of radar and optical observables which occur during reentry. It was hoped that these observables would provide a means of distinguishing the reentry vehicle from decoys. Lees and Hromas had written several papers on the subject¹⁷. One day in 1966 Dr Charles Hebel and Ray Markle of Bell Labs came to see Les Hromas, whose work on the wake they knew, and offered TRW a sole source subcontract to their Reentry Measurements Program Phase B (RMPB) contract with the Army. The purpose of the contract was to fly a full scale instrumentation package on about 10 Atlas flights. TRW did not say no thanks. The project that we got was called Reentry Measurements Instrumentation Package (RMIP). It was as large as the major satellite projects at TRW, but it was always a bit of a step child because it was not a satellite project.

On RMIP I became Assistant Project Manager for Experiments. I played the role of Chief Scientist. I was supposed to make sure that the needs of the experiments did not get overwhelmed by hardware systems considerations. Tommy Thompson, who was in Les Hromas’ organization, helped define the experiments and write specifications for them. The hardware systems people wanted the requirements for the experiments very quickly. They did not want to hear from the scientists after that unless something went wrong.

The experiments consisted of instruments flush mounted on the sides of the reentry vehicle to measure properties in the body boundary layer and instruments at the base of the vehicle to observe the wake properties. In addition to the stationary wake instruments a wake probe was deployed into the wake with data transmitted back to the vehicle along a long cable which was cut at the end of the deployment. Wake probes were deployed at three altitudes during reentry. Three different heat shields were flown on the Atlas vehicle: silica phenolic, teflon, and beryllium with a graphite tip. We set up a Principal Investigator (PI) system for the experiments. Dr. William Shackleford was the PI for UV and Visible Spectrometers, Dr. Ron Watson was the PI for Infrared Interferometers, Dr.John Chang was the PI for the Wake Probe Experiments, and Dr. Gerherd Grohs was the PI for Electrostatic Probes. We also had Acoustic Sensors, Radiometers and various housekeeping measurements. I found that for any obscure scientific subject I could always find someone at TRW who was an expert on that subject.

In the flight program there were several Atlas failures or reentry vehicle anomalies, but when we were able to obtain data the TRW experiments were highly successful. The data, especially the optical data, were still being used well into the 1990s. A project this elaborate will probably not be repeated.

In my spare time I got interested in the electrostatic probe experiment. These probes were flush mounted in an array on the sides of the vehicle. I predicted the decay of boundary layer electron and ion density with downstream distance in the boundary layer that would be measured with these probes ¹⁸ This turned out to correlate well with the data. Eric Baum did some more detailed analysis for the flush probes¹⁹²⁰²¹. He also analyzed the conical electrostatic probes that were deployed on the wake probe²². The theory correlated well with data of Scharfman at SRI. Dr. Andrew Hammitt using a different theoretical approach also got good correlation with the data. The experiments are described in an AIAA Journal article²³. Dr. Baum did some related work with Bob Chapkis²⁴. Dr. Gerhard Grohs²⁵ studied charged particle generation on a Silica Phenolic heat shield. A nonequilibrium analysis of trace species reactions in the turbulent boundary layer is described.

Dr. Baum revisited the near wake problem working with Jack Ohrenberger²⁶. At high supersonic flow speeds a lip shock forms as the flow turns to form the free shear layer. This changes the character of the local flow field. In addition to the lip shock formation the analysis includes the growth of the mixing layer into the rotational inviscid wake of the boundary layer, the recirculation region, and the wake shock in the wake neck region.

Stanley Howard²⁷developed a simplified technique for determining the electron density and the resultant radar cross section in a reentry wake. The method is based on correlations of previous flow field calculations when these are available. Where calculations are inadequate correlations of flight data are used.

I became involved in many other interesting projects, but my participation in reentry analysis, even as an observer, declined after the early 1970s.

References