Chapter 24: ►

_Ullage and Capsules_

We gathered close around Charlie Feltz’s desk in the garret. In two months our team had grown, seriously crowding our temporary quarters. Feltz now had two assistant project engineers, Bud Benner, a 33-year-old Pennsylvanian, capable, ambitious and relatively new to the company, and Raun Robinson, an old hand who had been around North American since the beginning of World War II. L. Robert Carmen had finally broken his “dreamer” partnership with NACA’s Hubert Drake. Now Carmen, who had helped Drake conceive the five-engine rocket-mother plane idea in 1950, was a member of the X-15 team. He was way out most of the time, too far into space for us, but destined to make one crucial suggestion that would pull us out of a deep hole.

We were all still new to the project and new to each other, feeling our way carefully, sizing up the talents and weaknesses of the individual players. It is not easy to start from scratch and organize a major-league team. In the field of rocket airplanes there were no minor leagues to draw upon. The major teams at Bell and Douglas which had preceded us in history had long ago drifted apart. Except for Carmen and myself, no one on the X-15 team had any experience whatsoever with rocket airplanes.

What we lacked in experience we made up in spirit. Although we still had not yet been completely “recognized” by North American--the dark secrecy surrounding our project hurt us from this standpoint--each of us knew, or was beginning to wake up to the fact, that we were working on something very special and important. In no sense was the approach routine. Feltz set the pace. He worked twelve to fifteen hours a day seven days a week; the rest of us fell into step without complaint. Although overtime was normally paid for extra working hours, no man on the X-15 team got it.

“All right,” Feltz said to the group around his desk. “Here is the bad news. In two months we have jumped from a 28,000-pound airplane to a 31,000-pound airplane. That’s three thousand pounds added weight.”

Someone let loose a long, low whistle. All of us knew the plane had been getting heavier, but this was the first time Feltz had totaled it up.

“To make matters worse,” Feltz added, “the specific impulse of the XLR-99 engine has dropped, according to Reaction Motors. It’s down from 278 to 269.” Specific impulse was our technical way of stating the efficiency of the engine, hence airplane performance.

“What does that mean in velocity altogether, Charlie?”

“Maximum velocity has slipped from 7200 feet per second to 5700. That’s about twenty per cent loss in speed, a little over a complete Mach number,” Feltz replied. When Feltz was glum, he could be glummer than any man I ever met. He reached his nadir that morning.

The engine under discussion, the XLR-99, was a customer-furnished item over which we had no control. If it failed to live up, it was not the fault of North American or our team. At that point North American’s Rocketdyne Division had built more rocket engines than any other firm in the free world. In one of its original X-15 proposals, North American had suggested an NAA-built Army Redstone rocket engine as a power-plant for the X-15. In that year, 1955, NAA tested the Redstone engine perhaps 5,000 times with singular success. But the Air Force picked Reaction Motors to supply our engine. There were good reasons for this decision. RMI had long experience in building rocket-airplane motors, reaching back to the early days of the X-1. Furthermore, North American’s Rocketdyne Division was very busy designing and building new engines for the Air Force ballistic missiles Atlas and Thor. The Air Force was reluctant to dilute the division with still another complex engineering project.

It was no easy task the Air Force assigned RMI. In many ways the RMI engine project for the X-15 was as revolutionary as the X-15 itself. The customer hoped to wipe out all past weaknesses of rocket-airplane engines. The goal was to come up with a “dream” engine many times as powerful as any in the past, and throttleable as well. The demands placed on RMI from the standpoint of reliability and precision were unprecedented. In our eagerness and search for perfection, we frequently became impatient with RMI. The fact that RMI was a small company facing a tremendously complex job on a fairly modest budget, with a thin line of engineering talent, rarely entered into our sharp discussions. Lying a full continent’s distance away and completely beyond our jurisdiction, RMI naturally became a favorite whipping boy in our camp. We blamed them unfairly in some instances. Later they were absorbed by Thiokol, a large company specializing in design and production of solid-propellant rocket engines for ballistic missiles.

“I think we can get some of this back,” Feltz said, referring to the lost velocity.

We leaned over a drawing which Feltz had spread across his desk. For background Feltz began to explain the external shape of the X-15, changed from NACA’s original views.

“On the X-1 and X-2 they mounted the maintenance tunnels on top and bottom,” Feltz said. Maintenance tunnels were housings or large pipes through which wiring, control cables, and plumbing were routed. Like missiles, the main body of a rocket airplane consists of a series of fuel and oxidizer tanks, as large in diameter as the fuselage. The wiring and plumbing cannot run through the sealed tanks; it must go around. Thus the tunnel concept was born years before.

“We found out in the wind tunnel that if we shift these tunnels from the top and the bottom to the side,” Feltz continued, “we can fair them out like a wing stub, running the length of the fuselage. This surface will add to the lift and give us more efficiency. Besides that, the tunnels being at eye-level will make for easier maintenance. We tried running them all the way out to the nose, but we got a severe pitch-up in the wind-tunnel tests. So we just cut them off here, right behind the cockpit.”

The new X-15 tunnel concept, the idea of George Owl, was absolutely ingenious. There is no other way to describe it.

“Now, back to the weight,” Feltz went on. “We have still another problem. NACA is demanding a three per cent fuel ullage. Three per cent of eight tons of fuel is a lot of weight. It’s damned near five hundred pounds.”

“Ullage? What the hell is that?” one of the younger engineers asked.

“Ullage is the allowance to be made for the fact that no tank can be completely filled,” Feltz explained. “In other words, we get five hundred pounds shaved off the total fuel supply. That means two or more seconds less burning time on the engine.”

We were all mentally calculating the performance penalty.

“Now, the big weight increase on the airplane itself comes from the customer. The ablating leading edges and nose are out. They believe these might make the plane unstable. At least, the wind-tunnel tests seem to indicate that. So from here on, the leading edges will be solid Inconel X. That will add considerably to the airframe. In addition, there’s some more instruments to go in, and more dampers for the control system.

“To get the performance back, we’re going to add six inches to the diameter of the fuselage and lengthen the tanks within the airplane. That will give us 2500 pounds more fuel capacity. But that’s as far as we can go with it. If we get any bigger, the weight of the tankage and fuel already begins to offset the gain of the added fuel. It’s a point of diminishing return. With the bigger fuselage and some ideas I have to save weight in the landing gear, I figure we can get the velocity back up to 6600 feet a second. That’s a net loss of only half a Mach number at maximum speed--down to Mach 6.5.

“But I want to tell you right now,” Feltz went on seriously, “I don’t intend to add another ounce to this airplane. It weighs 31,000 pounds now. It will weigh that when we roll her out. That means all you people have to trim every doggone thing out we can.”

“Say, Charlie,” an engineer said. “You know we got a space between frame 210 and 220 that you can see through. If you don’t watch out, someone’s going to stick something in there.”

“How big is that space?” Feltz asked.

“About ten cubic inches, I’d say.”

“Well, now,” Feltz said, “I just might cut a few inches off the length of this danged airplane. That’ll get rid of the space. No one can put something there if the space is gone.”

The meeting broke up in gales of laughter. But the engineers were soon glum again, busy at their desks figuring new ways to save weight. I hung behind. Feltz had indicated he wanted to talk to me.

“Scotty,” he said, propping his feet on a corner of his desk, “we got more problems. This one could really bust us for good. Take a look at this.”

He handed me a letter addressed to North American from a high-ranking Air Force general. I scanned through it hurriedly, stunned by the contents. The letter said that under new Air Force policy _all_ Air Force aircraft would be equipped with “escape capsules” rather than ordinary ejection seats. An escape capsule could assume many forms. Basically it was a “can,” as we called it, in which the pilot could enclose himself before ejecting from a disabled airplane. In theory the capsule would protect the pilot from wind-blast, heat, and high G forces associated with modern high-speed escape. The Air Force policy change had been prompted by experiences such as that of North American test pilot George Smith who had bailed out from an F-100 at supersonic speed. The blast tore the skin from his face. It was a miracle that he lived, really.

“Does this mean us, too?” I asked.

“It says _all_ new Air Force planes. The Air Force is paying for this one.”

“How much will this cost us?” We referred to weight like money.

“Twelve hundred pounds at least, to start,” Feltz said.

“That’ll ruin us.” I mentally estimated the total added weight to the plane--over eight thousand pounds. It would cost us at least a Mach number in performance, maybe more, and I knew it would take years to develop the capsule.

I could see in my mind the new problems the capsule would generate. Set within the cockpit, all the wires, controls, and plumbing would have to pass through it. It would have to be big or heavy enough to withstand the impact with the earth to avoid breaking the spine of an escaping pilot. It would require automatic ejection and automatic separation devices, and a parachute that would deploy automatically. In short, the capsule meant not only added weight, but greatly increased complexity, a dozen more things that might go wrong.

“This capsule thing,” I said. “It looks good on the surface, I know. But has anybody ever really engineered this thing out? We had a capsule nose on the Skyrocket but knew from the wind-tunnel data that if you separated the nose from the fuselage, the G force would be so great it could kill you. I made up my mind I would never use the Skyrocket capsule. I would ride the ship down and bail out. The X-2 has a capsule nose. It will probably kill the pilot, too.”

“You don’t have to convince me, Scotty. The way I look at it, if something goes wrong, the cockpit of the X-15 is the safest place to be, at least for a while. It’s going to be pressurized with non-inflammable nitrogen gas. You can’t have a fire. You can’t have a fire in space, anyway, because there’s no oxygen to feed it. The cockpit is stressed for plenty of G forces. If you are moving at maximum speed, that in itself means nothing is wrong. If something goes wrong, it means inevitably that the plane will slow down fast. So what’s wrong with just staying in the cockpit until you get down low enough and slow enough to eject?”

“I agree,” I said. “What can we do about it?”

“Well, we’re not going to get exception to an Air Force policy ruling with a phone call. The way I see it, we’ve got to engineer this thing out with a fine-tooth comb. Since this falls into the pilot’s realm, I think it would be a good one for you to take on. Call on anyone in the plant that you need for help. We’ve got to shoot this down or we’re dead.”

I turned to with a vengeance. I asked a half-dozen engineers to set to making studies on the big electronic brain in the main plant. Meanwhile I searched all the technical literature, pulling any and all engineering studies of escape systems. In the end, our team put in a total of 7000 engineering man-hours on this study. When it was completed, I was more convinced than ever that a capsule escape system was no good for the X-15. It might be suitable for combat-type airplanes. But for the X-15 it was superfluous.

In Santa Barbara, California, a few weeks later I presented the complete study to a gathering of Air Force and industry big shots. The presentation, probably the most thorough ever assembled on this subject, critically analyzed all escape capsule concepts as applied to the X-15. In every case, as I showed in chart upon chart, they were found wanting. Capsule development would increase the weight of the X-15 from 31,000 to about 40,000 pounds and delay the completion date perhaps years. The cost in terms of money would be enormous and unless a more powerful engine were used--at a cost of more millions--the X-15 would never be more than a Hangar Queen. And finally, I concluded after two hours at the lectern, the pilot would be no better off than he would be in the X-15’s special ejection seat. The audience, I hoped, was impressed.

A few weeks after that, in July, 1956, our customers came to North American for the first formal cockpit inspection. By then we had finished the cockpit mock-up, complete with instruments and a control system. The X-15 cockpit had no capsule escape system. It was rigged with the original X-15 ejection seat, a specially-designed affair with a new type of pilot-restraint harness and small stabilizers to “weather-vane” it into the wind blast and prevent fatal tumbling or oscillation. A small solid rocket, developing the thrust of the engine in the F-86, would blast the seat up and behind the X-15. The seat, without a formal reversal of Air Force policy, passed the inspection with flying colors. There was no alternative, really. Tied to the XLR-99 engine as we were, if the customer had insisted on a capsule for the X-15, would have killed the ship right then.

After the customers departed, Feltz said to me:

“Scotty, you really earned your pay on that one.”

“Somebody’s got to be stubborn and hold the line,” I said. “It might as well be me.”