_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.”