In the previous episode of Know Your Plane I’ve talked about the Lockheed F-104 Starfighter,
but in this episode I want to talk about the most remarkable version of the aircraft:
the NF-104, a Starfighter equipped with a 6,000 pounds of thrust rocket engine that was used
by the United States Air Force for research and the training of many of its astronauts.
In this video I will cover the following topics: the differences between the NF-104 and a standard
F-104, its specifications, history and some trivia.
Modifications
The most obvious difference between this version and the early Starfighters is the Rocketdyne
AR2-3 rocket engine and its exhaust mounted at 9 degrees above and behind the standard
General Electric J79 engine.
This particular rocket engine was chosen due to its compact size, multistart and variable
thrust capabilities, and also because it used JP-4, the same fuel as the J79 engine.
Another reason for its choice is that it was also reliable, having been previously tested
by the American Navy’s F-4s and Air Force’s F-86s.
Through a mixture of JP-4 and hydrogen peroxide the Rocketdyne engine gave the F-104 an extra
6,000 pounds of thrust for a duration of about 100 seconds.
This engine also had a considerable service life of about 4 hours.
In order to compensate for the space lost due to placement of the rocket engine, the
NF-104s were also fitted with the larger tail of the F-104G.
Another external change to the aircraft was the addition of the Reaction Control System,
RCS for short, which increased the wingspan of the aircraft by 4 feet and included a total
of 12 motors around the aircraft powered by hydrogen peroxide, the same ignitor used by
the engine, to give the pilot control of the aircraft at very high altitudes where there
wasn’t enough air for the aerodynamic controls to be effective.
4 of these motors were placed on the wing tips to control roll and the other 8 motors
were placed on the nose to adjust the aircraft’s pitch and yaw.
Note that for safety there were two motors to provide that same movement in case on of
them failed.
The RCS configuration came from the X-15, reason why the NF-104 got the nickname “Poor
man’s X-15”, and one disadvantage of the system was that the motors were fully ON or
OFF, with no capability of thrust modulation.
I should also mention that there was a Starfighter known as JF-104 that was modified and operated
by NASA that did have a similar RCS system, however NASA’s version of the system had
modular thrust.
Other than the engine and the Reaction Control System, there were several modifications made
to the aircraft including the removal of all the military systems, reason why this aircraft
was named NF-104, with the letter N referring to “non-standard”.
The radar had been removed for the installation of the RCS on the nose, the fiberglass radio-dome
was replaced with a metal nose to protect the hydrogen peroxide tanks and the cockpit
was pretty much redesigned to accommodate the needs of the missions, most importantly
to optimize the cockpit to the Reaction Control System and its displays.
An azimuth reference instrument became the principal reference to pilots of the NF-104
and replaced the traditional attitude indicator.
They also installed sensors and cameras in the cockpit so that instructors could review
the performance of students after flights.
Special measures such the installation of a one-hour battery were also taken into account
since the pilots were supposed to shut down their engine during the climb which would
affect systems in an regular aircraft.
Because of that a nitrogen pressurization system mounted on the gun bay would take over
once the bleed air pressurization system was deactivated with the shutdown of the engine.
Lastly, another notable feature of the NF-104 was an improved engine inlet optimized for
high altitude, high speed recovery, reason why the inlets of the NF-104 are 20 inches
longer than a standard F-104’s.
Specifications
The NF-104 is considerably longer than a standard
F-104 due to the rocket engine and the yaw and pitch sensors on the nose.
The aircraft is also 4 feet wider due to the addition of the RCS motors on the wings, something
that also helped the NF-104 to get to extra lift and compensate for its heavier weight.
History
In the early 1960s the space race between the United States and the Soviet Union was on.
In the Western side there was the prospect of training the first American astronauts
for space missions and even an eventual militarization of space, the new frontier for the United
States Air Force with the X-20 Dynamic Soarer program.
But before this could all happen the Americans needed trained crews and a better understanding
of high altitude flying, thus they needed a training and research aircraft capable of
achieving these high altitudes.
In 1961 the American Air Force announced that it would re-designate its Flight Test Pilot
School at the Edwards Air Force Base as the Aerospace Pilot School, ARPS for short, with
the mission of conducting research at high altitude and also training America’s first astronauts.
In addition to the training program to become a test pilot, an additional 8-month advanced
space research course was set up.
One problem for the school was they they wanted to have an affordable trainer aircraft, North
American proposed a two-seater version of the X-15 but the ARPS favoured a proposal
by Lockheed which suggested a rocket assisted F-104 which had the advantage of taking off
by its own unlike the X-15 which needed a mothership.
Given that the school already had many Starfighters and that the F-104’s fight characteristics
resemble a low lift, high drag re-entry spacecraft the Starfighter was chosen and thus the NF-104
program was born.
The whole idea of using the NF-104 was so pilots could experience the four phases of
a space flight: boost to sub-orbit, zero gravity (which lasted for about 2 minutes), use of
the reaction control system and the re-entry.
And so the Air Force got three standard F-104A Starfighters that were sitting on storage,
namely the aircraft numbers #756, #760 and #762, and converted them into the NF-104 with
the help of Lockheed.
The first flight of the NF-104 was on July of 1963 by a Lockheed test pilot and all three
aircraft had been delivered to the air force by November of that year.
The students of the school already had some experience flying the standard Starfighters
on zoom climbs but the NF-104 took these climbs to the extreme thanks to the rocket engine,
but not every student got to fly it.
In order to fly a solo zoom climb in the NF-104 a student had to first practice for four hours
in a simulator, followed by three zoom climbs in a standard Starfighter with a gradual build
up of the zoom climb angle from 30 to 45 degrees, followed by some ground school and finally
some solo flights on the NF-104.
Zoom climbing in this aircraft was a demanding task because the operation included an extensive
checklist preparation, a pull up of 3.5-Gs at Mach 2.2 to an angle of 70 degrees and
the activation of the rocket engine.
With such a steep climb and the use of a pressure suit the pilots had no visual indication of
the horizon, thus they had to do all of the climb while flying on instruments to make
sure they were on the correct path.
They also needed to pay close attention to the exhaust gas temperature indicator and
adjust the throttle of the J79 engine accordingly, from the initial pull-up at full afterburner
the throttle would eventually be set to military power and even shut down as the plane climbed.
Upon reaching high altitudes the controls of the aircraft would become unresponsive
due to the lack of dynamic pressure, this is when the pilot had to switch from the regular
flight stick to the Reaction System Control to adjust the movement of the aircraft in
the near vacuum conditions and make sure the plane was in a good trajectory for re-entry.
And in this very complex process many incidents happened.
On October 23, 1963 Canadian test pilot for Lockheed, Jack Woodman, was performing a flight
test on the NF-104 where the RCS had been wrongly fueled with nitrogen instead of Hydrogen Peroxide.
During the zoom climb to 118,000 feet, when he activated the RCS to pitch down the airplane,
all the nitrogen was expelled and it wasn’t enough to pitch down the nose of the plane
because the system wouldn’t work unless it was fueled by hydrogen peroxide.
Unable to change the attitude of his aircraft and with a pitch up angle of 70 degrees, Woodman
decided to relax and enjoy ride to see what would happen.
The aircraft entered a spin and Woodman would report by radio at every 5,000 feet of altitude
lost, starting at about 115,000 feet.
By 60,000 feet of altitude he managed to get the nose of the aircraft down and only at
35,000 feet he had made a successful recovery of the airplane - this being considered by
some as the longest spin at high altitude, one that took over 80,000 feet to recover.
But worse than regular spins are the flat spins.
During a practice flight to break the world altitude record on December 10 of 1963 the
NF-104 #762 piloted by Chuck Yeager ended up entering a flat spin.
Unable to recover from the spin that started at about 104,000 feet, Chuck made a successful
ejection at 11,000 feet and landed safely with the exception of severe burns when the
seat motor hit his helmet.
While investigating the cause of the accident, they found out that the NF-104 had different
spin characteristics when compared to a regular F-104 because at high engine RPMs the aircraft
would oscillate in a spin due to the gyroscopic effect instead of entering in a flat spin,
however because the NF-104 had to reduce engine RPM to comply with the exhaust gases temperature
limitations its flight characteristics favoured a flat spin as it happened when Yeager tried
to correct his pitch with the RCS that didn’t respond.
In order to prevent further accidents it was recommended that on future zoom climbs the
pilot must maintain at the apex of the climb a minimum speed of Mach 1.0, a dynamic pressure
of 20 pounds per square foot and a maximum climb angle of 50 degrees.
As good as the new guidelines were they wouldn't be able to prevent the design flaws of the NF-104.
The oxidizer for the rocket engine, hydrogen peroxide, was very corrosive and the fuel
tank material wasn’t resistant enough, for this and other reasons including the leaking
of hydrogen peroxide to the wing, the two remaining aircraft were grounded for a considerable
time between 1964 and 1968.
Eventually on June 15 of 1971, the NF-104 program lost its second aircraft when the
rocket engine exploded during activation.
Immediately after the explosion the pilot shut down the rocket engine, declared an emergency
and managed to safely land the aircraft.
From what you can see in this picture the NF-104 #756 lost a large section of both the
rudder and the engine to be eventually scrapped, although NASA did take advantage to get the
RCS from the aircraft to perform some tests on their own aircraft.
With only one aircraft left, the #760, the NF-104 program came to an end and the last
surviving aircraft is currently on display at the USAF Test Pilot School.
From 1963 to 1971 the NF-104s, also known as Aerospace Trainer or AST for short, were
flown for approximately 50 different pilots, with many of them becoming astronauts, on
a total of 302 flights.
Trivia
The accident that Chuck Yeager had with the
NF-104 #762 is portrayed in the 1983 movie “The Right Stuff”, but here is the thing,
instead of having the NF-104 as you can see by the lack of the rocket engine exhaust the
movie features the F-104G, also known as the super Starfighter.
It is also true that the American military has never operated the G version of the Starfighter,
the one aircraft in the movie is actually German because many German pilots would train
in the United States because of the good flying weather.
In this case, even though all the aircraft were owned by the German government they would
be painted in the colours of the American Air Force.
This brings us to the end of our episode on the NF-104 high altitude research and trainer
aircraft of the United States Air Force.
Huge thanks to my supporters on Patron for helping me get my books about this very unique
variant of the Starfighter.
Hope you have enjoyed this video and see you guys next time!