zoom climbs

Zoom Climbs – The Highest Life and Death Jet Flights to the Edge of Space.

In Planes, Videos by Paul ShillitoLeave a Comment


On the 31st of August 1977, Aleksandr Fedotov, flying a modified MiG-25RB with the more powerful Tumansky R-15BF2-300 engine’s, reached the record height of 123,523 feet or 37,649m and that record still stands today.

This marked the end Of the Super high altitude zoom climbs, a golden age of altitude records that had been started back in the 50s as a test regime by the Americans and ended up as part of the research into Space Flight but what is a zoom climb?

Back in the beginning of the jet age a few years after WW2, the threat of nuclear war was on the rise, and the main forms of delivery by both sides were by bombers flying at high altitudes.

To defend against this, both in the US, UK and Europe before the advent of effective surface-to-air missile defences, interceptor fighters would scramble once enemy bombers had been detected by radar.

Their job was to get from the runway to the altitude of the bombers as soon as possible, so as to intercept them before they reached their intended targets. If they took too long in reaching the required height even by a few seconds they could miss the chance of an interception.

As time went by, aircraft technology increased the height that the bombers were able to fly at which in turn required the intercept fighters to also fly higher to reach them.

Soviet bombers like the TU-95 could cruise at 45,000 feet and the high-altitude version, the TU-96 could reach 56,000 feet putting them at the limit of Western fighter’s altitude range.

During the early 1950s, several methods were developed for jet fighters to use a manoeuvre that would give them a boost in altitude that could be considerably higher than their normal flight ceiling.

I managed to find a copy of a report, which I have a link to in the description, written by Wing Commander L. Kelly at the College of Aeronautics, Cranfield in England in April 1952 in which he outlined the optimum climb technique for a jet-propelled aircraft. 

He introduced the concept of “energy height” but the procedure is basically like this.

A jet aircraft would take off and reach its maximum fight ceiling and fly at full speed, then it would pull up into a very high-angle climb and trade the kinetic energy that had been built into potential energy that would propel the aircraft to a higher altitude than that would be available by relying on the engines alone, a sort of slingshot manoeuvre.

Ideally, you would have a very fast jet with a high power-to-weight ratio. In the US in the late 1950s this would have been the Lockheed F104 starfighter, and in England slightly later the English Electric Lightning, and in the Soviet Union in 1959 the Sukhoi SU-9.

Zoom climbs are today commonly called “unrestricted climbs” and are often done by modern fighters to accelerate to a high speed near the ground and then pull the aircraft vertically or nearly vertically to quickly climb to the aircraft’s cruising altitude.

Back in the 50s, these were performed at high altitudes as part of the testing of new aircraft designs.

The F-104 Starfighter was designed as an air superiority fighter with a large powerful J79 jet engine and very short stubby wings which were mounted farther back than other fighters. This gave it very good supersonic and high-altitude performance at the expense of turning and low-speed performance.

It was also very light at just 6,350 kilogrammes empty weight, which gave it a very good power-to-weight ratio and it had a ceiling height of 73,000 feet or 22,000 metres.

As the space race started, the Aerospace Research Pilot School (ARPS) at Edwards Air Force Base, California was started.

Here the USAF would use the North American X15, a hypersonic rocket-powered aircraft as part of the X-plane series of experimental aircraft for astronaut training, but there was a need for a low-cost training vehicle that could simulate part of the X15’s performance and flight characteristics before the astronauts got in the X15.

The USAF bought 3 standard F-104 starfighters and had them modified by Lockheed to include a Rocketdyne AR2-3 rocket engine fitted at the base of the tail fin which would give an extra 6600 lbs of thrust.

With a standard F-104, the J79 engine with afterburner generated 15,000lbs of thrust and could take the plane to 60,000 feet but the engineers confidently thought that with the extra 6600 lbs of thrust from the rocket engine, it should be able to go between 120,000 to 140,000 feet. This could become the ideal Zoom flight platform.

In addition to the rocket engine, a reaction control system consisting of eight pitch and yaw thrusters, the same as used on the X-15 for use when it was at very the high atmosphere and normal control surfaces were ineffective.

They also removed unnecessary equipment like radar and guns to reduce weight, replaced the fibreglass nose cone with an aluminium one to house some of the reaction thrusters and installed a nitrogen-pressured cabin because the normal air bleed from the engine would not be available after it cut off in the climb phase.

These would become the Lockheed Aero Space Trainer or AST and were designated as the NF-104A which would go on to set the early high-altitude records.

However, one of the problems with the NF-104 was just how well the small wing would work in the very thin air above 100,000 feet. Although the F-104 was designed to be a high-speed high-altitude interceptor, when you tried to do other things with it, it became very unforgiving. Pilots would say that if the engine flamed out it would just fall to the ground with about as much glide as a set of car keys.

A typical flight profile for a high altitude flight would be to climb to 35,000 feet or 10,700 metres and accelerate to Mach 1.9 where the rocket engine would be ignited, and enter a short dive, on reaching Mach 2.1 the aircraft would be pitched up at 70° angle and the J79 afterburner would be throttled down once it reached 70,000 feet, then the engine fuel was cut at 85,000ft to stop it from overheating and damaging the turbine blades.

The Aircraft would continue on a ballistic path to a maximum height and descend back down to dense air where the engine could be restarted using the windmill technique.

In May 1958, an NF-104A reached an altitude of 91,247 feet or 27,811 metres at Edwards Air Force Base setting a new altitude record.

On September 4th 1959, a modified Soviet Sukhoi SU-9 set a record altitude of 94,659 feet or 28,852 metres using a zoom climb.

In December 1959 an early version of the McDonald Douglas F4 Phantom II during its proving phase accelerated to Mach 2.5 at 47,000 feet and then zoom climbed to 90,000 feet at a 45° angle. The pilot then shut off the engines and glided over the peak altitude of 98,557 feet before restarting the engines at 70,000 feet.

All of these manoeuvres were carried out by highly skilled test pilots, but it was still a very dangerous manoeuvre to try and pull off. And things could and did go wrong.

On the 10th December 1963, Chuck Yeager, who was the commander of the Aerospace Research Pilot School and the first man to go faster than the speed of sound was looking to set a world absolute altitude record in an NF-104A, which had been set just 4 days earlier by Major Robert W. Smith at 120,800ft, which is still the highest altitude attained by a US aircraft from a runway take off.

This was his fourth attempt to try and break the record but up until that point he had been unable to get above 108,000 feet.

On the last flight, He climbed to 35,000ft at Mach 0.85, then lit the afterburner which would increase for J79 turbojet engine’s thrust to 15,000 lbs.

Once he reached Mach 2.2, Yeager lit the rocket engine which would add another 6600 lbs of thrust pushing him to approximately 108,000 feet. But that was not enough for the aircraft to make effective use of the RCS and on the way down the NF104 was incorrectly positioned at only minus 50° nose down rather than the required minus 70° nose down and the aircraft entered a flat spin.

Without the required airflow, the jet engine could not be started and without the engine running there was no hydraulic pressure to power the flight control surfaces and he was unable to regain control of the aircraft.

The flight data recorder on board would indicate that the aircraft made 14 flat spins from 108,000 feet to the desert floor.

Yeager said he tried everything he could think of including using the drogue shut but to no avail. He rode out 13 of the 14 flat spins before bailing out and the aircraft crashed into the desert. Although he survived, he was badly burned by the ejection seat launch rocket when he was struck by the seat which was falling along with him.

In the following investigation, data from the flight recorder showed that the plane had been operating correctly with no mechanical failures and that this was down to pilot error as Yeager had not followed the instructions to keep to a 3.5 G climb and gain and maintain the 70-degree climb angle on all the previous flights as well the crash one.

After the explosion of the rocket engine on the 2nd NF-104 and the positioning of the third on a permanent pedestal at Edwards Air Force Base, this put an end to the AST programme and further zoom flights would be carried out using standard F-104s.

But Zoom climb would still prove to be deadly a few years later when in Nov 1968 Maj. Kermit L. Haderlie, a pilot at the Air Force Aerospace Research Pilot School flew an F-104C into a zoom climb.

He started at 45,000 feet and at 50,000 feet confirmed that his pressure suit was inflating properly.

At 63,000ft the positioning facility called Haderlie to order the standard afterburner shut down, four seconds later the aircraft was seen rolling inverted and Haderlie radioed that he had “lost his glove” as he passed 66,000ft.

The chase crew tried to regain contact but the data recorder showed that he passed thru 69,400 feet before descending inverted with full afterburner.

As he passed 44,000ft the chase crew told him to pull out, pull out… Eject! Eject!. But there was no reply.

As the aircraft hurtled down at Mach 2.2, it exceeded the thermal barrier, the F-104s canopy and skin overheated and as it hit the denser atmosphere the aircraft disintegrated.

In the following crash investigation, it was found that the mechanism that attached Haderlie’s glove to the pressure suit had a design fault allowing the glove to fly off his hand as the pressure in the cockpit dropped.

At 66,000 feet in the depressurized cockpit, this would have caused an explosive decompression in his suit where he could only stay conscious for a few seconds. He did not have the time to eject but even if he had, it was unlikely he would have survived at more than 1750 miles an hour.

At the time, approximately 400 of these pressure suits were in use and the USAF instituted an emergency safety procedure of taping over the connections and ordered a glove-to-pressure suit connector redesign to prevent additional accidents.

Although zoom flights continued, none of them got near the record set by Major Robert W. Smith at 120,800ft until Aleksandr Fedotov and his modified MiG-25RB set the record height at 123,523ft almost 14 years later.

And that’s where it has stayed and probably will do as modern fighter designs no longer go for the most powerful engines available like the MiG-25 did to allow for longer range and greater manoeuvrability.  

Although rocket planes like the X-15 or Virgin Galactic have flown a lot higher these are effectively spacecraft that were lifted by other aircraft before their solo flights and did not take off from a runway under their own power and therefore don’t qualify for jet altitude records.

So, thanks for watching and I hope you enjoyed the video and a big thanks goes out to Patreons for their ongoing support.

Paul Shillito
Creator and presenter of Curious Droid Youtube channel and website www.curious-droid.com.

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