Swedish Zero "g" flights

Letter from Marta Bohn Meyer, Operations Engineer and Victor Horton, Flight Test Engineer to the Chief, Dryden Aircraft Operations Division, dated 2 March, 1982.

During the period of February 8 through February 26, 1982, a concentrated effort was made to accomodate the flying of a zero "g" experiment in F-104 825 for the Swedish Space Corporation (Government of Sweden). The actual flying was to have been accomplished from February 16 through February 19, but mechanical problems with the aircraft delayed the flying one week.

A chronological order of events is: 

February 8 
Experiment package arrived from Sweden to allow time to inspect and/or correct any differences which existed and bring all components up to NASA OFRF flight quality standards. (Several anomalies were found on the five printed circuit boards and modifications had to be made to the techniques used to safety and fasten hardware to the mounting plate.)

Second from the left: Marta Bohn Meyer, fourth from the right Sven Wallin Swedish Space Corporation,
second from the right materials scientist Hamid Shahani.

February 9
A mini Flight Readiness Review convened. In addition to the above problems, they flagged a potential problem with the seven sealed lead-acid batteries used to power the experiment and requested further research inta the matter (memo 000-22/82). Work was continuing to sort out some last minute and unexpected problems with the telemetry system (the aircraft had undergone a complete avionics-instrumentation modification in late 1980, but the instru-mentation system required to support the zero "g" experi-ments had not been exercised since the completion of that modification). A post flight inspection of the aircraft revealed problems with the zero "g" engine presently installed in the aircraft and the engine had to be changed (only two zero "g" J-79 engines are set up for these flights although a standard J-79 engine requires only an additional nozzle pump to be brought to a zero "g" configuration). The engine change work began immediately.

February 10
Swedish representatives arrived (Rolf Jönsson, project manager, Sven Wallin, Sounding Rockets; Hamid Shahani, Royal Institute of Technology). After brief discussions on the requirements for support of the experiment, they assisted in re-conditioning the experiment to meet the stringent DFRF flight standards. A cali to the battery manufacturer confirmed suspicions that the lead-acid batteries used to power the experiment would/may explode if subjected to a complete vacuum environment. (Complete loss of aircraft pressurization during the profile would result in essentially a vacuum throughout the aircraft although the compartment where the experiment is mounted is normally pressurized to 3 psi.) Consequently, the batteries were removed.

February 11
DFRF Battery Shop was asked to provide battery power suitable for the experiment, and using the spare set of nicad batteries for one of the other F-I04 aircraft, was able to supply a battery source within two hours. Engine change was completed, and a ground run showed no anom­olies. A Functional Check Flight was made at about 3:30 p.m. and at the conclusion, the engine and aircraft were declared fully flight worthy. Performed a complete experiment system check for the FRR Committee and to verify that the telemetry system interfaced properly with the experiment package.

February 12
First dataflight early in day. Discovered problems >with telemetry reception and quality of the signal displayed for the pilot. Front cockpit display reversed resulting in a lower quality profile. In fact approximately a 1/4 "g" was held throughout the profile.Experiment results were surprisingly good. On final approach, the pilot had to use afterburner to maintain speed to make the runway for a landing. Engine run showed no problems with engine during ground check. Discussion on what to do about engine problem. Telemetry reception problem was tracked down to several small items (one being the location of a VHF antenna too close to the TM antenna and possibly blocking out much of the signal) and these items were corrected.

February 15
National holiday

February 16
After much discussion and with the engine manufacturer's concurrence, the decision was made to remove the engine. This required a replacement and G.E. had to install a nozzle pump on a previously non-zero "g" engine. This was done but no ground test cell runs were made prior to engine installation in the aircraft.

February 17
Engine installation and ground run completed by early afternoon. A Functional Check Flight was made and the telemetry reception was checked. The telemetry reception was improved considerabley but the pilots displays were still of poor quality. Post flight checks on the engine showed a severe oil leak, location unknown.

February 18
Day spent running the engine with GE supervision to find the oil leak. In late p.m. it was found to be at the interface between the engine and the newly installed nozzle pump.

February 19
After the gasket was replaced, a small leak continued to be present in the same position. February 22 Nozzle pump was once again removed and two gaskets were installed between the pump and the engine. Engine run was completed and the oil leak was gone. The source of the poor quality displays in the cockpit was finally tracked to improper instrumentation pallet installation in the aircraft. This was corrected.

February 23
Two flights made-for data purposes, 0900/1300take-offs for 25 minutes each. Initial conditions:

Experiment turned on at 480, off at 2 g on pullout.Had trouble in attaining zero longitudinal acceleration. Peak altitudes 65 - 70000 feet. Attained 50 - 59 seconds of low "g". Experiment functioned properly with good test results.

February 24
Three zero "g" flights made, approximately 0830 take off for 20 minutes. Initial conditions:

Approximately 1030 take off for 20 minutes. Initial conditions:

Approximately 1230 take off for 20 minutes. Initial conditions:

Cirrus layer from FL 250 to 240 existed for all three flights. Had to accelerate in it and do the pull up IFR. This was somewhat distracting to the pilot. Attained from 45 - 60 seconds of low "g" in all axis. Instituted a call from the ground at approximately 55 seconds to terminate the flight because of the 60 second no oil pressure limit on the engine. Longitudinal axis still a problem due to lack of thrust at altitude. Peak altitude of 72000 feet.

February 25
Pilot had to go to Forth Worth for F-16 simulation.

February 26
Three zero "g" flights made. Approximately 0900 take off for 20 minutes. Initial conditions:

Approximately 1030 take off for 20 minutes.

Approximateiy 1230 takeoff for 20 minutes. Initial conditions:

Attained 60 seconds of low "g" in all axes on all three flights.

General comments:
The inability to obtain zero longitudinal acceleration is due to low engine thrust at the high altitude. The technique used for the last three flights was to pull the throttle back to min. afterburner for 10 - 5 seconds and then use max afterburner for the remaining portion of the low "g" trajectory. This produced .01 to .03 "g" in the Ax axis. It also gave time to observe the engine oil pressure and although it does go to 0 psi, periodically the pressure rises to 20 - 30 psi as the pump catches a slug of oil. Reduced throttle motions ease the pilots task in the An axis as each increase or decrease in thrust requires a correction in the normal acceleration. A wing rock is generally encountered "on top" of about + 10 at about .5 cycle/second.

A short memo will be prepared to send to future experimenters using our facility, stating the items that the FRR generally questions such as fasteners, circuit board, batteries, etc. This will be in addition to the normal DFRF process specification. Efforts will be made to inspect the experiment prior to delivery at DFRF. Attachment 1 is a stripchart for a typical zero g-profile flight, this one flown at 1230 on February 26, 1982.

Marta Bohn Meyer, Operations Engineer

Victor W. Horton Flight Test Engineer