KC-135

Zero Gravity Trainer

At NASA's Johnson Space Center, there is a microgravity research aircraft, a KC-135 (a predecessor of the Boeing 707 which is typically used for refueling military aircraft) nicknamed the "Vomit Comet." The KC-135 is a four-engine turbojet. It is used to fly parabolas to investigate the effects of "zero" gravity.

Investigators who fly experiments on the KC-135 must have a valid Air Force Class III medical, with NASA Physiological Training (which consists of an eight-hour training course and a high-altitude chamber run for hypoxia symptom training).

The day before the flight a Test Readiness Review is conducted. The investigators bring the hardware they'll be flying on the "K-bird" to building 993 at Ellington Field (near NASA JSC), where safety inspectors review the documentation and each investigator gives a briefing on his experiment. Typically, all the other investigators huddle around and watch each other's briefings. During the briefing, the investigator gives an overview of the experiment and the equipment, tells about the planned sequence of events, and describes how the equipment will be mounted inside the vehicle. (Equipment must be mounted to withstand 9 G crash loads.) The safety inspectors might ask questions about the medical effects of the procedures, or about how the wires will be run, etc. They frequently give the investigators last-minute suggestions for improving safety or convenience of the experiment. At the end of the inspection, the investigators are given a go/no-go, or are told what modifications need to be done before flight.

Later that day, the investigators will have access to the aircraft to mount their hardware, install any wiring, and tap into the aircraft's 120 VAC power lines.

The next morning, the aircraft is open at around 7 AM. Investigators arrive, make any last-minute adjustments to their experiments, then go into building 993. About 8 AM, there is a pre-flight safety video which reminds investigators about safety equipment and procedures. After the video, the investigators don flight suits (if you don't have your own, you'll be issued one) and heavy boots (ditto). Barf bags are handed out. (The best place for a barf bag is in the pockets on your chest, sticking out a little bit, with the top open. When the urge to hurl comes, it will come fast. Most of the investigators take their anti-nausea drugs, usually Scop-Dex, a mixture of scopalamine and dexadrine.

About 9:30, the investigators start making their way out to the aircraft. Ground support equipment (GSE) provides electrical power and (blessedly) air conditioning to the aircraft before flight. After a bit of milling around, the investigators take their seats at the rear of the aircraft, while Linda Billica and Bob Williams direct traffic and generally help out. Dr. LaPinta takes his seat. The NASA videographers check their camera mountings (if any), and strap in. Bob is the head of the team, Linda is his able assistant. Dr. LaPinta provides medical assistance before, during and after the flight. There's almost always a NASA photographer on board, sometimes two, for taking still photos and video during the flight. There's a pilot and a co-pilot up front, but we investigators never see them.

The following information is for use when designing or calculating information for the safety review of experiments flying on the KC-135 aircraft.

Design Requirements

This is a very brief outline. See the Glenn Safety Manual and other applicable documents for the full requirements.

Structural and mechanical design of experiment hardware should provide a minimum factor of safety of 1.0 against yield failure of the material and/or 1.5 against the ultimate failure of the material, whichever is limiting. Components prone to brittle failure (such as optical windows) shall be designed with a minimum factor of safety of 10.0 against the ultimate strength of the material. The worst case operating temperature expected to be seen by the material must be taken into account.

Each pressure vessel and pressurized system shall be designed to 4 times the Maximum Allowable Working Pressure (MAWP) and certified by testing to 1.5 MAWP. Pressure relief devices based on the MAWP and having a flow capability suitable for preventing overpressurization must be used. Systems inherently low in energy or commercial systems having a record of operation without serious incident need only to be shown to work without failure when exposed to the worst case operational conditions.

Electrical systems shall be designed and documented with components and wiring suitable for the expected voltages and currents. Each circuit shall have one or more overcurrent protective devices to prevent exceeding the design limits. Circuit steady-state loads shall not exceed 80% of the overcurrent protective device limit.

The Qualified Operators List (QOL) required for the safety permit will consist of the flight crew manifest. If any non-flying persons will operate the equipment for ground checkout they should be added to the QOL.

Free float or stowed for takeoff/landing experiments need only meet worst case microgravity loads (3g any axis) but must meet cabin mount loads when stowed. (Design to cabin loads or stow in approved container)

Cabin Volume

Use 4346 ft**3 for calculations
Exchange rate is 3 minutes for entire volume

Cabin Pressure
Nominal cabin altitude is 8000 ft (10.9 psia) during parabolic
maneuvers

Worst case if depressurized use 3.5 psia

Cabin Temperature
Normal operating temperature 50-80 deg F

Ground temperatures may exceed these limits

During a flight, which lasts about two hours, total, we usually fly 40 parabolas. In this aircraft, tail number NASA 930, we've done as many as 101, depending on fuel and gross weight limitations. More parabolas mean more flexing of the airframe and more stress. We could fly 200 maneuvers, but we'd use the airplane up.

After the flight, we fly back to EFD, usually arriving around 12. We change out of our flight suits, and we talk about how our experiments went. (We don't usually talk about who got sick and who didn't.) The more adventurous (or more masochistic) go to Pe-Te's Cajun Barbeque for lunch. The investigators remove their equipment from the aircraft -- it must be out by about 3 PM.

Interesting fact about the KC-135

• The KC-135 Vomit Comet was used to film the weightless scenes for the movie Apollo 13. Ron Howard, his crew, and the cast spent more than half a year renting space and time on the plane to produce the movie.
  

Other KC-135 Related Pages

• Experiments using NASA's KC-135 Reduced Gravity Simulation Aircraft
• KC-135 Stratotanker
• USAF FACT SHEET 92-21 KC-135 Stratotanker
• JSC Reduced Gravity Program Page.
• Facilities used by the ABF