September 6, 1999 NASA's Vomit Comet: Hitchin' a Ride on a Buckin' KC-135

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by	Peter W. Yost

About the Author ... Peter Yost earned his Private Pilot certificate the old-fashioned way by handpropping a Taylorcraft BC-12D at a small grass airstrip in southeast Pennsylvania. He has since added a Commercial Glider rating to his collection. As an aerospace engineer, Pete works in developing safety equipment for civilian and military aircraft. So far in his career, he has worked on projects with all branches of the military, with NASA and most major aerospace companies in the U.S., and with companies in England and in Russia. He has been a member of the EAA, Soaring Society of America, and the American Institute of Aeronautics and Astronautics for over 20 years. Much of Pete's flying these days is of the short, up-and-down variety as a tow pilot with the Philadelphia Glider Council.

NASA Gives Students A Taste Of Space Flight — 30 Seconds At A Time

The engines are screaming at full power as I pull myself up off the floor against the 1.8 Gs and manage to look out the side window. What I see is not the typical view from a four-engine transport category aircraft: the starboard wing with the two Pratt & Whitney engines is tilted upwards at a steep 50 degree angle, the blue waters of the Gulf of Mexico over 30,000 feet below. If I were in any other plane of this size at this angle, I'd start asking the Almighty forgiveness for all the stupid things I did in my life. But today it won't be necessary. A few seconds later I hear the engines throttle back and NASA test director John Yaniec shout, "Hear we go, over the top." You feel it first in your stomach, just like the top of the roller coaster, but then the rest of your body catches up and the next thing you know you are weightless and floating free. The whoops and hollers of my fellow passengers on the NASA KC-135A say it all: This is what it feels like to really fly, to float free just like in those youthful dreams where you spread your arms and soared through the sky.

This time I'm not dreaming, but instead flying on the NASA plane with the nauseous name, the Vomit Comet. I'm participating as the journalist invited by a team of students from my alma mater, Purdue University, to cover their flight, and put my digestive tract in peril for AVweb readers. The Purdue team is one of 32 selected from colleges throughout the U.S. to participate in the Reduced Gravity Student Flight Opportunities program for three weeks this August. Funded by NASA and administered by the Texas Space Grant Consortium, the program allows college students to design and build an experiment for testing in zero gravity aboard a NASA KC-135A. The KC-135A simulates spaceflight by flying a series of parabolas that produce about 30 seconds of zero-G at the top of each parabola.

Not Just Another Junket

Started in 1995, the primary goals of the program are to involve students in research areas of interest to NASA while providing them with an invaluable educational experience. To be selected to fly aboard the KC-135A, the teams must create an idea for an experiment and then prepare a detailed proposal for review by NASA. The proposal is evaluated on scientific merit, design feasibility, fabrication, and compliance with NASA experimental protocols. NASA doesn't want this experience to just to be a joy ride for the students, and only the best of the proposals are selected to fly. The objectives are for the teams to conduct real science, learn as much as they can, and then share their experience with fellow college students and also primary and secondary students from their local area.

The team that invited me is one of four from Purdue University, West Lafayette, Ind., that had been chosen for test flights this August. Purdue has a reputation for one of the best aerospace engineering programs in the country (Hmmm, I guess that's why they accepted me), and has produced more graduates who have gone on to be astronauts, including Neil Armstrong, than any other college. My Purdue team included Nick Sadaah, from Oklahoma City, Okla., the team leader and a co-op student working at NASA's  Johnson Space Center (JSC) during the summer; Curt Peternell, Ft. Atkinson, Wis.; Rob Whiteman, Walled Lake, Mich.; and John (J.D.) Yamokoski, Tampa, Fla. Nick is an aerospace engineering major, with Rob, Curt, and J.D. majoring in mechanical engineering.

Their experiment was titled "Investigating The Use Of Piezoelectric Actuators To Actively Dampen Vibrations In Microgravity." I won't go into great detail about their experiment, but basically they were trying to use small, lightweight piezoelectric actuators to dampen the vibrations of a beam floating in zero-G. The beam simulated a spacecraft fuel tank that is subjected to vibrations when fuel sloshes around inside after an orbital thruster fires. If the vibrations are bad enough in the tank, it can disrupt experiments being conducted onboard the spacecraft. In their experiment for the KC-135A, the actuators were used to induce accelerations at the same frequency as the initial beam vibrations, but out of phase to dampen them. Think of the active noise reduction principle that makes your pricey pilot headset work, but apply it to accelerations in a beam instead of sound waves, and you get the idea.

All four team members and this experiment flew in March, but the team hadn't received good results. They took at hard look at what went wrong with their experiment, made changes, and applied to fly again this summer. NASA encourages this type of repeat effort, as very few research experiments work perfectly the first time. From my own experience as an engineer, this is usually what happens in the "real world." I used to say that we never learned anything unless we broke something during a test program.

Professor Steven Collicott, from the school of Aeronautics and Astronautics at Purdue University, was the faculty advisor present in Houston to watch the Purdue teams in action and offer last minute advice and encouragement. Professor Collicott has been the driving force behind the program at Purdue since 1996 when two students approached him with their desire to participate. So far nine Purdue teams have had their proposals selected by NASA, with over 30 students getting the chance to fly. The program has become so popular that aero engineering students can now chose it as a three-credit elective.

Professor Collicott feels that participation can help jump-start a student's career, saying, "As the years go by I think they will appreciate more of the months that lead up to the flight, not just remembering the flight, but the whole engineering education and experience. If you talk about what's going to help someone's resume jump out of a stack of 500, this type of experience is what employers look for when hiring engineers."

Some of the Purdue students who participated in the program have since gone on to work for NASA and are currently working on the International Space Station.

An Ordinary Tanker With An Extraordinary Mission

N931NA, the KC-135A that I flew on, is the eighth aircraft that NASA has used for reduced gravity research, beginning with a C-131 back in 1957. NASA also used a C-135 before switching to the KC-135A, and 931NA is the fourth KC-135A to be used for this purpose. Over the years, the NASA planes have flown over 80,000 parabolas, the equivalent of almost five weeks in space. The Boeing Company's model 367-80 was the basic design for the KC-135A Stratotanker as well as the later 707 commercial airliner. The first production Stratotanker was delivered to the Air Force in 1957, and 931NA was the second-to-last one ever made, with production ceasing in 1963 after more than 600 were produced. The majority of Stratotankers still flying in the Air Force or Reserves have been modified with turbofan engines that are more powerful, quieter, and fuel-efficient. Re-engined Stratotankers were designated either the KC-135E, R or T models.

Surprisingly, 931NA has not seen any major modifications since NASA began operating it in 1995. Unlike the existing Air Force models, it is still equipped with the original Pratt & Whitney J-57 turbojet engines. This was clearly evident by the four black exhaust trails streaming behind the ex-tanker, plus the high noise levels as it flew in and out of Ellington Field (EFD), where it is based. The refueling boom, minus the maneuvering "wings," is still attached to the underside of the aft fuselage. Inside, the 60-foot-by-10-foot main cabin is padded all around to protect wayward floaters, with about 25 seats remaining in the back for passenger use during takeoff and landing. There have been some minor modifications to the hydraulic system to keep the pressure from dropping to zero during the periods of zero-G, but the structure itself has not been strengthened, as the NASA missions are still within its normal certified operating envelope of +2.5/-1.0 Gs.

Although most people are familiar with pictures of astronauts being trained in 931NA, this makes up only about 20% of the aircraft's missions. Scientists and engineers from NASA and other organizations use the majority of its flight time for microgravity research. 931NA is also used as a "pathfinder" aircraft whenever the 747-carrier plane has to fly a shuttle back to the Kennedy Space Center (KSC) from a landing at Edwards AFB. 931NA flies 15 minutes ahead of the 747/Shuttle combo to warn of any rain or turbulence that could threaten the shuttle's delicate heat tiles. It is also used to calibrate the microwave landing system at the KSC runway in Florida, so you can see that this plane is not just a one-trick pony. Even though NASA has not made any structural mods to 931NA, they do have a very thorough maintenance and inspection program. This includes a Phase I inspection every 180 days, at which time it can take four or more days to perform a detailed non-destructive inspection analysis of critical structural components, searching for any evidence of fatigue. From what I could see after being in and around the plane for five days, it looks well cared for by the full-time four man crew from NASA maintenance contractor DynCorp.

Parabola Flying 101

I was curious to discover what it was like to handle such a large plane during the parabolic maneuvers, so I sat down and talked with Stephanie Wells, the NASA pilot who commanded my flight. Stephanie, an Iowa State University graduate with a degree in meteorology, was one of the first women accepted for flight training by the U.S. Air Force back in the '70s. In her ten years of active duty in the Air Force, she was an instructor pilot on T-37s and T-38s before moving on to fly the C-130. She joined NASA thirteen years ago, but continued to fly in the reserves, commanding the largest aircraft in their inventory, the C-5A Galaxy. According to Stephanie, a checkout in 931NA requires two weeks of Air Force ground school and simulator work, and then a return to EFD for a local check ride in the copilot's seat. There are eight NASA pilots (four pilots, four copilots) qualified to fly 931NA, but just like in the airlines, you have to wait until a pilot retires or leaves before you can slide into the left seat. Then, five more flights are required before you can start carrying experiments in the back. Stephanie is one of the "newbie" Vomit Comet pilots, with about 300 parabolas to her credit in the six months since she has been checked out.

According to Stephanie, a typical flight begins with a morning takeoff from EFD heading out over the Gulf of Mexico. The flights are always conducted under an IFR flight plan and handled by Houston Center, with the majority of the flight conducted in a warning area that extends from 60 miles south of EFD to 120 miles south of EFD. As far as weather minima are concerned, NASA won't fly parabolas in the clouds or if there is moderate turbulence forecast or encountered. Stephanie said that turbulence could affect the quality of the parabolas and thus the experiments in the back. Although the KC135A has a gross weight capability of 290,000 lbs., NASA flies the parabolas at 150,000 lbs. or less. At this weight you don't have any fuel in the forward or aft fuselage tanks to slosh around and screw up your ability to fly a smooth parabola.

Stephanie described how she begins the parabolas after leveling off at FL250. "The autopilot is engaged for roll and yaw. The pitch part of the autopilot is disengaged so we can hand fly just the pitch. The autopilot is actually hooked up into the navigation system so it keeps us on the track. All you have to worry about is pitch. The right-seater runs the throttles to make sure the energy level is right. We enter the parabola indicating 350 knots, which is right up near the redline speed. Start your pull-up at 1.8 Gs, bleeding off 100 knots in the pull, start your pushover at about 240 knots, and at this point you're about 50 degrees nose-high. Push over smoothly, and it takes about 10 seconds to go from 1.8 Gs to zero-Gs. Stabilize at zero-G as you go over the top until you are about 40 degrees nose-low. You have to push the yoke pretty far forward, but it's not a heavy push because you're pretty slow by then. Then once you're in the zero, it's not heavy at all, you just make small adjustments to the pitch. To finish the parabola, you try to hit 350 knots on the way down before pull-up," and then the whole process begins again. Each parabola takes about 10,000 feet from pull to top of the parabola, and they fly 16 parabolas on a straight track before making a 180-degree turn to fly the remaining 16.

When asked how she would compare flying the KC-135A to the C-5, Stephanie said, "The C-5 has much better handling qualities. The KC-135A can be a handful. It has negative Dutch roll qualities, and when it hits turbulence or crosswinds or gusty winds you have to learn how to overcome those. But it's an honest flying airplane and you really feel the airplane when you're flying it. It's a pretty heavy feeling compared to a hydraulically controlled airplane." In addition to flying 931NA, Stephanie also flies T-38s and Gulfstream G-IIs for NASA. I'm sure Stephanie Wells could have made more money flying for the airlines, but I got the feeling she's having much more fun flying for NASA. As Stephanie put it, "It's a cool job, a very good job."

No Zero-G Recess Until You Complete All Your Homework

Preparation to take on the challenges of the Vomit Comet started several months before when I was required to submit medical papers to NASA proving I was fit enough to fly on their plane without croaking. NASA requires all those who fly on 931NA to meet the medical requirements of an FAA Second Class flight physical. Easy enough, since I already had a current Second Class medical. But, since I was an "old-timer" (over 35), NASA also wanted to see an acceptable chest X-ray and EKG. Once the NASA medical types approved my paperwork, I then had to go through a day of physiology training at Johnson Space Center (JSC) with the students and other journalists before I could fly. The training included classroom instruction on hypoxia, decompression sickness, and of course spatial disorientation and motion sickness.

Our physiology training culminated with a "ride" to 25,000 feet in NASA's altitude chamber. During our "flight" we reviewed emergency procedures in case we encountered a rapid decompression on the KC-135A, and we also got to see first-hand the effects of hypoxia. At 25,000 feet we were instructed to take off our oxygen masks for five minutes to experience the insidious nature of hypoxia. Some of my fellow chamber mates started acting a little goofy after only two minutes, but when questioned by the Chamber operator they swore they felt perfectly normal. I noticed a tingling in my cheeks and ears about a minute before we put our masks back on, which turned out to be my signs of hypoxia. (If you are a GA pilot, I would highly recommend an altitude chamber ride so you can learn to recognize your own symptoms of hypoxia. Check with your local FSDO, as some can arrange chamber rides at a nearby military altitude chamber for civilian pilots.) After successfully completing the chamber ride and passing a written test, NASA finally gave me the green light to fly on the KC-135A.

Mr. Pete's Wild Ride

After all the briefs, training, hype, and nervous anticipation, I'm finally onboard and experiencing weightlessness for the first time. With Stephanie Wells at the control yoke, smoothly pushing us over the top on the first parabola, I'm watching my fellow free-floaters trying to adapt to zero-G. Your first reaction is to start flailing your arms and legs, as if you are swimming. Of course air is a lot less viscous than water, so unfortunately all that flailing doesn't really help. You quickly learn to plan your moves so you have a way to stop, such as grabbing support straps or even your fellow floaters. As each parabola was finishing up, test director John Yaniec would shout "Feet down, coming out!" Like a tumbling cat that always tries to land feet-first, those who were upside down had to do some quick aerial contortions to insure they didn't land on their heads. Usually you only had three-to-five seconds of warning before the pull-out, and one time I almost landed on top of an experiment before I pushed off the wall and twisted my body to just miss it before tumbling onto the padded floor.

After about 12 parabolas I was getting a little better at controlling my body, so I became a little bolder and started flying across the cabin. When the astronauts are training on the KC-135A, there are usually no more than ten people onboard and no hardware strapped on the floor, so they can fly all over the cabin. On our flight there were about 24 passengers, plus eight experiments of various sizes, so you couldn't float very far without banging into someone or something. However several times I had a clear path to float across the cabin, which I found was very easy to do with just a light push. I soon got to where I could push off one wall, fly across the cabin Superman-style, do a quick flip at the other wall, and fly back to where I started.

In spite of the aircraft's reputation for trying even the strongest of stomachs, I felt pretty good the whole flight, thanks to several precautions, including my secret weapon, the Woodside Biomedical ReliefBand (see sidebar). Making the experience much more pleasant for us all was the fact that only two people on our flight actually barfed, so you didn't have to worry about dodging floating chunks or getting a whiff of that unpleasant smell. Throughout the flight I kept an eye on J.D. as he conducted the Purdue experiment, and I was happy to see that he had a smile on his face, which meant he wasn't getting sick. After about 25 parabolas I noticed J.D. had finished his data collection and was having some fun with his own aerial gymnastics.

After parabola number 30, Yaniec shouted, "Next stop, the moon!" Wells flew this parabola at a higher entry speed, which produced only 1/6 Earth's gravity, but lasts longer — about 45 seconds. On this parabola I felt like the Apollo astronauts on the moon, making giant leaps with each stride. The final parabola was flown to simulate Mars' gravity (1/3 Earth's), prompting some of the students to show off their "strength" by performing easy pushups with two students on their back. Finally, Yaniec called "That's a wrap!" after parabola 32, signaling an end to our high-flying adventure. We returned to our seats for the flight back as Yaniec announced that with only two persons sick, our flight was one of the best he had ever seen. After landing at EFD, we exited 931NA to the applause of other students and NASA personnel who had watched our flight on the live downlink. Walking down the stairs from the plane, some of my fellow floaters triumphantly waved their empty barf bags to show that at least on this flight, our stomachs had beaten the infamous Vomit Comet.

Coming Back Down To Earth

Sitting in the hanger after the flight getting my "land legs" back again, all I could think of was what a great experience this program provides for students. Based on my own engineering experience, the Reduced Gravity Student Flight Opportunities program gives students an excellent feel for the struggles and triumphs associated with solving "real world" engineering problems. Talking to the Purdue team after their flights, they told me how the experiment worked much better than their first flight in March. According to Curt, "We got some fantastic data on this flight compared to the flight in March." Nick, J.D., Curt and Rob told me that all the hard work was definitely worth it. Rob, a senior planning to apply for the Rhodes scholarship, added, "Without a doubt I'd fly again. It's well worth all the work that goes into it, about a semester's worth of constant work, a lot of all-nighters." The team summed it up by saying that they gained invaluable experience, have a nice addition to their resumes, and most of all, had a blast when they finally got to fly.

Purdue students Curt Peternell (L) and Rob Whiteman "overfly" their experiment.

Along with the student's efforts, NASA and the Texas Space Grant Consortium should also be commended for not only offering such a unique program but for also providing an enjoyable experience during the students' two week stay in Houston. When teams weren't working on their experiments or flying, they could take tours of JSC facilities, attend lectures by astronauts and engineers, or just unwind at informal picnics. As for me, it was a chance to share the enthusiasm of young people with their entire careers ahead of them, and also experience flight totally unencumbered, just as I had always dreamed. Who knows, when the International Space Station is fully operational, NASA may give students the chance to test their experiments in real weightlessness sometime in the next century. If that happens, you can bet I'll be badgering Purdue to once again be invited to report first-hand on the exploits of our next generation of engineers and scientists.

Special thanks to Nick, Rob, J.D., Curt and Professor Steven Collicott from Purdue. NASA KC-135A test directors Jim Withrow, Judy Rickard, and John Yaniec. KC-135A pilot Stephanie Wells, and former astronaut Steve Nagel. Finally Debbie Mullins from the Texas Space Grant Consortium. To learn more about the Reduced Gravity Student Flight Opportunities program, check the program's web site.

The Purdue zero-g teams are always looking for corporate sponsorship to help defray the costs of hardware, with sponsors getting their logo on any experiments that fly. For more information on sponsorships, contact Professor Steven Collicott.