Monday, April 14, 2014

Ferrying the Shuttle - Part One: How Do You Ship A Space Shuttle?

In April of 1972, John Young and Charlie Duke landed Apollo 16, the next to last Apollo lunar mission, in the Descartes highlands. 
It was during one of the moonwalks that the word came from Mission Control in Houston - Congress had approved the NASA budget for fiscal year 1973 by a margin of 277-60. That vote included funding for the Space Shuttle. Upon hearing the news John Young remarked “the country needs that shuttle mighty bad”.  

Getting the funding for the shuttle was the first step.  Getting the detailed design finalized and the shuttle on to development was the next critical phase at a pivotal time for the U.S. space program. One more lunar mission was still funded. The Skylab and Apollo-Soyuz Test Project were still NASA objectives for the seventies, but the long term future was for the decade to close out with the shuttle taking flight. 

As the Space Shuttle concept was close to being finalized, NASA has a pretty good idea of how the shuttle would get into space. Gone were ideas of a fully reusable flyback booster,  It would ride two solid rocket boosters strapped to a giant external tank that fueled three onboard shuttle engines. The journey from ground to space would be full of obstacles for the shuttle program and delays were commonplace.  The shuttle program would have to deal with issues around the tiles used for thermal protection and problems with the powerful Space Shuttle Main Engines (SSMEs). All of it would add to the budget overruns and delays resulting in the shuttle waiting until the beginning of the next decade to fly. 

What was less clear at the time was how the shuttle would move between earthly destinations. If the Space Shuttle did not have a way to conduct atmospheric tests or go from the factory where it was built or a return landing at Edwards Air Force Base to its home and launch site at the Kennedy Space Center, the shuttle might never reach space. Several concepts had emerged on how the shuttle would “shuttle” between California and Florida, and perhaps other destinations or alternate landing sites. 

In June of 1973, NASA seemed to have the answer.  Unlike previous spacecraft, the shuttle was a hybrid, part space vehicle, part aircraft.  What was never in question in the shuttle design is that it would launch like a rocket and land like an airplane. Since it had flight control surfaces and was intended to function like an aircraft on its return from space it made sense to leverage those capabilities and fly the shuttle like an airplane when the destination was not space.  It seemed the perfect plan, fly the shuttle as an airplane when it needed to move around within Earth’s atmosphere. 

This was not exactly a new or revolutionary idea for the shuttle.  Early in the design phase serious consideration was given to equip the shuttle with onboard jet engines that would be used on the return from space to give the shuttle flight capabilities consistent with an airplane. One concept was to stow the engines near the back of the fuselage behind the cargo bay and deploy these engines when the shuttle reached a safe operational altitude upon return from space. These engines would allow for a higher level of maneuverability and not rely on the shuttle functioning solely as a glider.  It was an idea born out of safety.  Surely the shuttle could not fly from space to ground only as a glider. The pilots had to have capability for powered flight if they needed to go around on an aborted landing or choose and alternate landing site. 

As NASA, the Air Force and the Nixon Administration haggled on costs and payload specifications, jet engines as a component of the final shuttle had to go. Internally stowed engines that would deploy within Earth’s atmosphere would add too much weight. The engines, the fuel, the additional controls, wiring, and backup redundancy would add thousands of pounds of weight to the shuttle and drastically reduce the amount of payload it could haul to orbit.  While jet engines as an operational component of the shuttle were off the table there was still the issue of how to transport the shuttle between destinations on earth.  For that problem, the use of jet engines in some capacity on the shuttle was still in play. 

On June 26, 1973, that problem appeared to be solved. NASA announced that had selected the Pratt & Whitney TP33-P-7 engine to install on the shuttle for both atmospheric testing and for use when the shuttle needed to move between ground destinations.  The TP33-P-7 engine was already tested and proven on the Air Force’s C-141 Starlifter and had 21,000 pounds of take-off thrust.  NASA agreed to purchase 25 of the engines. At the time it joined the shuttle’s development and production, the main engines, and the integration of the shuttle systems to become the fourth major contract awarded in the shuttle program.  At least on paper, things were on the move in the Space Shuttle program. 

Read Ferrying the Shuttle - Part Two:  The Orbiter is the Ferry

Resources used in this article are courtesy of NASA.Gov, NASA History Office, United States Congress, Committee on Science and Astronautics - 1974 Status Report

Ferrying the Shuttle - Part Two: The Orbiter is the Ferry

It did not take long to figure out that the concept of ferrying the shuttle like an airplane between ground destinations was not going to fly.  The numbers just were not adding up, but the risk factors were rising.  The idea was to install six airbreathing jet engines beneath the shuttle. The engines would be fed by a fuel tank that would be placed in the aft section of the payload bay. The shuttle would be configured in this mode for atmospheric tests and when the shuttle needed to ferry itself from California to Florida.  The Space Shuttle would then fulfill the dreams of many a science fiction fan by having a spacecraft takeoff and land from a conventional runway. However, in this mode the destination would never be space.  

The shuttle was by 1970’s technology standards the most complex flying machine humans had ever built. Its engines, operating parameters, and thermal protection system would be at the far end of machine performance. The shuttle would have two and a half million moving parts with one thousand of those rated as a single point of failure. Just one failure out of a thousand parts could cause a loss of crew and vehicle. When the last shuttle flew, it still held the title of most complex flying machine, and arguably the most complex machine ever built. Now, with the idea of using the shuttle to transport itself another level of complexity was added.  The Orbiter has often been referred to as a ‘flying brick”. Under this plan, the “brick” would get jet engines and that was not going to be as easy as first thought. Soon, holes in the plan began to appear. 

The design of the orbiter would now have to account for the systems and structure to attach and detach jet engines and fuel. This meant the Orbiter would need to carry dead weight to space. Weight to support the engine pylons, added strengthening for stress and load bearing on the vehicle and the systems required to operate the vehicle in “airplane mode”.  This meant controls and systems for thrusters, engine performance, and a host of other electronics and subsystems.  It would also mean that the Space Shuttle Main Engines (SSMEs) would need to be removed before the ferry flight to save weight and balance the vehicle. If engineers could work out those details there was the issue of hot exhaust and particulate coming from the engines and potentially damaging the orbiter’s sensitive tiles and systems. Placement of the engines would also be an issue as there were potential problems with the Orbiter’s center of gravity. 

Finally, if everything came together as envisioned, the devil would be not only on the details, but in the execution. For the plan to work skilled technicians would have to install and uninstall this added equipment after each ferry flight. That would mean time, expense, and more hands in and around the Orbiter before and after ferry flights. It also would mean more inspections and checkouts of more systems. Parts, equipment, and personnel would need to be transported across the country.  Not known at the time the idea was first put forth would be how fragile the Obiter would actually be to operate and maintain.  Considering the extremes the vehicle was designed to withstand, in many ways it was as fragile as glass. The slip of a tool by a technician could easily crack the fragile tile system. 

The logistics and risk of making NASA's Orbiter a ferry were mounting. To add to it were the performance numbers and range of a “jet-equipped” Space Shuttle. The shuttle could only carry enough fuel to feed those thirsty six engines for slightly more than 400 miles under typical conditions. It would require a 10,000ft runway for takeoff and had a maximum operating ceiling of 10,200ft. It could only sustain a failure in one of the six engines and even then would have to land at the nearest available field. An emergency landing at a field that did not meet specifications could spell trouble. There were few ways to lighten the Orbiter to use a shorter runway. 

Jet engines on an Orbiter could have been tested on the Enterprise. Enterprise was not a "space ready" vehicle, but rather a test article that had planned to be converted to a full operational shuttle at a later point in the program. Improvements in the construction of the subsequent shuttles lightened the vehicle and made converting the Enterprise costly and impractical. There would have been little value in the plan other than to test the vehicles aerodynamics and handling capabilities. It would not have been a good test of actual conditions when the shuttle finally flew. The Soviet version of the Space Shuttle did take this approach and their test article shuttle, the OK-GLI, was equipped with tail mounted jet engines. The OK-GLI flew 25 times in this configuration and like the Enterprise was not a "space-ready" vehicle. The Soviet's motivation for testing their version of the Orbiter was also due to the lack of a ferry airplane. By the time the Soviet shuttle was under development their "piggyback" aircraft, the AN-225 was still being built. The Soviet's could not wait and opted for a "jet powered" shuttle, but only for atmospheric testing. 

It was all adding up to a complicated and risky approach. The Orbiter flying itself had so many drawbacks that there had to be a better way.  The idea of having the country’s newest space vehicle built at a cost of one and a half billion dollars each flying with such slim margins for error were unacceptable. The search was on for Plan B. 

Read Ferrying the Shuttle - Part Three:  Hitching a Ride

Resources used in this article are courtesy of NASA.Gov, NASA History Office, United States Congress, Committee on Science and Astronautics - 1974 Status Report

Ferrying the Shuttle - Part Three: Hitching a Ride

If the shuttle could not fly itself across the country then how do you get a spacecraft that weighs as much as a Boeing 757 from California to Florida? One idea proposed by Lockheed was to "tow" the Orbiter behind a C-5. This method would be used to ferry the vehicle and for drop tests. The plan called for a 1000ft cable linking the two, but doubts about the tension in the cable and the effect of jet wash on stability quickly nixed the idea.  Despite the fact that Lockheed had faith in the concept, much like the other options on the table it seemed too risky an approach. In the event that the cable had to be detached in an emergency, or that it snapped in flight, the Orbiter would become a piloted glider with few options to maneuver to a suitable landing site. The outcome likely would have been the loss of the vehicle and possibly the loss of crew.  NASA was searching for the middle ground trying balance the safety and protection of their newest spacecraft against the expense. 

The inspiration that would lead to the final design would come from the X-15 program. It was suggested that perhaps the vehicle could be stowed under the wing of an aircraft and transported from site to site or used for drop tests similar to previous NASA test vehicles. The Orbiter is not an X-15, it is large and it is heavy. No aircraft existed that could carry the shuttle tucked under its wing like the X-15 was under a B-52. There was no shortage of other ideas.  There was the concept of designing the Orbiter with removable wings or wings that could be “changed out” for ferry flights. That seemed to be too expensive and would likely add weight to support the attach and detach support structures. Given the size and weight of the Orbiter it would have to be centered underneath an aircraft.  The only way that could be reasonably accomplished would be to build a plane with a twin fuselage joined by a straight center connecting wing and hang the Orbiter right in the middle. 

As the brainstorming continued, four options for this model were kicked around by engineers. The first would involve building a brand new one of a kind aircraft from scratch,  but using “off the shelf” components. In this model, the ferrying aircraft would not need a twin fuselage, but would be low mounted in the center of the aircraft and the cockpit would be above the Orbiter. Picture the second story cockpit of the 747. Another concept dubbed the "long leg" approach called for center mounting the Orbiter underneath a C-5. To make room for the shuttle the C-5 would be equipped with extremely long landing gear to provide adequate clearance under the aircraft. Other ideas for ferrying a low, center-mounted shuttle involved using two 747s or two Air Force C-5 Galaxies.  Under this plan, the 747s or C-5s would have a left and right fuselage and outer wing joined by a massive straight center wing. By any standards, this would have been the world’s largest aircraft, but it was not to be.
Another possible selling point to the low mounted Orbiter in this "twin" configuration was that it might have been possible to use the same aircraft to transport the shuttle's external fuel tank on a separate flight. Some proposals called for such a capability. 

The odds of getting one C-5 from the Air Force was unlikely, the chance of getting two, then cutting them in half was going to be out of the question.  The going rate for a single C-5 was not too far off from the cost of the Orbiter.  For NASA, the 1970’s was all about change, small change.  The budget was not going to allow for large expenditures, if the Shuttle needed help getting around it would need to do it in the least expensive way possible. The only remaining option now was the “piggyback”.  Putting the Orbiter on top of a plane and letting it catch a ride seemed the simplest method.  The 747 and the C-5 seemed the logical choice, but could they safely carry a Space Shuttle?

Resources used in this article are courtesy of NASA.Gov, NASA History Office, United States Congress, Committee on Science and Astronautics - 1974 Status Report

Ferrying the Shuttle - Part Four: The Shuttle Carrier Aircraft

The bragging rights to carry the Shuttle on its back came down to the Air Force’s C-5 and Boeing’s 747. The C-5 was a potential budget buster for NASA. It was a large and expensive aircraft that entered production in 1968.  The initial version of the aircraft had developed problems with cracks in the wings and it appeared that at least in the near term were likely to suffer a higher rate of fatigue. The aircraft also had a high wing design and a large T-tail which while not enormous drawbacks could have proved problematic. It should be noted that the Soviet Space Shuttle Buran successfully rode atop the high wing AN-225.  The AN-225 has similar features to the C-5, but varied in design of the vertical stabilizer. While the C-5’s job was to haul heavy cargo, given the expense it was overkill for what NASA needed.  

It would be a combination of factors that would ground the C-5 from consideration and push the 747 to the forefront. The availability, acquisition, operating, and maintenance costs of the C-5 were too big an obstacle for NASA to overcome. On June 17, 1974, less than a year after NASA signed the contract to put jet engines on the Orbiter for ferry and atmospheric testing flights, the debate was settled. In a press release NASA announced that Boeing 747 would now get the call and become the Shuttle’s chauffeur when it needed a ride. 

Solving the problem of how to ferry the shuttle was perhaps a foreboding sign of how challenging getting the Space Shuttle off the ground would turn out to be.  In less than a year, the Orbiter went from having jet engines installed between missions to ferry itself to riding on atop a 747.  Now, all NASA needed to do was find a bargain priced 747 and make sure the modifications to the aircraft could be made and that the plan would work.  

In that same year, NASA acquired a used 747 from American Airlines for $16 million. The aircraft, built in 1970 for passenger service, would become NASA 905 with tail number N905NA.  It was put to work immediately as part of a study to understand the effect of wing vortices on trailing aircraft.  In 1976, it was sent to Boeing to undergo modifications in preparation to carry the shuttle on the first approach and landing tests. The modifications included stripping the interior to reduce weight, adding the Orbiter’s attach points and strengthening the fuselage. Its only job would be to carry the spacecraft anytime it needed to move from west to east, or on its last flights take the Orbiters home to their final resting place. In 1998, NASA acquired a second used 747 from Japan Airlines, It would become N911NA. 

In 1977, Enterprise was became the only shuttle to ever ride the 747 and then get released into the air to glide to a landing. That test proved that the Orbiter could safely function as a glider and did not need the support of jet engines. The rest of the shuttle fleet hitched a ride from the Rockwell factory in Downey, California, from landings at Edwards Air Force Base, and once from the White Sands Missile Range. 

The use of the 747 would add a measure of safety to transporting the Obiter, but was not without limits. NASA originally projected that the pair would have a range of 2320 nautical miles. That would be sufficient to make the cross country trip non-stop. It would turn out that the mated 747 and Orbiter had a range of about 1000 nautical miles and a ceiling of 15,000ft.  The range was double the jet engine concept's performance, but less that half of what was needed to fly coast to coast without a stop for refueling.There would be a speed limit imposed on the 747 when the shuttle was riding along and reminders for the crew not to exceed a maximum of 250 knots. Any time the shuttle was being ferried the route was carefully planned to avoid weather and allow for contingencies in the event of an onboard emergency. When mated to the Orbiter, the Shuttle Carrier Aircraft was always preceded by an advance scout plane flying approximately 100 miles ahead to check for adverse weather conditions and turbulence.  A transatlantic abort never occurred in the shuttle program, but it would have been a challenge for NASA.  When Enterprise went on a European tour to England, Germany, Italy, and then to Canada it would prove that the 747 could carry an Orbiter back from an overseas trip. A typical route might take the pair from continental Europe to England, then on to Iceland, then Canada, and finally to the United States. However, the Enterprise was not a "fully weighted" shuttle and even with this lighter load the mated ships would require several stops on a planned route to make it back. In the event of an operational shuttle aborting to a European landing site the shuttle would have to undergo modifications prior to being mated to the Shuttle Carrier Aircraft. This would have meant taking everything possible from the payload bay. The removal of cargo, the robotic arm, or any extra components that would reduce the weight of the shuttle and increase the range of the 747. 

A mate-demate scaffolding like structure was built to lift the Orbiter on to the back of the 747. These mate-demate structures we housed at the Kennedy Space Center and Armstrong Flight Research Center (formerly Dryden).  Only one time in the operation phase of the program were the devices not used. That occurred on STS-3 when Columbia landed at the White Sands Test Facility, Special equipment was shipped by train to the site to lift the Orbiter on to the 747. It would not be used again until the flight ferry flights of Discovery, Endeavour, and Enterprise to museums in Virginia, California, and New York. The older NASA 905 would be the workhorse from start to finish.  The newer NASA 911 would retire early and NASA 905 would take finish the job it started three decades earlier without a hitch giving the Orbiters one last ride.  

The Space Shuttle Orbiter sitting atop a Boeing 747 was a memorable sight.  Its last trip made a lasting impression as it performed flybys to crowds in cities like Houston, Austin, Washington, and New York that were awed by the sight of these two historic feats of engineering that were dependent on each other.  Starting with the very first space shuttle, Enterprise to the last shuttle Endeavour, every Orbiter needed the Boeing 747 Shuttle Carrier Aircraft (SCA) to go anywhere but space.  After more that 30 years of service their work was done. With nowhere left to fly the Shuttle Carrier Aircraft NASA 905 joins the Orbiters in the history books and will now go on display at the Johnson Space Center in Houston, Texas.   

Figuring out how to ferry a Space Shuttle got off to a rough start, but in the end it turned out to just turned out to be all in a day's work for the Boeing 747.

Shuttle Carrier Aircraft Performance Specifications (Courtesy of NASA)

Airspeed limits with, and without an orbiter: 250 knots or Mach 0.6
Altitude: Typical cruise with orbiter, 13,000-15,000 ft; typical cruise unmated, 24,000-26,000 ft. 
Minimum temperature at altitude 15 degrees (F) (-9 degrees C)
Range: Typical mated, 1000 nautical miles (with reserves); maximum unmated, 5500 nautical miles

Resources used in this article are courtesy of NASA.Gov, NASA History Office, United States Congress, Committee on Science and Astronautics - 1974 Status Report