Space Shuttle program

Space Shuttle Columbia, 1981 (NASA)

NASA's Space Shuttle program is an ongoing endeavor, started in the late 1960s, that has created the world's first partially reusable space launch system, and the first spacecraft capable of carrying large satellites both to and from low Earth orbit. Each shuttle is designed for a projected lifespan of 100 launches. The original purpose of the program was to ferry supplies to a space station. In reality, the Shuttle is the United States' sole manned launch vehicle and has totally dominated NASA's operations since the mid 1970s. With the construction of the International Space Station the Shuttle has finally begun to be used for its original purpose. In January 2004, it was announced that the Shuttle fleet would be replaced by 2010.

Table of contents

1 Components
2 Statistics
3 Shuttles
4 The Shuttle decision
5 Shuttle development
6 The Shuttle in retrospect
7 Shuttle description
8 Shuttle accidents
9 Previous Programs
10 External links

Components

The Space Shuttle consists of four main components:

the reuseable orbiter itself
a large expendable external fuel tank (ET) containing liquid oxygen and liquid hydrogen (at the forward and aft ends, respectively) for the three main engines of the orbiter; it is discarded 8.5 minutes after launch at an altitude of 109 kilometers and breaks up in the atmosphere upon reentry; the pieces fall in the ocean and are not recovered)
a pair of reusable solid-fuel rocket boosters (SRB); the propellant consists mainly of ammonium perchlorate (oxidizer, 70 % by weight) and aluminum (fuel, 16 %); they are separated two minutes after launch at a height of 66 km and are recovered after landing in the ocean, their fall slowed by parachutes

Initial plans for the so-called Space Transportation System included space tugs and extra fuel tanks for the orbital manuvering systems among many other concepts. None of them made it to actual hardware.

Statistics

Space Shuttle stack height: 56.14 m (184.2 feet) tall
Orbiter alone: 37.23 m (122.17 feet) long
Wingspan: 23.79 m (78.06 feet)
Weight at liftoff: 2,041,166 kg (4.5 million pounds)
Weight at end of mission: 104,326 kg (230,000 pounds)
Maximum cargo to orbit: 28,803 kg (63,500 pounds)
Orbit: 185 to 643 km (115 to 400 statute miles)
Velocity: 27,875 km/h (17,321 mi/h)

Space Shuttle Atlantis transported by a Boeing 747 Shuttle Carrier Aircraft (SCA), 1998 (NASA)

Shuttles

test vehicle only suitable for glide/landing tests:
- Enterprise
lost in accidents (see below):
- Challenger
- Columbia
in use:
- Atlantis
- Discovery
- Endeavour

The Shuttle decision

NASA had conducted a series of paper-projects throughout the 1960s on the topic of reusable spacecraft to replace their expedient "one-off" systems like Mercury, Gemini, and Apollo. Meanwhile the Air Force had a continuing interest in smaller systems with more rapid turn-around times, and were involved in their own spaceplane project called Dynasoar. In several instances groups from both worked together to investigate the state of the art.

With the major Apollo development effort winding down in the second half of the 1960s, NASA started looking to the future of the space program. They envisioned an ambitious program consisting of a large space station being launched on huge boosters, served by a reusable logistics "space shuttle", both providing services for a permanantly manned Lunar colony and eventual manned missions to Mars.

However reality was to interject and NASA found themselves with a rapidly plunging budget. Rather than stepping back and looking at their future as a whole given their new financial situation, they attempted to save as many of the individual projects as possible. The mission to Mars was quickly eliminated, but the Space Station and Shuttle continued on. Eventually only one of them could be saved, so it stood to reason that a low-cost Shuttle system would be the better bet, because without it a large station would never be affordable.

A number of designs were proposed, but many of them were complex and varied widely in their systems. An attempt to re-simplify was made in the form of the "DC-3" by one of the few people left in NASA with the political clout to pull it off, Maxime Faget, who had designed the Mercury capsule, among others. The DC-3 was a small craft with a 20,000lbs (or less) payload, a four-man crew, and limited manuverability. At a minimum, the DC-3 provided a baseline "workable" (but not terribly advanced) system by which other systems could be compared for price/performance tradeoffs.

The final defining moment was when NASA, in desperation to see their only remaining project saved, went to the Air Force for its blessing. NASA asked that the AF place all of their future launches on the shuttle instead of their current expendable launchers (like the Titan II), in return for which they would no longer have to continue spending money upgrading those designs -- the shuttle would provide more than enough capability.

The Air Force relucantly agreed, but only after demanding a large increase in capability to allow for launching their projected spy satellites (mirrors are heavy). These were quite large, weighing an estimated 40,000 lbs, and needed to be put into polar orbit, which requires more energy to get to than the more common LEO. And since the AF also wanted to be able to abort after a single orbit (as did NASA), and land at the launch site (unlike NASA), the spacecraft would also require the ability to manuver significantly to either side of its orbital track to adjust for the launching point rotating away from it while in polar orbit - in a 90 minute orbit Vandenberg would move over 1,000 miles, whereas in a "normal" equatorial orbit NASA needed the range would be less than 400. This large 'cross-range' capability meant the craft had to have a greater lift to drag ratio than originally planned. This required the addition of bigger, heavier wings.

The result was that the simple DC-3 was clearly out of the picture because it had neither the cargo capacity nor the cross-range the Air Force demanded. In fact all existing designs were far too small, as a 40,000 lbs delivery to polar orbit equates to a 65,000 lbs delivery to a "normal" 28 degree equatorial orbit. In fact any design using simple straight or fold-out wings was not going to meet the cross range requirements, so any future design would require a more complex, heavier delta wing.

Worse, any increase in the weight of the upper portion of a lauch vehicle, which had just occurred, requires an even bigger increase in the capability of the lower stage used to launch it. Suddenly the two-stage system grew in size to something larger than the Saturn V, and the complexity and costs to develop it skyrocketed.

While all of this was going on, others were suggesting a completely different approach to the future. They stated that NASA was better off using the existing Saturn to launch their space station, supplied and manned using modified Gemini capsules on top of the Air Force's newer Titan II-M. The cost of development for this looked to be considerably less than the shuttle alone, and would have a large space station in orbit earlier.

The answer within those groups dedicated to the shuttle was to show that as long as you have enough launches, the development cost of the system would be overwhelmed by the cost of the rockets you would otherwise throw away. One factor that needs to be considered is inflation though, and in the 1970s this was high enough that the payback from the development had to happen very quickly or that money would never pay for itself. In other words you needed a very high launch rate to make the system work.

But there was no way that a space station or Air Force payloads could demand such rates (roughly 1 to 2 per week), so they went further and suggested that all future US launches would take place on the shuttle, once built. In order to do this the cost of launching the shuttle would have to be lower than any other system with the exception of the very small, which they ignored for practical reasons, and very large, which were rare and terribly expensive anyway.

With a baseline project now gelling, NASA started to work though the process of obtaining stable funding for the five years the project would take to develop. Here too they found themselves increasingly backed into a corner.

With the budgets being pressed by inflation at home and the Vietnam war abroad, Congress and the Administration generally couldn't care less about anything as long-term as space exploration and were therefore looking to make further deep cuts to NASAs budget. But with a single long term project on the books, there wasn't much they could do in terms of cutting whole projects -- the shuttle was all that was left, cut that and there would be no US manned space program by 1980.

Instead they looked to reduce the year-to-year costs of development to a stable figure. That is, they wished to see the development budgets spread out over several more years. This is somewhat difficult to do--you can't build half a rocket. The result was another intense series of redesigns in which the re-usable booster was eventually abandoned as impossible to pay for. Instead a series of simpler rockets would launch the system, and then drop away for recovery. Another change was that the fuel for the shuttle itself was placed in an external tank instead of internal tanks from the previous designs. This allowed a larger payload bay in an otherwise much smaller craft, although it also meant throwing away the tankage after each launch.

The last remaining debate was over the nature of the boosters. NASA had been looking at no less than four solutions to this problem, one a development of the existing Saturn lower stage, another using "dumb" pressure-fed liquid fuel engines of a new design, and finally either a large single solid rocket, or two (or more) smaller ones. The decision was eventually made on the smaller solids due to their lower development costs (a decision that had been echoed throughout the whole Shuttle program). While the liquid fueled systems provided better performace and enhanced safety, delivery capability to orbit is much more a function of the upper-stage performance and weight than the lower. The money was simply better spent elsewhere.

Shuttle development

The shuttle program was launched on January 5, 1972, when President Richard M. Nixon announced that NASA would proceed with the development of a reusable low cost space shuttle system.

The project was already to take longer than originally anticipated due to the year-to-year funding caps. Nevertheless work started quickly and several test articles were available within a few years.

Most notable among these was the first complete Orbiter, originally to be known as Constitution. However a massive write-in campaign on the part of fans of the TV show Star Trek convinced the White House to change the name to Enterprise. Enterprise was rolled out on September 17, 1976 and later conducted a very successful series of landing tests which was the first real validation of the gliding abilities of the design.

The first fully functional shuttle orbiter was the Columbia, which was delivered to Kennedy Space Center on March 25, 1979 and was first launched on April 12, 1981 with a crew of two. Challenger was delivered to KSC in July 1982, Discovery was delivered in November 1983, and Atlantis was delivered in April 1985. Challenger was destroyed in an explosion during launch in January 1986 with the loss of all seven astronauts on board, and Endeavour was built as a replacement (using spare parts originally built for the other orbiters) and delivered in May 1991. Columbia was lost, with all seven crew, in a re-entry mishap on February 1, 2003.

The Shuttle in retrospect

Whilst the shuttle has been a reasonably successful launch vehicle, it had been unable to meet its goals of radically reducing flight launch costs, as each flight costs on the order of $500 million rather than initial projections of $10 to $20 million.

Although the design is radically different than the original concept, the project was still supposed to meet the upgraded AF goals as well as be much cheaper to fly in general. What went wrong?

One issue appears to be inflation. During the 1970s the US suffered from the worst inflation in modern history, driving up costs about 200% by 1980. In contrast, the rate between 1990 and 2000 was only 34% in total. This has the effect of magnifying the development costs of the shuttle tremendously.

However this doesn't explain the high costs of the continued operations of the shuttle. Even accounting for inflation the launch costs on the original estimates should be about $100 million today. To explain this you have to look at the operational details of maintaining and servicing the shuttle fleet, which have turned out to be tremendously more expensive than anticipated.

When originally conceived the shuttle was to operate similar to an airliner. After landing the Orbiter would be checked out and start "mating" to the rest of the system (the ET and SRBs) and be ready for launch in as little as two weeks. Instead this turnaround process in fact takes months. This is due, in turn, to the continued "upgrading" of the inspection process as a result of hardware decisions made to reduce short-term development costs which resulted in higher maintenance requirements which where exacerbated by the fallout from the loss of Challenger. Even simple tasks now require unbelievable amounts of paperwork. This paperwork results from the fact that, unlike current expendable launch vehicles, the Space Shuttle is manned and has no escape systems to speak of and therefore any accident which would result in the loss of booster would also result in the loss of the crew which is, of course, unacceptable. Because loss of crew is unacceptable, the primary focus of the shuttle program is to return the crew to earth safely, which can conflict with other goals, namely to launch satellites cheaply. Furthermore, because there are cases where there are no abort modes, no potential way to prevent failure from becoming critical, many pieces of hardware simply must function perfectly and so must be carefully inspected before each flight.

The result is a massively inflated manpower bill. There are 25,000 workers in shuttle operations (perhaps an older number), so simply multiply any figure that you choose for an average annual salary, divide by six (or 4 or 7...launches per year), and there you have it.

The lessons of the shuttle have been seen as different depending on who you ask. In general, however, future designers look to systems with only one stage, automated checkout, and in some cases, overdesigned (more durable) low-tech systems.

Perhaps the most annoying aspect of the shuttle system is to consider the Air Force participation. While the blame rests solely at the feet of NASA for getting them involved in the first place, it was the Air Force requirements that drove the system to be as complex and expensive as it is today. Ironically neither NASA nor the Air Force got the system they wanted or needed, and the Air Force eventually threw in the towel and returned to their older launch systems and abandoned their Vandenburg shuttle launch plans. The capabilities which most seriously hobbled the Shuttle system, namely the 65,000 payload, large payload bay, and 1000 mile cross-range, have in fact, except for the payload bay, never been used.

Shuttle description

The Space Shuttle consists of four main components; the reuseable orbiter itself, a large expendable external fuel tank, and a pair of reusable solid-fuel booster rockets. The fuel tank and booster rockets are jettisoned during ascent. The longest the shuttle has stayed in orbit in a single mission is 17.5 days, on mission STS-80 in November 1996.

The Shuttle has a large payload bay taking up much of its length. The payload bay doors have heat radiators mounted on their inner surfaces, and so are kept open while the Shuttle is in orbit for thermal control. Thermal control is also maintained by adjusting the orientation of the Shuttle relative to Earth and Sun. Inside the payload bay is the Remote Manipulator System, also known as the Canadarm, a robot arm used to retrieve and deploy payloads. Until the loss of Columbia, the Canadarm has only been included on missions where it will be used. Since the arm is a crucial part of the Thermal Protection Inspection procedures now required for shuttle flights, it will likely be included on all future flights.

The Space Shuttle system has had numerous improvements over the years.

The Orbiter has changed its thermal protection system several times in order to save weight and ease workload. The original silica-based ceramic tiles need to be inspected for damage after every flight, and they also soak up water and thus need to be protected from the rain. The later problem was initially fixed by spraying the tiles with Scotchgard, but a custom solution was later developed. Later many of the tiles on the cooler portions of the Shuttle were replaced by large blankets of insulating felt-like material, which means huge areas (notably the cargo bay area) no longer have to be inspected as much.

Internally the Shuttle remains largely similar to the original design, with the exception that the avionics continues to be improved. The original systems were "hardened" IBM 360 computers connected to analog displays in the cockpit similar to contemporary airliners like the DC-10. Today the cockpits are being replaced with "all glass" systems and the computers themselves are many times faster. The computers use the HAL/S programming language. In addition several improvements have been made for safety reasons after the Challenger explosion, including a crew escape system for use in situation that require the Orbiter to "ditch". With the coming of the Space Station, the Orbiter's internal airlocks are being replaced with external docking systems to allow for a greater amount of cargo to be stored on the shuttle mid-deck during Station resupply missions.

The Space Shuttle Main Engines have had several improvements to enhance reliability and power. This is why during launch you may hear curious phrases such as "Go to throttle-up at 106%". This does not mean the engines are being run over-limit. The 100% figure is the power level for the original main engines. The actual engine contract requirement was for 109%. The original flight engines could handle 102%. The 109% number was finally reached in flight hardware with the Block II engines in 2001.

The external tank was originally painted white to protect the insulation that covers much of the tank, but improvements and testing showed that it was not required. This saves considerable weight, and thereby increases the payload the orbiter can carry into orbit. Additional weight was saved by removing some of the internal "stringers" in the hydrogen tank which proved to be unneeded in flight. The resulting "light weight external tank" has been used on the vast majority of shuttle missions. STS-91 saw the first flight of the "super light weight external tank". This version of the tank is made of the 2195 Aluminum-Lithium alloy. It weighs 7,500 lbs less than the last run of light-weight tanks. As the Shuttle cannot fly unmanned, each of these improvements have been "tested" on operational flights.

And, of course, the SRBs have undergone improvements as well. Notable is the adding of a third O-ring seal to the joints between the segments, which occurred after the Challenger accident.

A number of other SRB improvements were planned in order to improve performance and safety, but never came to be. These culminated in the considerably simpler, lower cost, probably safer and better performing Advanced Solid Rocket Booster which was to have entered production in the early to mid 1990s to support the Space Station, but was later cancelled to save money after $2.2 billion had been spent. The loss of the ASRB program forced the development of the SLWT, which provides some of the increased payload capability while not providing any of the safety improvements. In addition the Air Force developed their own much lighter single-piece design using a filament-wound system, but this too was cancelled.

Shuttle accidents

Two shuttles have been destroyed, both with the loss of all astronauts on board:

Challenger - lost 73 seconds after lift-off, January 28, 1986
Columbia - lost during re-entry, February 1, 2003: see Space Shuttle Columbia disaster