The article highlights key points of a special National Research Council (NRC) committee on Space Shuttle upgrades’ ‘Upgrading the Space Shuttle’ report. The NRC committee reviewed two $1 billion-plus proposals for changing what NASA uses to propel the orbiters to space, and found that these could be broached only if more flights were planned, much more design review was done, and the Shuttle would be in service after 2012. The committee used NASA's grouping of the proposed upgrades into phases, with one being the least expensive, time-consuming, and risky, and four being the most costly, long term, and risk-prone. The committee studied another upgrade that would eliminate a hazardous material. The upgrade would modify the Shuttle's orbital manoeuvring and reaction control systems to use liquid oxygen and ethanol propellants instead of current engines' toxic N2O4 and monomethyl hydrazine propellants. ## Article Looking for. a way to feel old? Neither was I. But face this fact: The Space Shuttle program is less than three years away from being 30 years old. Most of the design work commissioned by NASA was done in the 1970s, when engineers actually hit the drawing board instead of the keyboard and mouse and when engineering software was something much closer to a novelty. The integrity and safety of the Space Shuttle is no laughing matter. Thousands of living examples of our nation's and the world's greatest engineering talent pour themselves into this endeavor. And if you thought that devising new components and subsystems for this first-ever reusable space vehicle was difficult at the onset, there's an argument to be made that repairing, maintaining, and improving the Shuttle is even tougher. That's because every change in design means a change in the set of risks for astronauts, and every change means accounting for each decision that came before it. After 30 years of engineering, that's a lot of decisions and one onerous risk assessment. So, NASA doesn't just count on blue sky researchers at the NASA Langley Research Center, or the double Ph.D.s that its largest contractors, like Boeing, employ. It counts on the type of people who not only hold or held (some are retired) positions like these, but who also sit on the Aeronautics and Space Engineering board of the National Research Council, a function of the National Academies of Science and Engineering. In January, a special NRC committee on Space Shuttle upgrades issued its "Upgrading the Space Shuttle" report. The 11-member team was given this task by NASA last spring, then spent most of the summer conducting interviews and on-site visits, and the fall writing up 25 recommendations, with assessments. NASA and the committee are dealing here with a new reality. To put it bluntly, their world no longer revolves around the Shuttle. Throughout the 1990s, stern assessments have been made of space program spending (as in most government programs because of political pressure). Early in the decade, national policy stated that money could be allocated only if it were tied to upgrades that helped the Mir and International Space Station efforts. By 1996, scrutiny became tight enough that a "design freeze" on the Shuttle was in place and the program was, to a degree, outsourced. Contractors Boeing Co. and Lockheed Martin Corp. formed their United Space Alliance Corp. joint venture and took a six-year,$7.4 billion engineering services contract. The committee found that this arrangement creates weak incentives for upgrade work and the report says that no USA Corp.-financed major upgrades are under way. Furthermore, the money NASA allocated for fiscal 1999 for safety and performance upgrades is about a third of what it spent 15 years ago.

Now, NASA awaits "a high-level national policy decision" on whether the Shuttle will operate at all after 2012. Much depends on developments in the X-series vehicles. Also, much depends on a NASA-sponsored space launch architecture study, which is near completion under Dan Mulville, NASA's chief engineer, according to Bryan O'Connor, the coml11ittee chair and an aerospace safety consultant.

The decision had been planned for 2000, and there is a good chance it will come after the November elections of that year. Such realities prompted the committee to clearly state that all of these engineering decisions are made in full recognition of the. uncertain policy environment for NASA.

## Engineering Assessments

Given these factors, the committee used NASA's grouping of the proposed upgrades into phases, with one being the least expensive, time- consuming, and risky, and four being the most costly, long term, and risk-prone. Upgrades that further the International Space Station goals are a primary focus and so are considered phase one. They will be completed by 2000.

Chief among ISS-focused work is to increase the Shuttle's payload capacity. NASA has already spent $200 million for a super lightweight tank, upgraded to lightweight crew seats, and nude other minor weight reductions in trajectory and propellant fuel reserves. The Shuttle is clearly positioned as a servicing vehicle. A recurring theme in the committee's report is eliminating toxic materials, or going to less volatile ones. The obvious goal of removing obsolete components also figures in, and this is already occurring within the program. The committee revisited the topic of protection from micrometeoroids and orbital debris, which another NRC committee h ad addressed a The multitiered staging facility for STS-79's solid rocket booster shows the size and complexity of engineering the Shuttle's propulsion. The NRC committee reviewed two$1 billion-plus proposals for changing what NASA uses to propel the orbiters to space, and found that these could be broached only if more flights were planned, much more design review was done, and the Shuttle would be in service after 2012. year earlier. The current com committee deferred to that report, but found it worth putting on the record that the risk from debris should be lessened. The Shuttle orbiters will be modified during 1999 and 2000 to protect the radiators and the leading edges of the wings from meteoroids and debris.

The more drastic phase three upgrades were also studied. For one, the committee looked very closely at the Shuttle's auxiliary power unit (APU).

Each Shuttle orbiter has three APUs, which are used to power the vehicle's hydraulics during ascent and reentry. The APUs use hydrazine propellant to drive a high-speed turbine that produces hydraulic power. The hydrazine fuel is toxic, and it has already presented other problems, including a fire involving the fuel after the landing. of the STS-9 mission.

NASA is studying a number of options for replacing the APUs with an electric system, including different battery chemistries and ultracapacitors to provide energy storage and peak power supply. Most of the electric systems under consideration would weigh slightly more than the current APUs, but would be less toxic.

Along with probabilistic risk assessment (which some of the committee members specialize in), the committee suggested a study of the cost of spare parts for the current APUs. The committee also raised the idea that it could pay to upgrade the hydrazine-driven units that power the solid rocket booster's thrust vector control system at the same time.

For the Space Shuttle's main engine, NASA has been investigating a new channel-wall nozzle to replace the current nozzle, which is the projected opening that directs the flow of fluid into the engine's open space at the rear of the Shuttle.

Employing a process for this nozzle that was developed in Russia and used for the Russian RD-0120 rocket engine, flat stock is roll-formed into a conical shape, which serves as the nozzle liner. The liner is slotted to form channels for the nozzle's liquid hydrogen coolant to flow through. Next, a jacket is installed over the liner and welded at the ends. The entire assembly is then furnace-brazed. The channels in the liner take the place of the 1,080 tubes that regeneratively cool the current nozzle.

The channel-wall nozzle is a relatively simple design that has fewer parts and welds than the current complex main engine nozzle. The nozzle takes two and a half years to build, costs $7 million, and is flown 12 to 15 times because of safety concerns related to hydrogen leaks. Not only did the committee endorse this upgrade, but after a recent main engine failure during test firing was attributed to the current nozzle, replacement with the channel-wall nozzle was also endorsed by NASA's Mishap Investigation Board. The committee raised concerns about depending on foreign production, an issue that has always loomed large for American defense and aerospace. Another structural study surrounds proposed extended nose landing gear. The proposed extension would include a new middle segment for the landing gear, a redesigned upper strut housing, and a gas supply cylinder for pneumatic actuation. The upgrade would add approximately 70 to 90 kg to the landing gear system, but would either increase the safety margins during Shuttle landing or allow the Shuttle to land with a higher maximum weight at current safety margins. The committee concluded that the expense would be warranted only if future plans called for heavier landing weights. Maintenance on the orbiter's fuel cells is currently running at about$15 million per year, and they do not live up to their billed operational time between repairs and maintenance. Ninety-six fuel cells in three stacks convert hydrogen and oxygen into electrical power, water, and heat via an alkaline electrolyte.

With continuing overhauls and repairs, the current inventory of fuel cells could support current Shuttle flight rates beyond 2012. If the flight rate increases to 12 per year or more, addition al fuel cells will be needed. Two distinct upgrades—longer life alkaline fuel cells and proton exchange membrane fuel cells—are being considered to replace the current cells.

International Fuel Cells (the long-time supplier) and Boeing propose the use of longer-life alkaline fuel cells. The modified cells would operate at reduced reactant temperatures and would be designed to resist corrosion and improve reliability.

## PEM Fuel Cells

Proton exchange membrane (PEM) fuel cells use a moist polymer membrane as the electrolyte. Although PEM cells were flown in space before alkaline fuel cells, alkaline systems were chosen for the Apollo program and then the shuttle.

The NASA Johnson Space Center in Houston is evaluating prototype PEM fuel cells from four different vendors for several purposes, not just a Shuttle upgrade. The committee studied another upgrade that would eliminate a hazardous material. The upgrade would modify the Shuttle's orbital maneuvering and reaction control systems to use liquid oxygen and ethanol propellants instead of the current engines' toxic N2O4 and monomethyl hydrazine propellants. The proposed upgrade would involve replacing the current engines. The forward reaction control system would be connected to new common propellant storage tanks. NASA expects to be ready for a decision on whether to proceed with the upgrade by nlid-2000.

The upgrade has some disadvantages. The liquid oxygen propellant would require additional tanks, insulation, and thermal controls. Structures and other subsystems in the vicinity of the liquid oxygen may also require thermal protection. And abandoning hypergolic fuel could reduce reliability and mean more maintenance, as would be the case with several of the changes inherent in this proposal. The NRC committee said further study is warranted.

As with the channel-wall nozzle, a second cooling apparatus could be upgraded. The water membrane evaporator is being considered as a replacement for the orbiter's flash evaporator system, which cools the orbiter during ascent and reentry and also provides supplemental cooling in orbit. The flash evapora tor is experiencing corrosion, which creates a risk of freon leaks.

The water membrane evaporator takes advantage of the hydrophobic, or water-repelling, nature of microporous Teflon to evaporate water while maintaining excess liquid water in a hydrophilic layer behind the hydrophobic layer.

The only phase four upgrades briefed to the committee were two new first stage booster concepts: the five-segment reusable solid rocket booster, and the liquid fly-back booster.

Committee chair O'Connor explained that the boosters (which propel the spacecraft but fall away and are recovered) run on solid fuel and cannot be throttled. " In other words, once those big fire crackers light off, they burn until the fuel is spent. Even if the crew or mission control wanted to turn them off or adjust the thrust level to accommodate some emergency, they cannot. Liquid rocket boosters, on the other hand, can be shut off or throttled to allow for more failure tolerance.

"A liquid design also allows the range safety officer to shut off the rockets rather than blow them up and kill the crew, as he has to do with today's solids [in the remote case where the Shuttle l11.ight veer off course and pose a threat to populated land masses]. There are lots of other differences, but that one is probably the most important from a safety point of view," he said.

Each concept represents a major programmatic and technical undertaking.

Any new booster design, no matter how many safety and reliability enhancements it contains, will necessarily pose additional risk to the first few crews who fly it. NASA is not likely to, and the Committee agrees it should not, enter into any major new booster program without substantial national need for the promised performance enhancements and cost savings.

With that said, the committee gave its evaluation of the five- segment reusable solid rocket booster.

This upgrade, informally proposed by Thiokol Propulsion, would modify the Shuttle's four booster segments and add a fifth. The proposed upgrade also would modify the nozzle and insulation and alter the grain of the solid fuel to provide a more risk-tolerant thrust profile. Both Thiokol and Boeing have funded preliminary designs, estimated benefits, and examined systems integration issues. Estimated total costs for the upgrade are in the range of $1 billion with an estimated four-year schedule from authority to proceed until the first flight. On the surface, the five-segment booster appears to be a relatively straightforward approach to increasing the performance of the Shuttle's boosters. The extra performance from this upgrade could either allow the Shuttle to carry heavier payloads, eliminate the need to throttle the main engines during ascent (thus improving safety), or minimize or eliminate a high-risk launch abort mode. A more radical approach comes in the liquid fly-back booster (LFBB). The proposed upgrade would replace the Shuttle's two solid rocket boosters with winged liquid-fueled boosters that would automatically fly back to the launch site (using conventional gas turbine engines) after they have used up their rocket fuel and separated from the orbiter. The proposers of the upgrade believe that the fly-back boosters would improve safety by reducing or eliminating the need for some high-risk abort modes, save$400 million per year in operations costs (with seven Shuttle flights per year), and increase the Shuttle's payload capacity. The proposers also predict that the LFBB would enable a three-week turnaround time between missions, and (with three sets of LFBBs) could allow the Shuttle to fly 15 times per year.

The committee said many options seem to have been left off the table, like liquid, ocean- recovered boosters instead of ones that fly back. It added that the high development cost of the liquid fly-back booster might not receive funding in the uncertain policy environment.

It's an interesting time for NASA. A key focus at the time of writing is the Italian-made space station module being processed at the Kennedy Space Center in Cape Canaveral, Fla. The Shuttle is considered as much a workhorse as a triumphant national as et. Aerospace engineering is a very different proposition than it was in 1972, when President Richard Nixon authorized the Shuttle program. Engineers most often now work for the one or two survivors of the intensely consolidated defense and aerospace industries. One thing is clear: Space flight in the 21st century is going to be whole different ball game. The United States and the world have the Shuttle to build on.

Editor's note: Technical details for this article come directly from the NRC's report, "Upgrading the Space shuttle." Also, computer users with a cable modern or fast LAN connection can take full advantage of the 400,000-plus photos and digital movies at the NASA Image Exchange (http://nix.nasa.gov).