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Is The Shuttle Fatally Flawed

The lateral configuation makes for a dangerous launch with the fuel tank above the crew cabin making any ejection system overly complex and party why such an escape system was dropped.
by Josef Pinkas
Prague - Mar 3, 2003
After the recent loss of the Space Shuttle Columbia, Americans and Israelis are mourning the tragic deaths of the seven-crew members. This is a tragedy not only for them but also for all of us, because the brave astronauts sacrificed their lives for the progress of all mankind.

The inquiry into Columbia's loss is still far away from its final conclusion, but many questions have arisen about the safety of all Space Shuttle launch systems. Even if the cause of the fatal accident is found directly in the Shuttle spacecraft itself, the real root of its problem could be in the basic design philosophy of its launch system.

This design has a primary influence on safety and effectiveness. The aim of this article is to discuss the main problems with the Shuttle system's design in terms of crew safety and cost efficiency, and also to consider whether it could have been possible to choose another way at the time of system design.

Shuttle Launch Systems Problems In Terms Of Safety:

  1. The spacecraft is positioned laterally, alongside the main LOX/LH2 tank and solid rocket boosters. In the case of any major explosion of the tank or booster, the Shuttle crew has practically no chance of survival. This was the case in the loss of the Challenger in 1986.

  2. Installation of the three main LOX/LH2 SSME engines directly into the Shuttle poses the possibility of a fatal disaster in any case of engine or fuel turbine explosion.

    Furthermore, this particular configuration results in the tremendous vibrations of the working engines being directly transferred to the structure of the Shuttle, its systems and crew compartment.

    These vibrations greatly contribute to material fatigue of the structure and increase the possibility of the loss of some of its crucial belly tiles.

    Additionally, the main engines are operating at chamber temperatures of thousands of degrees centigrade for about 480 seconds, while the feed pipes supplying LOX/LH2 to the engines have extremely low negative temperature. These two facts mean that the Shuttle must withstand major temperature stress to its structure and systems with potentially negative consequences.

  3. The three SSME engines have a combined thrust of almost 700,000 kgf (7 times higher than the weight of the Shuttle). This thrust must be transmitted through the Shuttle structure to the LOX/LH2 tank, because from a mechanical point of view, the Shuttle is carrying a huge fuel tank on its belly and not vice versa.

    It means that immense stress on the Shuttle structure from the SSME engines during launch creates elastic deformations including deformation of its belly, again increasing the possibility of tile detachment. It also means that the structure of the Shuttle must be much more rigid and heavy than if the launch system was carrying the Shuttle.

    This weight factor, combined with the mass of three SSME engines enhances the stress to the structure during re-entry. Nevertheless, the main problems during re-entry could well be caused by the immense load on the Shuttle structure that occurs during its launch into orbit.

  4. The lateral position of the Shuttle during launch enables pieces of debris or ice from the external tank to impact the Shuttle, possibly with very serious consequences.

  5. It is difficult to stop solid rocket boosters after ignition and therefore it is difficult to effect launch abort in the critical phase after ignition of engines and before liftoff.

Shuttle launch system problems in terms of cost and mass efficiency: The first question is, why was such a flawed design approach - one that compromised the safety of the crew- chosen during the early stages of development?

There are several reasons:

  1. NASA management and also designers wanted to have Shuttle system components reusable in as much as possible.

  2. Most of the US engineers working on the project, with the exception of Werner von Braun, were much more experienced with LOX/LH2 and Solid Rocket engines than with LOX/ KEROSENE engines. This situation resulted in the adoption of a strange combination of LOX/LH2 engines, with very good specific impulse, with solid rocket boosters which had never before been used for manned space travel and had very poor specific impulse. The result was that the combined mass efficiency of the Shuttle system is no better than that of the SATURN 5 system, designed 45 years ago.

    US engineers succeeded in convincing most of the decision-making people at NASA that the LOX/LH2 engine was more modern and its specific impulse (higher than that of LOX/ KEROSENE) was crucial for system-wide mass economy. Because of its better specific impulse, especially in a vacuum, the LOX/LH2 engine is very favourable for the upper stages of the rockets, where dimensions of the LH2 tanks do not present a problem.

    However, the specific impulse of LOX/LH2 is substantially lower at sea level than in a vacuum, and the mass of the huge LH2 tank including insulation is much higher than the mass of a corresponding kerosene tank.

    To use LH2 in the first stage of system or in the stage burning all the launch time, as in the case of the Shuttle, it brings enormous problems with dimension of the LH2 tank, with its foam insulation, in handling the huge volume of extremely low-temperature LH2 fuel.

    These problems can be manageable with classic rockets like DELTA IV but in the Shuttle System the use of LH2 has practically excluded the possibility to situate Shuttle in an on-axis position at the top of the system (meant without main engines).

    The lateral configuration of the Space Shuttle means that the vector of booster thrust does not correspond with the vector of the SSME engines. The angle between them presents losses. After separation of the boosters, it is necessary to ensure vector of SSME thrust to point to the centre of gravity of the complex Shuttle fuel tank, because the fuel tank's centre of gravity continuously changes its position as it empties.

    In a lateral configuration, it is necessary to have a main engine installed in the Shuttle, otherwise the mass efficiency of the system becomes even worse (see notice about BURAN below) All of the above mentioned technical factors substantially degrade the advantages of the higher specific impulse of the LOX/LH2 engines, and the necessity to install them directly onto the laterally mounted Shuttle presents very dangerous compromises for the safety of the crew.

  3. It has been proven that the launch of heavy payloads to LEO is much more economical using expendable rockets than by the Shuttle system. For example, the almost 50-year-old Russian PROTON rocket with a total launch mass 3 times lower can place in LEO almost the same cargo as the Shuttle, at a cost almost 10 times lower. The cost of other expendable boosters like ARIANE, ATLAS, DELTA is only a little higher.

Therefore the Shuttle could have been designed for transport of astronauts and small payloads only (this is usually the case anyway) and be much smaller and much more economical.

- An extreme example of such an economical system is the Russian MASK Spaceplane, the draft project of which was finished in 1988. This Space vehicle was based on the An-225 Mriya carrier aircraft, the largest in the world, and a reusable MAKS Orbiter with two RD-701 tripropellant engines.

It was expected that MAKS could reduce the cost of transport to earth orbit by a factor of ten, but development resources have not available found in Russia since 1990.

Could another configuration have been chosen at the time of Shuttle basic design?

Assuming that the required LEO payload of the system is the same (app.100,000 kg), it is clear that the total lift-off mass of any system designed at that time would be approximately the same as that of the Shuttle system - 2,300,000 kg - and total required thrust during launch would also be approximately the same. Did the US have at that time any other options besides the SSME and SRB engines?

The combine liftoff thrust of two SRB + three SSME is 3050.000 kgf. This represents approximately the thrust of four LOX/ KEROSENE F1 engines used before for the SATURN 5 moon rocket. The F1 engine was very reliable and powerful - 789.000 kgf of thrust, but of course, its specific impulse was not high (vac/sl: 304/265s) and its Chamber Pressure low (70 bar).

For comparison, SRB has only 269/237s - while on the other hand the SSME has 455/363s. Nevertheless, it is necessary also to calculate the major disadvantage of SRB - very high empty mass in comparison with the mass of F1 + its empty fuel tanks, not mention the very negative environmental impact of SRB engines.

It is possible to imagine such competitive design some 30 years ago: During many years of Space Shuttle system development, specific impulse of the F1 engine has been improved to the level of the Russian RD171 engine (337/309s), to be reusable and its thrust has been increased by 10%.

This means that four boosters with F1 engines can be used during lift-off instead of two SRB boosters and three SSME Shuttle engines. After the boosters burn out, the central core with one F1 engine is ignited to carry the Shuttle in an on-axis position into orbit.

The following design is now being used: a small platform carrying the Shuttle is situated on a relatively short central core, which is surrounded by a cluster of four boosters.

The Shuttle is of the same operational capacity but without SSME engines - consequently with lower empty mass and higher payload - making it similar to the Russian BURAN.

The Shuttle is equipped with a powerful rescue system designed to fire off the crew cabin in any emergency situation during launch. The four main boosters are recovered and reused, like SRB boosters. Only the core engine and its tank are expendable. Recovery of boosters by parachute could be even easier than SRB because of its much lower empty mass

Conclusion: The use of LH2 as fuel has brought about an inevitable system configuration that in many aspects has compromised crew safety. If LOX/kerosene fuel is used in boosters and also in central core carrying on an axis-situated Shuttle, for the price of one expendable F1 engine, all very serious safety compromises (see points 1-5) could have been removed and crew safety multiplied many times.

The development and operational expenses could have been substantially reduced, the system very easily adapted for launch of unmanned payloads. Much cheaper, much easier in handling and much more environmentally friendly (during lift-off) storable fuel could have been used.

It is really very hard to understand, why such a good and powerful engine as the F1, developed for billions of US$ during the moon race was discarded after only three years of use. Was it only because the F1 was developed under Werner von Braun leadership and American engineers wanted to prove that they were able to develop something better? It is now a very serious question which engine is better.

Similar Shuttle System in Soviet Union: It is interesting to compare the development of a similar system in Soviet Union. Soviet engineers at the end of the nineteen-eighties had developed a very good reusable LOX/kerosene engine- the RD171 (better than F1), and also boosters with this engine, much more efficient than the SRB.

They had a very good Space Shuttle, the BURAN, without main engines installed - safer for the crew. No single tile was lost during its maiden flight. They could easily have designed the centre core with the same engine RD171 and put the BURAN in an on-axis position at the top of the system.

This configuration was seriously considered. But in a dubious effort enforced by the political sphere to have "technological parity" with the US in the development and use of LOX/LH2 engines, Russian technicians instead developed a central core of ENERGIA with LOX/LH2 engines similar to SSME.

Consequently, it was necessary to use a huge, high fuel tank like the US, and it was technically impossible to place BURAN above it in the on-axis position. The only option was to place BURAN in the same lateral position as the US Shuttle.

Such a position without main engines installed in BURAN worsened mass efficiency to the level of the US Shuttle system, even though much more efficient boosters were used. Moreover, the LOX/LH2 engines were expendable at this stage of development.

The result was a system with almost the same disadvantages as the US Space Shuttle system. Those problems were also factors in the eventual abandonment of all ENERGIA-BURAN projects.

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