Abstract
The Solid Rocket Boosters played an essential role in orbiting a Space Shuttle. However, their many pitfalls did erode safety margins, limiting the escape possibilities for the crew in the first two minutes of the flight. This chapter tells how NASA tried to develop liquid rocket booster, which would have increased safety and greatly improved performance.
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Notes
- 1.
To date, such system proved essential in two occasions. The first on September 26, 1983 when the rocket meant to launch the Soyuz T-10 mission caught fire on the pad. The second occurred on October 11, 2018 shortly after lift-off when during separation of the first stage boosters, one of them collided with the second stage. Within seconds, the crew of Soyuz MS-10 had already been vigorously pulled away from the ailing rocket.
- 2.
Early in the program, the African sites were dismissed as the logistics of returning an Orbiter from such sites was considered too burdensome, especially considering the rather politically unstable climate of that continent. For the majority of the program, a TAL abort could be performed to reach either one of three European sites located in Istress (France), Moron, and Zaragozza (both in Spain).
- 3.
Challenger is the only Space Shuttle that can claim to have performed an ascent abort. On July 29, 1985 she launched to carry out the STS-51F mission. Some five minutes into the ascent, the center main engine shut down and an ATO abort was declared. Albeit on a lower orbit, the mission took place.
- 4.
Considering that the RTLS mode has a rather “aerobatic” profile, it was the least trusted by the astronauts. Despite the extensive effort devoted to it in training, no one expected to return alive from such an abort.
- 5.
It must be noted the SRBs were jettisoned as soon as the internal pressure had lowered to 50 psi. Although at this point a small fraction of the propellant still remained and was being burned, the thrust generated was so small that for all intentions and purposes, the SRBs could be considered incapable of providing any useful propulsion contribution. Hence, they had to be jettisoned, because the main engines would not be able to fly the desired trajectory while carrying what was now dead mass.
- 6.
In fact, from that first return to flight mission until the last flight in July 2011, a total of 110 flights, the SRB performed flawlessly.
- 7.
It is worth clarifying that contingency abort modes could be invoked before booster separation. For instance, had one or more main engine failed during the first two minutes of ascent, the crew would have initiated the appropriate contingency abort procedure. However, they still had to wait for booster separation before commencing to steer the Orbiter towards an emergency landing site or towards a point in the ocean where they could bail out.
- 8.
As a matter of fact, the first stage of SpaceX’s Falcon 9 rockets is ignited for a few seconds a few days ahead of the launch to ascertain the cluster of nine Merlin engines are fit for the launch. In addition, each engine is fired and tested at SpaceX’s McGregor testing facility in Texas before being shipped for final assembly on the first stage.
- 9.
As the External Tank was being depleted, it becomes lighter and lighter. Maintaining the same thrust settings resulted in an increase in the acceleration. Some ten seconds prior to MECO, the main engines were throttled down to 67% to prevent the acceleration rate to exceed the Orbiter airfrafme’s structural limit.
- 10.
In fact, in the case one or all main engines had gotten stuck in the thrust bucket and could not throttled up, procedures called for an RTLS abort soon after SRB separation.
- 11.
The extremely unusual cold temperature on the day of launch was one of the several factors that doomed Challenger. The cold temperatures had frozen one O-ring sealing the joint between two segments on the right SRB, allowing for the hot exhaust to im**e on the adjacent External Tank, like a blow torch. This lead the booster aft end to slam against the base of the External Tank initiating a chain reaction of events the lead to a rapid explosive disassembly.
- 12.
It is helpful to remember that the large size of the External Tank, and of the Orbiter/Booster vehicles devised in the early studies, was dictated by hydrogen being the lightest element in the periodic table, thus the least dense. Therefore, even in liquid form, hydrogen requires large tanks for storage.
- 13.
Typically, these burners or gas generators burn a small quantity of propellant to generate a stream of hot pressurized gases. Flowing though the turbine side of the turbopump, the spin the pump side increasing the pressure of the propellants being delivered to the engine’s combustion chamber.
- 14.
Often, such combination of propellants is simply termed “hypergolics”.
- 15.
Certainly, the most notable example of thermal insulation was that of the Space Shuttle External Tank whose whole exposed surface was covered with a sprayed-on foam insulation layer that gave it its distinguished orange appearance.
- 16.
An alternative designation is Refined Petroleum-1.
- 17.
In technical parlance this is referred to as “Max-q” and indicate a precise moment during the ascent, when the combination of speed and air density work together to produce the maximum possible aerodynamic loading that the rocket will experience during powered flight in the atmosphere. Beyond this point, even if the rocket is accelerating, the expeditious reduction in the atmospheric density means that aerodynamic loads precipitate to nothing leaving the structure of the rocket to deal with inertial loads. Typically, Max-q occurs when the rocket has reached supersonic speed. The formation of a conically shaped cloud of water vapour with the apex centred on the rocket’s nose provides a visible clue to when the rocket is experiencing Max-q. This is a consequence of the shock waves produced by the rocket nose which by compressing the air ahead causes the water vapour to condense.
- 18.
Designated in technical parlance as the ET GOX vent arm, it positioned a sort of suction cap on top of the External Tank to provide an escape path for gaseous oxygen built up within the tank below. In fact, despite the protective insulating foam and the cryogenic temperature inside the tank, some of the liquid would still boil off. This was not necessarily a bad thing as it kept the propellant slightly pressurized to prevent cavitation of the main engines’ turbopumps. However, to prevent tank rupture it had to be vented safely away from the External Tank. This is why, propellant loading continued up to two minutes before lift-off.
- 19.
Note that this shrinkage results from loading the ET-LRB attachment structure, while the LRB are still bolted to the launch pad and support the entire stack.
- 20.
This holds true up to the value of maximum angle of attack. Should the aircraft attitude move the wing chord beyond this value, the wing will stall producing a sudden loss in lift with the aircraft rapidly falling out. While recovering from a stall at high altitude is perfectly feasible and practised in training, stalling during landing close to the ground is as dangerous as it sounds, as there is not be enough time to recover from the fall and bring the wings to generate lift again before crashing.
- 21.
This strategy is known as “assisted airstart”.
- 22.
The way bank angle works is that it changes the vertical component of the lift produced by the wing so that the imbalance with the weight causes the vehicle to sink at a rate determined by the bank angle, generating the desired drag and maintaining aerodynamic and structural loads within the desired limits.
- 23.
Among them there was even the F-1 engine that powered the first stage of the Saturn V Apollo program!
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Sivolella, D. (2022). Boosting the Booster. In: The Untold Stories of the Space Shuttle Program. Springer Praxis Books(). Springer, Cham. https://doi.org/10.1007/978-3-031-19653-9_3
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