The Space Shuttle Main Engines

The space shuttle program, which began in the 1970s, dealt with a launch vehicle that was flanked by two large solid-fueled rockets. In the middle at the base were three liquid-fueled "main" engines, each having nearly 500,000 lb (2.2 million N) thrust and propelled by hydrogen fuel and oxygen at a mixture mass ratio of 6:1 (oxygen to hydrogen). Each of these space shuttle main engines (SSME's) has a dedicated pumping system, consisting of a low-pressure single-stage inducer-type pump for each propellant, which in turn feeds a high pressure pump. Three of these four pumps can be seen in the photograph of the engine in Figure 2. Located up above is the low-pressure hydrogen pump, with its large discharge line going off to the right and down to the high-pressure hydrogen pump below. Opposite, on the left, is the high-pressure oxygen pump, with its large inlet line also in view. Engineering of these high-pressure propellant pumps embodied a triple challenge in comparison to the F-1 pumps, because a) this engine was designed to be re-used, so the pumps had to have a life of 7.5 hours with 100 starts; b) the pumps for this engine have higher energy levels than the F-1 pumps, and, c) due to the mission profile, they have to be throttled back further from the design point. Each of these pumps is boosted by a low-pressure inducer-type pump to suppress cavitation enough to maintain performance3.

The propellant flow system is illustrated in the schematic diagram of Figure 3. (The pump speeds, pressures, and flow rates shown in this figure are somewhat lower than the design conditions of Table 1, which for the SSME pertains to the maximum engine thrust level.) All four of the pumps are driven by turbines and so are called "turbopumps." The system picks itself up by its own bootstraps, so to speak; each low-pressure turbopump boosts the flow to the corresponding high-pressure pump and is driven by the same fluid that it pumps. This driving fluid comes back from each high-pressure pump, the low-pressure oxygen turbopump (LPOTP) being driven by recirculated liquid oxygen, and the low-pressure hydrogen or fuel turbopump (LPFTP) turbine being fed by gaseous hydrogen heated by the thrust chamber, as indicated in Figure 3.

The high-pressure pumps are driven by turbines fed by "preburners," which are com-bustors that burn hydrogen-rich. Some of the fuel entering these combustors is the gaseous hydrogen coming from the LPFTP turbine exhaust, which cools the turbine housings on the way to the preburners. But most of the fuel supplied to the preburners is the 80 percent of the liquid hydrogen discharging from the high-pressure fuel turbopump (HPFTP), which first flows through the cooling passages of the nozzle walls. [Eleven percent of the oxygen is also fed to the preburners by way of the preburner boost stage that is a part of the high-pressure oxygen turbopump (HPOTP) package. Finally, the partially burned fuel passes as a hot gas into the main combustion chamber, where more oxygen is added and the pressure is 3,000 lb/in2 absolute (21 MPa)3,4.] Most of the remaining 20 percent of the hydrogen is that which was already described as cooling the main combustion chamber and, along the way, becomes gaseous and powers the drive turbine of the LPFTP. It also cools the hot-gas manifold and injector and pressurizes (in a small amount) the fuel tank. Approximately 75 percent of the liquid from the high-pressure oxygen turbopump (HPOTP) goes directly to the main combustion chamber, 11 percent to the preburners (as already stated), about 13 percent to drive the turbine of the low-pressure oxygen turbopump (LPOTP), and a small amount is sent to pressurize the tank.5 A brief description of each pump follows:

• The low-pressure oxygen turbopump (LPOTP) consumes 1,740 hp (1.30 MW) and runs at 5,450 rpm. It has a single-stage, axial-flow, inducer-type impeller that is 11.725 in (298 mm) in diameter and is driven by a six-stage liquid-oxygen hydraulic turbine. Pump head rise is 670 ft (204 m) so the HPOTP therefore operates without pressure

FIGURE 2 The space shuttle main engine (SSME) (National Geographic Magazine, March 1981: Jon Schneeberger/NGS Image Collection)
FIGURE 3 SSME liquid oxygen and hydrogen pumping arrangement (Courtesy of The Boeing company)

loss due to cavitation at all engine conditions. Specific speed Ns = 3,230 (1.18), which is low for an inducer, but this one performs a more substantial pumping task than do inducers running at high speeds on the same shaft as the high-pressure centrifugal stages they feed. The shaft has two ball bearings cooled by liquid oxygen, and the envelope of the package is 18 in (457 mm) in diameter X 18 in (457 mm) long.

The low-pressure fuel turbopump (LPFTP) is similar in concept to the LPOTP. It is a 2,900 hp (2.2 MW) machine that runs at 15,700 rpm. The inducer-type impeller is 12.014 in (305 mm) in diameter and is driven by a two-stage gaseous-hydrogen turbine. It ingests liquid hydrogen to the extent of 1 of the mass flow rate of oxygen. Due to the very low specific gravity of liquid hydrogen (= 0.0708 at atmospheric pressure), the head rise is more than 7,000 ft (2,100 m). Ns = 2,550 (0s = 0.93). The shaft has three ball bearings cooled by liquid hydrogen, and three seals limit leakage between the pump and the turbine before engine start and during operation.

Both low-pressure pumps exhibited suction-specific speeds Nss in excess of 35,000 (0ss = 13)4. They are shown schematically at the top of Figure 3, and the high-pressure pumps are below them.

The high-pressure oxygen turbopump (HPOTP) is shown in cross-section in Figure 4 together with the associated preburner unit. Inducers are shown in the detailed cross-sectional drawing of Figure 5 feeding the double-suction main stage that generates nearly 5,000 lb/in2 (34 MPa)—making this one of the world's highest-energy pumps (as defined in Section 2.1, Figure 32). And more than 3,000 lb/in2 (21 MPa) in addition to this pressure is generated in the small booster impeller (on the outer end of the turbopump as seen in Figures 4 and 5) that feeds about 11 percent of the main flow to the preburners for driving both high-pressure pumps. Both shrouds of the main impeller act as orificed balancing disks to keep all but a small preload off the two angular-contact ball bearings, which are cooled by liquid oxygen. Radial loads are minimized by the use of a vaned diffuser. The compact design and high speed (over 31,000 rpm) of this 28,000 hp (21 MW) machine result in operation between the first and second critical speeds. Several seals are utilized,

Ssme Turbopump
FIGURE 4 SSME high-pressure turbopumps (Courtesy of The Boeing company)
FIGURE 5 Cross-section of SSME high-pressure oxygen turbopump (Courtesy of The Boeing company)
FIGURE 6 Cross-section of SSME high-pressure oxygen turbopump (Courtesy of The Boeing company)

including a helium-purged seal that isolates the hot gas of the turbine from the cold liquid oxygen. Special material combinations are used to prevent sparking, ignition and explosion due to rubbing in the strong oxidizing environment of the liquid oxygen.

• The high-pressure fuel turbopump (HPFTP) is also shown in Figure 4. Additional detail is afforded by the cross-sectional drawing of Figure 6. No inducer is needed due to the pressurization of the LPFTP and the more favorable vaporization characteristics of liquid hydrogen. The pump has three identical impellers and diffusers, except that the third-stage diffuser discharges into a scroll or volute, and the third-stage impeller also acts as an orificed balancing disk, which removes all axial thrust (except for a small spring-preload force) from the angular-contact, liquid hydrogen-cooled ball bearings located at each end of the pump-and-turbine package. The head per stage is over 65,000 ft (20,000 m) because of the high pressure rise and low density of liquid hydrogen. This leads to an impeller OD tip speed of nearly 2,000 ft/sec (600 m/s)—probably the highest in existence for a pump and successfully deployed in titanium, which has the benefit of a higher strength-to-weight ratio than the more commonly used steels. The high speed (over 37,000 rpm) keeps this 77,000 hp (57 MW) machine small and light and necessitates operation between the second and third critical speed.

Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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