The successful explortion of space requires a system that will reliably transport payloads into space and return back to earth; without subjecting them an uncomfortable or hazardous environment. In other words, the space crafts and its pay loads have to be recovered safely into the earth. The space shuttle used at older times were not re-usable. So NASA invented re-usable space shuttle that could launch like a rocket but deliver and land like an aeroplane. Now NASA is planning to launch a series of air-breathing planes that would replace the space shuttle.A BRIEF HISTORY OF THE SPACE SHUTTLE
Near the end of the Apollo space program, NASA officials were looking at the future of the American space program. At that time, the rockets used to place astronauts and equipment in outer space was one-shot disposable rockets. What they needed was a reliable, but less expensive, rocket, perhaps one that was reusable. The idea of a reusable "space shuttle" that could launch like a rocket but deliver and land like an airplane was appealing and would be a great technical achievement.
NASA began design, cost and engineering studies on a space shuttle. Many aerospace companies also explored the concepts. In 1972 NASA announced that it would develop a reusable space shuttle or space transportation programme (STS).NASA decided that the shuttle would consist of an orbiter attached to solid rocket boosters and an external fuel tank because this design was considered safer and more cost effective.
At that time, spacecraft used ablative heat shields that would burn away as the spacecraft re-entered the Earth's atmosphere. However, to be reusable, a different strategy would have to be used. The designers of the space shuttle came up with an idea to cover the space shuttle with many insulating ceramic tiles that could absorb the heat of re-entry without harming the astronauts.
Finally, after many years of construction and testing (i.e. orbiter, main engines, external fuel tank, solid rocket boosters), the shuttle was ready to fly. Four shuttles were made (Columbia, Discovery, Atlantis, Challenger). The first flight was in 1981 with the space shuttle Columbia, piloted by astronauts John Young and Robert Crippen. Columbia performed well and the other shuttles soon made several successful flights.
The space shuttle consists of the following major components:
· Two solid rocket boosters (SRB) - critical for the launch
· External fuel tank (ET) - carries fuel for the launch
· Orbiter - carries astronauts and payload
THE SPACE SHUTTLE MISSION
A typical shuttle mission lasts seven to eight days, but can extend to as much as 14 days depending upon the objectives of the mission.
A typical shuttle mission is as follows:
1. Getting into orbit
1.1 Launch – the shuttle lifts off the launching pad.
1.2 Ascent.
1.3 Orbital maneuvering burn.
2. Orbit-life in space.
3. Re-entry.
4. Landing.
The difference between space shuttle and hypersonic planes is mainly in the first function that is getting into orbit. We will study only about the first function of the space shuttle.
1. GETTING INTO ORBIT
To lift the 4.5 million pound (2.05 million kg) shuttle from the pad to orbit (115 to 400 miles/185 to 643 km) above the Earth, the shuttle uses the following components:
· Two solid rocket boosters (SRB)
· Three main engines of the orbiter
· The external fuel tank (ET)
· Orbital maneuvering system (OMS) on the orbiter
Let's look at these components closely.
Solid rocket boosters
The SRBs are solid rockets that provide most of the main force or thrust (71 percent) needed to lift the space shuttle off the launch pad. In addition, the SRBssupport the entire weight of the space shuttle orbiter and fuel tank on the launchad. Each SRB has the following dimensions.
· Height - approximately 150 ft (46 m)
· Diameter - 12 ft (3.7 m)
· Weight:
o Empty - 192,000 lb (87,090 kg)
o Full - 1,300,000 lb(589,670 kg)
Thrust - 2.65 million lb (11.7 million N wing dimensions, parameters and parts:
Because the SRBs are solid rocket engines, once they are ignited, they cannot be shut down. Therefore, they are the last component to light at launch.
Main engines
The orbiter has three main engines located in the aft (back) fuselage (body of the spacecraft). Each engine is 14 feet (4.3 m) long, 7.5 feet (2. 3 m) in diameter at its widest point (the nozzle)
The main engines provide the remainder of the thrust (29 percent) to lift the shuttle off the pad and into orbit.
The engines burn liquid hydrogen and liquid oxygen, which are stored in the external tank(ET), at a ratio of 6:1. They draw liquid hydrogen and oxygen from the ET at an amazing rate equivalent to emptying a family swimming pool every 10 seconds! The fuel is partially the burned in a pre-chamber to produce high pressure, hot gases that drive fuel pumps. The fuel is then fully burned in the main chamber and the exhaust gases (water vapor) leave the nozzle at approximately 6,000 mph (10,000 km/h). Each engine can generate between 375,000 and 470,000 lb (1,668,083 to 2,090,664 N) of thrust; the rate of thrust can be controlled from 65 percent to 109 percent maximum thrust. The engines are mounted on round bearings that control the direction of the exhaust, which controls the forward direction of the rocket.
External fuel tank
As mentioned above, the fuel for the main engines is stored in the ET. The ET is 158 ft (48 m) long and has a diameter of 27.6 ft (8.4 m). When empty, the ET weighs 66,000 lb (30,000 kg). It holds about 1.6 million lb (719,000 kg) of propellant with a total volume of about 526,000 gallons (2 million liters).
The ET is made of aluminum and aluminum composite materials. It has two separate tanks inside, the forward tank for oxygen and the aft tank for hydrogen, separated by an intertank region. Each tank has baffles to dampen the motion of fluid inside. Fluid flows from each tank through a 17 in. (43 cm) diameter feed line out of the ET through an umbilical line into the shuttle's main engines. Through these lines, oxygen can flow at a maximum rate of 17,600 gallons/min (66,600 l/min) and hydrogen can flow at a maximum rate of 47,400 gallons/min (179,000 l/min). During the first few shuttle missions, the ET was painted white, but this was stopped to reduce the weight.
Orbital maneuvering systems
The two orbital maneuvering systems' (OMS) engines are located in pods on the aft section of the orbiter, one on either side of the tail. These engines are used to place the shuttle into final orbit, to change the shuttle's position from one orbit to another, and to slow the shuttle down for re-entry.
The OMS engines burn monomethyl hydrazine fuel (CH3NHNH2) and nitrogen tetroxide oxidizer (N2O4). Interestingly, when these two substances come in contact, they ignite and burn automatically (i.e., no spark required) in the absence of oxygen. The fuel and oxidizer are kept in separate tanks, each pressurized by helium. The helium is used to push the fluids through the fuel lines (i.e., no mechanical pump required). In each fuel line, there are two spring-loaded solenoid valves that close the lines. Pressurized nitrogen gas, from a small tank located near the engine, is used to open the valves and allow the fuel and oxidizer to flow into the combustion chamber of the engine. When the engines are shut off, the nitrogen goes from the valves into the fuel lines momentarily to flush the lines of any remaining fuel and oxidizer; this purge of the line prevents any unwanted explosions. During a single flight, there is enough nitrogen to open the valves and purge the lines 10 times.
Either one or both of the OMS engines can fire, depending upon the orbital maneuver. Each OMS engine can produce 6,000 lb (26,400 N) of thrust. The OMS engines together can accelerate the shuttle by 2 ft/s2 (0.6 m/s2). This acceleration can change the shuttle's velocity by as much as 1,000 ft/s (305 m/s). To place into orbit or to de-orbit takes about 100-500 ft/s (31-153 m/s) change in velocity. Orbital adjustments take about 2 ft/s (0.61 m/s) change in velocity. The engines can start and stop 1,000 times and have a total of 15 h burn time.
As the shuttle rests on the pad fully fueled, it weighs about 4.5 million pounds or 2 million kg. The shuttle rests on the SRBs as pre-launch and final launch preparations are going on through T minus 31 seconds:
1. T minus 31 s - the on-board computers take over the launch sequence.
2. T minus 6.6 s - the shuttles main engines are ignited one at a time (0.12 s apart). The engines build up to more than 90 percent of their maximum thrust.
3. T minus 3 s - shuttle main engines are in lift-off position.
4. T minus 0 s -the SRBs are ignited and the shuttle lifts off the pad.
5. T plus 20 s - the shuttle rolls right (180 degree roll, 78 degree pitch).
6. T plus 60 s - shuttle engines are at maximum throttle.
7. T plus 2 min - SRBs separate from the orbiter and fuel tank at an altitude of 28 miles (45 km). Main engines continue firing.
· Parachutes deploy from the SRBs.
· SRBs will land in the ocean (about 140 miles (225 km) off the coast of Florida.
· Ships will recover the SRBs and tow them back to Cape Canaveral for processing and re-use.
1. T plus 7.7 min - main engines throttled down to keep acceleration below 3g's so that the shuttle does not break apart.
2. T plus 8.5 min - main engines shut down.
3. T plus 9 min - ET separates from the orbiter. The ET will burn up upon re-entry.
4. T plus 10.5 min - OMS engines fire to place you in a low orbit.
5. T plus 45 min - OMS engines fire again to place you in a higher, circular orbit (about 250 miles/400 km).
SPACE PLANES AND REPLACEMENT OF SPACE SHUTTLE
To replace the space shuttle NASA is planning to launch a series of space planes that named as X series planes. Some X series planes are given below
· The X-37, which will test many space plane technologies, including re-entry capabilities.
· The X-34, a sub orbital vehicle that will test technologies to reduce cost, time and personnel for space launches.
· The X-33, a reusable launch vehicle (RLV) that is a prototype for a space shuttle replacement.
In this the third one that is X-33 is the one that will replace the space shuttle in the future. Despite the shuttle's many accomplishments, the fact remains that it is extremely expensive to launch into space. Each pound of payload in the shuttle's bay costs $10,000 to launch. According to NASA, each of the space shuttle's two solid rocket boosters carries about 1 million pounds (453,592 kg) of solid propellant. The large external tanks hold another 500,000 gallons of super cold liquid oxygen and liquid hydrogen. These two liquids are mixed and burned to form the fuel for the shuttle's three main rocket engines. The cost of this huge amount of propellant, and of recovering and replacing the solid rocket boosters for every mission is extremely expensive. NASA's solution to the problem is the X-33.
The X-33 is a prototype for a unique single-stage-to-orbit vehicle. Its wedge-like shape is unlike any spacecraft that has preceded it. At its base, the X-33 is 77 feet (23.5 m) wide, and the vehicle is 69 feet (21 m) long. The purpose of this design is to allow the spacecraft to hold all of the needed propellant onboard the ship, thus eliminating the need for solid rocket boosters. By eliminating the boosters and the main fuel tank, NASA will trim much of the liftoff weight that makes space shuttle missions so expensive. Launch costs for the X-33, or a derivative of the X-33, are expected to be only a tenth of the cost of launching the space shuttle.
Two more tests will follow, and successful testing could lead to a more efficient space-access vehicle. NASA officials say that the scramjet engine would be a major step forward for NASA and would arguably provide a safer, more flexible, less expensive way to get people and cargo to space.
HYPER SONIC PLANES WITH AIR BREATHING ENGINES
Living in air
The futuristic X-43A prototype looks like a flying surfboard. It’s thin, has a wingspan of 5 feet (1.5 m), measures 12 ft (3.7 m) long and 2 ft (0.61 m) thick and weighs 2,800 pounds (1,270 kg). A working version of the X-43A will be about 200 ft (61 m) in length but still relatively lightweight, en most unique feature of the X-43A is its engine.
abling it to carry heavier loads into space. But the most unique feature of the X-43A is its engine.
The best way to understand an X-43A’s air-breathing engine is to first look at a conventional rocket engine. A typical rocket engine is propelled by the combustion created when a liquid oxidizer and a hydrogen fuel are burned in a combustion chamber. These gases create a high-pressure, high-velocity stream of hot gases. These gases flow through a nozzle that further accelerates them to speeds of 5,000 to 10,000 mph (8,047 to 16,093 kph) and provides thrust.
The disadvantage of a conventional rocket engine is that it requires a lot of onboard oxygen. For example, the space shuttle needs 143,000 gallons of liquid oxygen, which weighs 1,359,000 pounds (616,432 kg). Without the liquid oxygen, the shuttle weighs a mere 165,000 pounds 74,842 kg . NASA has determined that it can easily drop the weight of a vehicle at launch if they were to take away the liquid oxidizer, which would quickly drop the weight of the vehicle to about 3.1 million pounds. That's still a heavy vehicle, but it would mean a huge reduction in cost of launching a vehicle into orbit.
Solution to this is its air-breathing engine. An air-breathing engine requires no onboard oxygen. The X-43A will scoop up oxygen as it flies through the atmosphere. In an Earth-to-orbit mission, the vehicle would store extra oxygen onboard, but less than what a space shuttle requires. The scramjet engine is a simple design with no moving parts. The X-43A craft itself is designed to be a part of the engine system: The front of the vehicle acts as the intake for the airflow, and so, if you remove the liquid oxygen, wouldn't the fuel be unable to combust and provide thrust? You have to think outside the normal operation of a conventional rocket engine. Instead of using liquid oxidizer, an air-breathing rocket, as its name suggests, will take in air from the atmosphere. It will then combine it with the fuel to create combustion and provide thrust.
An air-breathing rocket engine, also called a rocket-based, combined cycle engine, is very similar to a jet engine. In a jet engine, the compressor sucks in air. The engine then compresses the air, combines it with a fuel, and burns the product, which expands and provides thrust. A jet engine can only be used for up to Mach 3 or 4 before its parts will begin to overheat. In a supersonic combustion ramjet, or scramjet, an air inlet draws in air. The air is slowed and compressed as the vehicle speeds through the atmosphere. Fuel is added to the supersonic airflow, where the two mix and burn. Fuels most likely to be used with the air-breathing rockets include liquid hydrogen or hydrocarbon fuel
Taking flight
As mentioned before, scramjet-powered aircraft don’t carry oxygen onboard. That means that they can’t lift off like conventional spacecraft. The X-43A will require a booster rocket to get it up to a hypersonic speed, at which point it will be released and sent flying on its own. This rocket boost is necessary for the scramjet engine to work.
As efficient as air-breathing rockets are, they can't provide the thrust for liftoff. For that, there are two options being considered. NASA may use turbojets or air-augmented rockets to get the vehicle off the ground. An air-augmented rocket is like a normal rocket engine, except that when it gets a high enough speed, maybe at Mach two or three, it will augment the oxidization of the fuel with air in the atmosphere, and maybe go up to Mach 10 and then change back to normal rocket function. These air-augmented rockets are placed in a duct that capture air, and could boost performance about 15 percent over conventional rockets. Further out, NASA is developing a plan to launch the air-breathing rocket vehicle by using magnetic levitation (maglev) tracks. Using maglev tracks, the vehicle will accelerate to speeds of up to 600 mph before lifting into the air.
Following liftoff and after the vehicle reaches twice the speed of sound, the air-augmented rockets would shut off. Propulsion would then be provided by the air-breathing rocket vehicle, which will inhale oxygen for about half of the flight to burn fuel. The advantage of this is it won't have to store as much oxygen on board the spacecraft as past spacecraft have, thus reducing launch costs. Once the vehicle reaches 10 times the speed of sound, it will switch back to a conventional rocket-powered system for a final push into orbit.
Because it will cut the weight of the oxidizer, the vehicle will be easier to maneuver than current spacecraft. This means that traveling on an air-breathing rocket-powered vehicle will be safer. Eventually, the public could be traveling on these vehicles into space as space tourists.
Two more tests will follow, and successful testing could lead to a more efficient space-access vehicle. NASA officials say that the scramjet engine would be a major step forward for NASA and would arguably provide a safer, more flexible, less expensive way to get people and cargo to space.
The Marshall Center and NASA's Glenn research center Cleveland are planning to design a flight-weight air-breathing rocket engine in-house for flight demonstration by 2005.
That project will determine if air-breathing rocket engines can be built light enough for a launch vehicle.
Advantages
· Reduces launch cost
· Vehicle will be easier to maneuver the current spacecraft.
· Air-breathing rocket-powered vehicle will be safer
Conclusion
By using air breathing engine we can reduce the launch cost. Moreover air breathing rocket vehicle safer as compaired to conventional rocket.
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