Katerina Swanson
Launching a space shuttle requires a lot of fuel. The amount of fuel needed depends on the rocket equation, which states that the excess mass of fuel required for a craft heading into space increases exponentially. This is because most of the fuel is used to transport the fuel that will be burned later in the journey. The space shuttle has three main parts: an orbiter, an external fuel tank, and two solid rocket boosters. The external tank holds more than half a million gallons of self-combustible liquid, while the boosters contribute about two million pounds of rubbery aluminium fuel. The boosters are dropped into the ocean after two minutes and are reused. The external tank drops off after six minutes and disintegrates upon reentering Earth's atmosphere.
| Characteristics | Values |
|---|---|
| Weight of the shuttle on the launchpad | 4.5 million pounds |
| Weight of the shuttle when it reaches orbit | 10% of its launch mass |
| External tank capacity | 629,340 kg of liquid oxygen and 106,261 kg of liquid hydrogen |
| External tank weight | Not mentioned |
| Solid rocket boosters (SRB) weight | 500,000 kg each |
| SRB fuel | Ammonium Perchlorate Composite Propellant (APCP) |
| SRB burn time | 124 seconds |
| SRB specific impulse | 269 seconds |
| SRB thrust | 12.5 MN per SRB |
| Orbiter fuel | Orbital Maneuvering System (OMS) hypergolic liquid propellant |
| Orbiter OMS burn | 200-550 ft/s |
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What You'll Learn
The fuel tank and boosters
The external fuel tank is responsible for holding a significant amount of fuel, over half a million gallons of self-combustible liquid, to be precise. This fuel is essential for the shuttle's main engines, and it is burned by the shuttle during its journey. The external tank also provides propellant for the three Space Shuttle Main Engines (SSME) on the orbiter.
The two solid rocket boosters (SRBs) are equally important, each using around 500,000 kg of an 11-star perforated solid propellant cake of Ammonium Perchlorate Composite Propellant (APCP). This mixture includes ammonium perchlorate, aluminium, iron oxide, PBAN or HTPB polymers, and an epoxy curing agent. The boosters provide 85% of the thrust needed to lift the shuttle off the ground, generating approximately 12.5 MN of thrust per SRB.
The boosters have a burn time of 124 seconds, after which they are jettisoned and fall into the ocean, where they can be recovered and reused. The external tank, on the other hand, is jettisoned once it is empty, about six minutes after launch, and disintegrates upon reentering Earth's atmosphere.
The multistage vehicle concept, where powerful rockets are dropped sequentially or in sections when their fuel is exhausted, helps address the challenge of boosting "excess" mass in the form of fuel. This design allows for more efficient space travel, as towing empty fuel tanks adds unnecessary weight.
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The orbiter
The Space Shuttle orbiter is the spaceplane component of the Space Shuttle. Operated from 1981 to 2011 by NASA, the U.S. space agency, this vehicle could carry astronauts and payloads into low Earth orbit, perform in-space operations, then re-enter the atmosphere and land as a glider, bringing its crew and payload back to Earth.
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The role of oxidisers
In the context of space shuttle fuel, liquid oxygen (LOX) is commonly used as the oxidiser. It is stored in an external tank (ET) along with the fuel, providing structural support and attachment points for the Solid Rocket Boosters (SRBs) and the orbiter. This external tank is the "backbone" of the shuttle during launch, as it supplies the fuel and oxidiser under pressure to the main engines. The oxidiser-to-fuel ratio is carefully monitored to ensure efficient combustion and engine performance.
The Space Shuttle Main Engines (SSME) typically use a mixture of liquid hydrogen and liquid oxygen as propellants. This combination provides a high specific impulse, which measures the efficiency of rocket propellants. The specific impulse indicates how much thrust is obtained per unit of propellant consumed.
Additionally, solid rocket boosters also utilise oxidisers in their fuel composition. These boosters contain a solid propellant cake of Ammonium Perchlorate Composite Propellant (APCP), which includes ammonium perchlorate as the oxidiser. This mixture provides a significant amount of thrust during the initial stages of the shuttle's launch.
The choice between liquid and solid propellants, and therefore the specific oxidisers used, depends on various factors. Liquid propellants offer the advantage of moderating, stopping, or restarting combustion, which is useful for throttle control. Solid propellants, on the other hand, typically have higher thrust and are easier to store and handle, making them ideal for boost stages and when large amounts of thrust are required.
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The weight of fuel
The external fuel tank is a crucial component, holding over half a million gallons of self-combustible liquid fuel. This tank provides propellant for the three Space Shuttle Main Engines (SSME) on the orbiter. The two solid rocket boosters contribute significantly to the total weight of fuel, burning through approximately 500,000 kg of Ammonium Perchlorate Composite Propellant (APCP) each, providing 124 seconds of burn time.
The weight of the fuel in the external tank and boosters is not the only consideration. The Space Shuttle also carries Orbital Maneuvering System (OMS) hypergolic liquid-propellant rocket engines as stage 2. While the exact weight of this fuel is not specified, it is noted that the OMS burn typically uses between 200-550 ft/s, with additional fuel required for maneuvering during the mission.
The challenge of boosting the "excess" mass of fuel is a significant hurdle in space travel, as addressed by Tsiolkovsky's rocket equation. This issue is mitigated by the multistage vehicle concept, where powerful rockets are used to launch a smaller payload, and then sequentially dropped when their fuel is exhausted.
Furthermore, the weight of the fuel is not just a concern during launch. For a successful return to Earth, the shuttle must also carry enough fuel to slow down and land gently. This is a crucial consideration, as the shuttle is travelling much faster during its return and faces the challenge of managing heat and friction.
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Slowing down and stopping
Neil deGrasse Tyson gives an example of this problem:
> "Look at how much energy (and fuel) it took for the space shuttle to go from 0 to 18,000 mph. It required giant fuel tanks and a huge fireball shooting out the bottom for like 10-15 minutes straight. If you wanted to slow down from 18,000 mph to 0 mph without using friction, you'd need to use the same amount of energy. You might think that it would cost 2x the fuel to do a powered deorbit. The real problem is that if you wanted to use your engines to slow down, you'd have to carry all that fuel up into space. And fuel is heavy. So, now in order to launch the space shuttle along with all that extra fuel, you need even more fuel for the initial launch. I haven't done the math, but I wouldn't be surprised if it would require 5-10x as much fuel to launch a spacecraft+fuel that is capable of a powered descent. The launch cost would be significantly higher."
One way to avoid using so much fuel for slowing down and stopping is to use the Earth's atmosphere to create friction and slow down the shuttle. This is what the space shuttle does when it returns to Earth: it glides back to Earth unpowered, using the atmosphere to slow it down. However, this method has its challenges. The shuttle is travelling much faster during its return than during its launch, so heat and friction are much bigger problems. Additionally, the shape of the rocket is optimized for launch, not return, and powerful stabilization engines are needed to keep the main engine pointing in the correct direction to slow the shuttle down.
Another way to slow down and stop a space shuttle is to use a technique called "powered re-entry." This method involves turning the rocket nozzles backward so that they point in the direction of motion and igniting the fuel. This method also requires a lot of fuel, and the heavy weight of the fuel is a significant challenge. However, in the distant future, if fuel didn't weigh so much (for example, if we could put a fusion reactor on a ship), powered re-entry might become more common.
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Frequently asked questions
The amount of fuel needed depends on the type of shuttle and its mission. NASA's Space Shuttle used two Solid Rocket Boosters (SRB) and an external tank with more than half a million gallons of self-combustible liquid fuel.
The two Solid Rocket Boosters (SRB) used a total of 1,000,000 kg of Ammonium Perchlorate Composite Propellant (APCP), providing 124 seconds of burn time and 12.5 MN of thrust per SRB.
The external tank provides propellant for the three Space Shuttle Main Engines (SSME) on the orbiter. It holds more than 500,000 gallons of liquid oxygen (LOX) as the oxidizer and 106,261 kg of liquid hydrogen (LH2) as fuel.
100% of the SRBs and external tank fuel is used, and about 50% of the OMS fuel carried in the orbiter is used. By the time the shuttle reaches orbit, 90% of its launch mass has been left behind.
The amount of fuel needed is influenced by various factors, including the rocket's mass, efficiency, and external forces. Additionally, the need to transport excess mass in the form of fuel for later stages of the journey contributes to the challenge of determining the required fuel quantity.
