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Helium in Spacecraft and Rockets
Context:
Two NASA astronauts will stay on the ISS longer due to a faulty propulsion system on Boeing’s Starliner, which had helium leaks.
More on News:
- SpaceX’s Polaris Dawn mission has been delayed due to helium issues with ground equipment, while Boeing’s Starliner spacecraft made an uncrewed landing in a New Mexico desert late Friday.
- Similar helium leak problems have previously impacted ISRO’s Chandrayaan 2 and ESA’s Ariane 5 missions.
Helium, a seemingly humble gas with an atomic number of 2, is the second lightest element after hydrogen and plays a crucial role in the aerospace industry. Although non-toxic, it cannot be breathed alone as it displaces the oxygen necessary for respiration. By using helium, engineers can keep rocket designs lighter and more efficient.
Key Highlights:
- Helium’s primary appeal lies in its chemical inertness. Unlike reactive gases, helium does not combust or interact with other substances, making it a safe choice for sensitive environments.
- Additionally, helium remains gaseous at extremely low temperatures (-268.9°C), which is vital for rockets, where many fuels are stored at cryogenic temperatures.
- Rockets must achieve specific velocities and altitudes to reach and maintain orbit. Heavier rockets require more energy, increasing fuel consumption and necessitating more powerful, and thus more expensive, engines.
Applications in Spacecraft and Rockets:
- Helium serves two primary functions in rockets: pressurising fuel tanks and cooling systems.
- As fuel and oxidisers are burned in the rocket engines, helium fills the resulting empty space in the tanks, maintaining the necessary pressure to ensure uninterrupted fuel flow to the engines.
- Helium is also used in cooling systems to manage the extreme temperatures of rocket components.
Challenges:
- Due to its small atomic size and low molecular weight, helium tends to escape through tiny gaps or seals in tanks and fuel systems.
- Leaks are easily detectable due to the scarcity of helium in Earth’s atmosphere, making it useful for spotting faults in fuel systems.
Industry Response and Alternatives:
- The frequent occurrence of helium leaks underscores the need for better valve designs and more accurate valve-tightening methods.
- Alternative Gases: Some rockets have experimented with argon and nitrogen, but helium remains more prevalent.
- For instance, Europe’s Ariane 6 rocket used a novel pressurisation system converting a portion of its propellants into gas for pressurisation, which failed during a mission despite an otherwise successful launch.
Types of Propellants
- Liquid Propellants: Fuel and oxidizers are stored in separate tanks and combined in a combustion chamber. Allows throttling, stopping, or restarting of the engine.
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- Petroleum Fuels: RP-1, a highly refined kerosene
- Cryogenic Fuels: Liquid hydrogen and liquid oxygen
- Hypergolic Fuels: Hydrazine, monomethyl hydrazine, and unsymmetrical dimethyl hydrazine
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- Solid Propellants: Consists of a casing filled with a mixture of solid compounds. Burns from the centre out, with thrust controlled by the burn rate and shape of the propellant.
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- Homogeneous Propellants: Simple base (e.g., nitrocellulose) and double base (e.g., nitrocellulose and nitroglycerine)
- Composite Propellants: Mixture of oxidizer (e.g., ammonium perchlorate) and fuel (e.g., aluminium) with a polymeric binder (e.g., PBAN or HTPB)
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- Hybrid Propellants: Combines a solid fuel with a liquid oxidizer.
- Other Propellants:
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- Alcohols: Used in early rocketry (e.g., German V-2, USA Redstone), now mostly obsolete.
- Hydrogen Peroxide: Used in Britain’s Black Arrow rocket and as a monopropellant. Less toxic but unstable.
- Nitrous Oxide: Used as an oxidizer in hybrid rockets and amateur rocketry. Decomposes into nitrogen and oxygen.
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