Hypergolic vs non hypergolic ignition in rocket engines

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Hypergolic vs non hypergolic ignition in rocket engines

An early hypergolic-propellant rocket engine, the Walter 109-509A of 1942–45; source: Wiki

Hypergolic ignition uses propellants that ignite on contact, eliminating the need for a separate igniter, whereas non-hypergolic ignition requires an external ignition source like an electric spark or pyrotechnic to start the combustion. Hypergolic systems are simpler, more reliable for multiple restarts, and can use storable, room-temperature propellants. Non-hypergolic systems, often using cryogenic propellants like liquid oxygen, offer higher performance but require more complex ignition hardware. Common pairs of non-hypergolic propellants are LOX/RP-1, LOX/LH2 and LOX/CH4.

Thruster with Igniter vs. Hypergolic Ignition

 
How ignition works?
In the case of hypergolic ignition, the fuel and oxidizer components of a propellant spontaneously ignite upon contact, eliminating the need for an external ignition source. This characteristic simplifies rocket systems by allowing for quick, reliable ignitions and re-ignitions without the need for separate ignition devices. Hypergolic propellants are often used in spacecraft due to their storability and ability to provide high specific impulses. 

The main advantages of hypergolic propellants are that they can be stored as liquids at room temperature and that engines which are powered by them are easy to ignite reliably and repeatedly. Common hypergolic propellants are extremely toxic or corrosive, making them difficult to handle.

Example of such use is in rocket attitude control systems where reliability and simplicity are critical. 

Non-hypergolic propellants require an igniter (torch, spark, pyrophoric fluid, or electrical spark). Ignition systems add complexity but are routine for large main engines. 
They are safer to handle. Propellants are generally less toxic and easier to handle than hypergolic propellants. Non-hypergolic fuels can be more cost effective for certain applications. 

Unfortunately, complexity and additional weight of the ignition system require additional weight.  Also, there is a problem with re-ignition capabilities due to degradation over time of certain igniters, such as catalytic igniters. There is also a physical and chemical ignition delay time between the mixing of propellants and their ignition. 

Example Applications: Used in main rocket engines, often employing cryogenic propellants like liquid methane and liquid oxygen (LOX), as seen in the Starship rocket. 

Typical propellant examples and properties:
  • Hypergolic
    • Fuel examples: Monomethylhydrqazine, Unsymmetrical dimethylhydrazine, Aerozine-50
    • Oxidizer: Nitrogen tetroxide, MON variants
    • Density: about 780 - 900 kg/m³
    • Typical Specific Impulse Isp: 
      • small pressure fed orbital engines about 280-320 s (in vacuum)
      • Monoprop hydrazine thrusters about 200-235 s (in vacuum) 
    • Storage: Storable at room temperature for long periods (years)
    • Key factors: Reliable, immediate ignition, restartable
  • Non-hypergolic
    • Example: LOX/RP1 - modern booster engines
      • Density RP-1 about 810 kg/m³
      • Isp about 260 s (at sea level), 300-311 s (in vacuum)
    • Example: LOX/LH2 - high efficiency upper stages
      • Density LH2 about 70 kg/m³
      • Isp about 430-465 s (in vacuum)
    • Example: LOX/CH4 - modern reusable designs
      • Density Liquid Methane about 422 kg/m³
      • Isp about 330 - 360 s (in vacuum)
    • Storage: Cryogenic types, which need boil off management
    • Key factors: Higher peak performance possible, optimize performance, cost and reusability.
The simplest rocket engine cycle to spin up is a pressure-fed engine. Because the propellant is already stored at high pressure, simply opening valves allow propellants to flow into the combustion chamber at the necessary operating pressure. However this type is not enough to get the rocket to orbit from Earth's surface. 

Before an engine can be ignited the propellants must be mixed in the combustion chamber. This is done through injectors. The easiest engines to ignite are the hypergolic ones because, they simply ignite on contact. 


A diagram of a bi-propellant pressure fed engine; source: Wiki

The Soviet Union used and Russia still uses a pyrotechnic ignition system with wooden sticks in the Soyuz rocket's combustion chambers, which are ignited by pyrotechnic charges on the tips of the sticks. Then when these are ignited, the engine ignites. Of course, the main disadvantage is that it cannot be repeated in space. 

The mentioned birch sticks are clearly visible from the bottom of the rockets of this family; Image source: Yuri Lozga 

Russia Actually Lights Rockets With an Oversized Wooden Match, source: Popular Mechanics






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