Cryogenic fuelled liquid rockets use liquid hydrogen (LH2) as fuel and liquid oxygen (LOX) as oxidizer as we saw earlier. These liquid gases are stored on board at below minus 253 & 183 degree centigrade temperature respectively.
Cryogenic fuelled liquid rockets use liquid hydrogen (LH2) as fuel and liquid oxygen (LOX) as oxidizer as we saw earlier. These liquid gases are stored on board at below minus 253 & 183 degree centigrade temperature respectively.
Semi cryogenic liquid fuelled rockets use earth storable liquid fuel like kerosene and oxidizer LOX. Having understood the basic fuelling system in a rocket it is time now to understand the anatomy of a cryogenic rocket.
Now one can ask a question as to why the hydrogen and oxygen have to be liquefied increasing the complexity. The answer to this question is that there is a limitationof space and size on board. If these gases are stored in their gas form, the size and weight of the rocket will go unimaginably up and the rocket will not lift. By liquefying the gases, storage space on board can be drastically reduced to manageable level.
The cryogenic fuels are stored on board a rocket, in separate tanks. These liquid fuels are pumped out from storage tanks and injected into the combustion chamber in the gaseous form.
The gaseous mixture in combustion chamber is ignited by a pyrotechnic igniter. This process generates high pressure combustion gases which are expelled out through the nozzle connected to the chamber, giving the required thrust to move the rocket body up.
Now the first technological challenge in using cryogenic propulsion system is to produce and harness liquid hydrogen and oxygen in tanker type storage, transport these liquids to the launch site, interim storage at launch site and fill these propellants into the rocket at the time of launch count down.
The most difficult task is to handle hydrogen since it burns with even the slightest trigger of inadvertent ignition. The flame is invisible hence a great safety hazard. The gravity of the hazard can be visualized in the fact that a man will be burning even if no flame is seen.
The tanks are vacuum and thermally insulated to minimize the heat soak from environment. The temperature gradient between inside and outside of the tank walls are of the order of 280 degree centigrade. Many materials become brittle at LH2 temperature.
Welded and sealed joints may crack & leak due to the thermal gradient. Hence selection and development of materials, fabrication technology, and safety systems are some of the important technological challenges.
These technologies are to be developed in house as no country will part with this technology. The complexity in LOX systems is comparatively of lower order; nevertheless similar challenges do exist for LOX systems too. This is about the ground systems.
Onboard cryogenic propulsion system for a rocket can be broadly divided into two major sub-systems ie engine and stage. Both integrated together, along with other rocket systems, make the cryogenic stage.
The important engine subsystems consists of a combustion chamber including the injection & ignition system, a dynamically cooled nozzle, pumps to pump the LOX and LH2 and a turbine to drive the pumps, gas generator to drive the turbine, and electronic real time mixture ratio control system.
The stage system includes basically the onboard storage tanks for LOX andLH2, pressurization & feed systems for these, support structures, plumbing, fluid regulator and valve/safety system, thermal insulation system etc.
Here the mathematical modeling of dynamic fluid & thermal systems & software are some of the important technologically challenging tasks involved. Other technological challenges are the materials and fabrication technology which could sustain the high temperature gradients, dynamic seals and the joints.
India is developing its own cryogenic stage to propel its launch vehicles in the GSLV category and is not very far to achieve the goal. Currently Russian engine is used in Indian launch vehicles.
The cryogenic rocket engine is a classic example of systems engineering, enveloping mechanical, electrical, electronic, chemical, metallurgical, computer and structural engineering. Each of the engineering needs a very high order of specialisation.
Of course system engineering is on the top of all these engineering which integrates all of them along with the project management skills. (Concluded)
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