What could be the future of Cryogenic propellent thrusters? Green Fuel and storage technology development, how will that be useful? And other ways of propulsion

Well, propulsion is the main part of a rocket, it can be divided into three categories, escape propulsion, in-space propulsion, and deep space propulsion. 

Now there are different types of propulsion used to produce thrust for different uses. Like chemical thrusters are used for primarily all the launches to escape earth's gravity from the surface and get to an orbit. Basically, for launches, we use chemical thrusters because of how mature that technology is and how much thrust they can provide. Another way of Space propulsion technology is Electric thrust to propel space vehicles, they use ions, or plasma with magnetic fields to push the ions or plasma in one way to produce thrust towards that opposite way, and this type of technology is used for satellites mostly, for aligning orbit, and also use for deep space propulsion as, although they don't provide much thrust, they work for quite a long time for the fuel which is provided to Ion or Plasma thrusters, as in case of Chemical thrusters, they burn their fuel very quickly to produce a lot of thrusts. 

Now Chemical thrusts work by just simply chemical reactions to provide thrust, using a propellant and an oxidizer, from which combustion happens when the engine is fired, the propellant gets oxidized and produces a ton lot of gas which expands which results in thrust as all the expanded gas has to come out of the combustion chamber where the chemical reaction takes place, so the expanding gas comes out of the nozzle which ultimately provides thrust as the gases are coming out of the nozzle with so much force, How? Well, there are many types of thrusters. Firstly the type of fuel they are using, now Chemical-based thrusters can use solid fuel or liquid fuel, in the case of solid propellants the propellant and the oxidizer are mixed with each other in a propellant grain which when reacts provides thrust and another way, using liquid propellants as fuel, the propellant and oxidizer are contained in separate tanks and are pumped into a combustion chamber through pumps where they react, lets talk of Hydrolox (Cryogenic) and Methalox engines for example, in case of Hydrolox engines, Liquid Hydrogen, and liquid oxygen which when heated reacts the gases expand and a lot of thrusts is produced, and Methlox engines also work on the same principle but uses subcooled Methane in place of Liquid Hydrogen with Liquid Oxygen. Another type of chemical-based thruster is Kerolox, which uses Kerosene and Liquid Oxygen.

There are few advantages of using liquid fuel than using solid fuel as the amount of thrust with liquid fuel can be controlled by adjusting how much liquid is pumped.

Other propulsion technologies are using Nuclear propulsion, Tether propulsion, Solar Sail propulsion, etc. 

USING GREEN PROPELLANTS

GPIM

Well using liquid fuels like Kerosene or Methane can be toxic for the environment, so are there any green alternatives, which are just environment friendly but also are very effective at producing thrust. Well Green fuel is a new idea thrown around and may also be a reality in the future like ISRO is looking at using Hydrogen Peroxide as its fuel for its Gangayan Mission, well why Hydrogen Peroxide, well when in contact with a catalyst, Hydrogen Peroxide decomposes into a high-temperature mixture of steam and oxygen, and not just that a "green" fuel, well let's say less toxic fuel, Hydroxyl ammonium Nitrate monopropellant was used in a NASA mission, named GPIM (Green Propellant Infusion Mission) which was launched on SpaceX Falcon Heavy rocket, it was launched on 25th June 2019, it was launched just to see the practical capabilities of using this type of fuel. Well, the fuel is Hydroxyl Ammonium Nitrate fuel/oxidizer blend called AF-M315E, and this offers many advantages for future satellites, longer mission durations, maneuverability, increased payload space. AF-M315E, Hydroxyl Ammonium Nitrate, NH₃OHNO₃, offers 50% higher performance for a given propellant tank, compared to a conventional Hydrazine system. It can be used in both solid and liquid forms and decompose by only using a catalyst.

Green propellants is a broad category of many propellants which are non-toxic, or say less toxic, being used in liquid, solid, hybrid, mono or bi-propellant engines.

These can offer less pollution, toxicity, or better performance and storage ability. The main expectancy of using these types of propellants is them being less toxic that reduces hazards and safety precautions, low environmental impact, reduces production and operational costs, and also reduced complexity which comes with using liquid propellants. 

Well but there already exists much cleaner and cost-efficient methods of propulsion, like plasma propulsion, which just uses the high exhaust velocity of the propellant, well they are very well suited for long-distance  interplanetary space missions and instead of just burning the entire  fuel in a short period of time for thrust like the chemical thrusters, as ion propulsion engine ejects individual atoms at velocities higher than the traditional engines and can work for a long period of time, weeks or longer, provided what propellant is provided, and it eventually propels the spacecraft to much larger velocities than the traditional engines. 

Well but  these perks come with some difficulties, like the thrust is very less compared to chemical thruster and also one more is the erosion of electrodes due to plasma, which is currently being worked on by ZARATHUSTRA to develop better plasma engines, read about it in this article.

WHAT ARE THE THINGS WHICH ARE CRUCIAL LONG TERM CRYOGENIC PROPELLANT STORAGE IN SPACE?

Well if we talk of strong or using cryogenic fuel in space, say Lunar surface at a Moon Base or space transportation, or spaceports or any other place where the appliance of Cryogenic fuel is needed, well for that new technologies would be needed and the hurdles we are facing today would be needed to be fixed.

Passive Thermal Control - The propellant and oxidizer boil off really quickly into their respective gases because they are stored at very cold temperatures and at very high pressure. So Multi-Layer Insulation has influenced the passive thermal control system. And other things also attribute to the propellant boil-off loss, like MLI (Multi-Layer Insulation) attachment to the tank, materials used, tank to vehicle support structure and attachments, tank size, configurations, insulation penetrations, insulation venting provisions for launch and ascent, flight, and surface environment, vehicle orientation, coatings and materials on the surface, a lot of other things which affect the boiling off of propellant.

Active Thermal Control - Active Thermal control combines the passive thermal control technology with active refrigeration with cryocoolers which allow the fuel to be stored for periods of months to years with reduced boil-off losses.

Now the cryocooler developed doesn't eliminate the boil-off loss, 20K LH₂ cryocoolers with a sufficient cooling capacity of 20 Watts which eliminates the boiling off of LH₂, don't exist, so they still need to be developed. 80K Cryocooler for LO₂ and LCH₄ have been developed and also have been on many satellites, but the integration of these cryocoolers is still a technological issue as the cryocoolers to propellent tank integration techniques for large space-based storage systems still needs to be developed, other than that, distributed cooling shields with MLI also needs development. And to get to Mars 20K 20 Watt LH₂ would be required.

Pressure Control - Controlling cryogenic propellant tank pressure in low gravity with minimum boil-losses can be accomplished by TVS (Thermodynamic Vent System, consisting of pumps for circulation and mixing, heat removal valves, vent lines, and such. Low Gravity fluid dynamics and heat transfer for specific TVS designs can be a big hurdle for controlling pressure as the integration of TVS with the fuels.

OTHER ADVANCED WAYS OF PROPULSION

SOLAR SAILS: Well this is just a concept of propulsion, using a big photon reflector, basically a sail, a very thin sail made up of a very light but durable material, which gets propelled by using light, the power source for a solar sail is the Sun, the incoming sunlight propels as it gets reflected by the sails, as momentum is transferred, ok so basically photons have momentum, p = h/ʎ ( h = Planck's constant, ʎ = photon wavelength) although not having mass, so the momentum the photon posses gets transferred to the sail when the light hits the sail and gets reflected, which ultimately causes propulsion when a large number of photons hits the sail, so its needed to be very light, and also very thin, and the propulsion is so low that a very big sail, to even propel some grams, which makes it not the best way of propulsion. So will this method ever able to see the light of the day, or light in space, or sail the light of the day, get it? Anyways, this method might be used in upcoming future. 

TETHER PROPULSION: A tether propulsion consists of a long, thin wire which is connected to an orbiting satellite or vehicle, the movement of a wore through a magnetic field produces electric current the current on the wire produces a magnetic field around the wire which interacts with Earth's magnetic field, so a force is exerted on the tether which can be used to raise or lower the satellites orbit. In theory Tether propulsion can be used in proximity of any planet with a significant magnetosphere, like Jupiter. And researchers at the Marshal Center are also researching on electrodynamics tethers for future missions to Jupiter and its moons.

NUCLEAR PROPULSION: Nuclear energy can be used to heat hydrogen propellant to achieve impulse higher  than the best hydrlox rockets, NTR (Nuclear Thermal Rockets) offers high thrust propulsion, and can be used outside Earth's atmosphere. Interesting fact is that the capabilities of using nuclear energy for rocket propulsion were recognized before the discovery of Nuclear Fission. 

FUSION PROPULSION: Fusion based nuclear propulsion has the potential of fast interplantary transportation, a fusion reaction gives out a enormous amount of energy which can be ejected from th engine to provide thrust, a lot of it. USing this type of propulsion we can reach Mars in just three months. But its just an concept which maybe will not become a possibility, but who knows it might just be possible, first we haven't built a fusion reactor on Earth yet, only then we can think of integrating it with rockets, in the next decades or centuaries.

LASER PROPULSION: Laser propulsion uses a beam-powered propulsion system where the energy source is a remote laser system which can direct a beam towards a Solar Sail which will ultimately powers the solar sail to high velocity.

Well that is it for this one, there are more technological advancements in using cryogenic engines and other ways of propulsion, so many there will be another part of this article later, as we know we have missed on many other topics, so do love to get your, the readers feedback. 

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