The choice of rocket motor propellants for terrestrial rockets
Rocketry is definitely in vogue these days. There are many rocket companies developing small rockets that aim to reach space one day.
Rocketry, one may say, has transformed completely since the early days. An orbital class booster used to be single use only and spent very little of its time in altitudes where breathing air was an option. So obviously they used liquid propellant for density and performance considerations and carried liquid oxidizer for the same considerations. Although they have to carry oxidizer on board that remains a necessity for higher altitudes and overall performance.
This is not true the same way for today’s rockets. Booster reuse is now a must since there’s no physical reason not to have it and since it’s been proven that it can be done, now it’s a must. But that redraws the entire picture. At most altitudes for takeoff and landing air or oxigen is very much available on the outside and can be used as an oxidizer.
That, however, still carries the problem of performance. A liquid propellant, liquid oxidizer engine will always be orders of magnitudes better performer than air breathing ones due to the well optimized nature of mixing these in liquid form and burning it. Chemistry at it’s best, one might say. Burn efficiency will be for all purposes 100%. Directing all that chemical energy to produce all the thrust in exactly the same direction as the rocket travels is of course still a huge challenge and to maximize exit velocity of those gases is still in the realm of serious rocket engineering. This is no amateur project although the physical or checmical nature of the problem is actually quite simple. This is serious rocket science or more accurately rocket engineering. All that energy getting released in a very confined space is seriously dangerous and requires proper setup and a testing range.
So how do you translate this same problem for the terrestrial rocket realm. Not needing to carry oxidizer is a great weight saving. Propellants like RP-1 is dense, energy rich and does not really need cooling due to the marginal gain it would provide.
So that’s all fine but choosing a high performer air breathing turbo engine is still a challenge. With rocketry you want to maximize performance and minimize weight. High energy combustion with high combustion pressures requires relatively strong chamber walls to safely contain it that in turn increases weight. But there’s a similar problem for airplanes that carry air breathing turbo engines and weight is still a very important consideration.
So why not use Turbojet engines like airplanes do? A rocket is very lean in its form factor. T/W is a challenge when you carry both propellant and oxidizer on board. When you can get rid of the oxidizer requirement this challenge becomes a lot easier. A modern AeroJet engine for model use weighs around 1 kg. The propellant you carry weighs also around 1 kg. The engine has a thrust of around 6 kgs. So for a T/W of 2 you still have plenty of weight to make use of. The weight of the rocket body itself will not be much since light materials are used. The same is true for the flight control electronics. The choice of attitude control brings in another consideration. Engine thrust vectoring is probably not a good choice here since gimbaling an engine that’s almost the same width as the rockeet body you have little room left for gimbaling range.
The most efficient method for attitude control can mimick that of larger rockets. RCS thrusters work with compressed gas. The weight of the high pressure tank in such a small form will not add much to the overall weight. Compressed gas also carries lot of energy and all you need to do is to actuate small valves to steer the rocket in the desired direction. Another advantage is that the placement of the thrusters can be optimized to be as far of the rocket’s center of mass as possible. This will provide maximum torque for steering.
So overall with this back of a napkin analysis or not even that, this engine, propellant and attitude control choice seems to be optimal for terrestrial rockets that spend most of their time near the Earth’s surface. In this sense they are more airplanes than not.
Consideration of extreme weather is a huge advantage. A rocket body exposes the least amount of surface area in the direction of travel. Very high winds and precipitation will have the least effect on a rocket body travelling in that direction. A drone or helicopter will never have the same amount of control authority in those conditions since they rely much more on the air itself to gain that authority. When conditions change for the worse and become chaotic. it becomes a disadvantage. Rely on your surroundings too much and you’ll be in trouble. That’s why a rocket will always be the most optimal form of surviving adverse conditions.
This is our starting point for our research and experimenting with small sized rockets to provied extreme weather tolerant shipping for terrestrial uses.