Hydrolox rocketry, due to hydrogen's low density, requires boosters for takeoff. I tried to solve this problem by developing high T/W hydrolox engines. Additionally, I proposed a 3-stage architecture to reduce each stage's propellant ratio requirement. Lately, I proposed a hybrid hypersonic missile architecture. This made me iterate on the hybrid solid booster idea and incorporate it into my hydrolox rocket.
The idea is to have solid propellant inside a combustion chamber, and we inject liquid oxygen onto it to combust. Unlike side-strapped solid boosters, the solid propellant would be contained inside these special engines. Given that we only need high thrust during takeoff and we throttle down the engines as we accelerate, this negates the need for bulky solid boosters. Unlike side boosters, using pure aluminum and oxygen results in a much higher T/W ratio and higher total thrust. In order to solve the liquid clogging of aluminum oxide at the nozzle, I propose to add High-Density Polyethylene (HDPE) to the pure aluminum. The exhausted steam and carbon dioxide from HDPE combustion wash away the heavy liquid particles. HDPE also covers the aluminum from ambient oxygen so that it remains in its unoxidized form before the engine fires. The hydrogen content of the HDPE also lowers the hydrogen requirement of the rocket.
Let me clarify the design. We have the first stage, which is the atmospheric elevator. This takes the two-stage hydrolox rocket to 100 km altitude. I call it Stage Zero. This zero stage has a pure vertical flight trajectory. Because drag gets lower as the rocket ascends, the rocket does not need to have a high aspect ratio. This allows a wider rocket diameter, allowing more engines to be placed on the bottom of the rocket. I propose most of the engines of the Zero stage to be of this Al-hybrid design. Because this stage fights against gravity, it should generate high thrust quickly. Unlike the later stages where specific impulse is important, Stage Zero requires high volumetric thrust. The considerably higher volumetric density of aluminum compared to hydrogen turns the rocket into a hypersonic ballistic missile. In order to increase total thrust from these solid hybrid engines, I propose them to be tall, like 5 meters (depending on the rocket payload capacity). As is classic with all my rocket designs, even the hybrid engine will utilize an aerospike engine. The toroidal channels of the aerospike will be filled with Al+HDPE, like in solid boosters. This layout reduces the dead mass of the solid booster shell and results in an altitude-compensated nozzle.
As with my previous hydrolox rocket, the expansion cycle of liquid oxygen will be used to generate electricity, which will be used to pump the propellant into the engines. Unlike a pure hydrolox rocket, most of the fuel, which is Al+HDPE, will already be in the combustion chamber and will require no pumping. I still use some hydrogen with oxygen to initiate combustion within the hybrid engine. Unlike fuel-rich combustion, it will be oxygen-rich to combust the solid propellant. The higher mass of the combustion will give very high thrust and accelerate the rocket more aggressively than any liquid engine can do, including methalox engines.
With all-aerospike engines and no gimbal, we need differential throttling to control the rocket. This will be done with compact hydrolox engines, so their contribution to total thrust will be very low. As a result, the rocket's Stage Zero will require considerably less liquid hydrogen. Coupled with the very high density of aluminum, the stage's dry mass will be considerably low. The stage will have tall hybrid engines making up the engine bay, along with a couple of small hydrolox control engines. One note to this design: after stage separation and Stage Zero's descent back to the launch site, a hybrid engine will be used to shed the stage's velocity. As I mentioned earlier, the stage will have a considerably high diameter, which will allow it to shed its velocity much higher in the atmosphere and have a considerably lower terminal velocity. With all this given, the stage's mass penalty due to stage recoverability will be considerably low compared to other recoverable rockets. For the final seconds of the landing, the hydrolox engines will be used for a smooth landing.
The kinetic energy delivered by Stage Zero will allow the first stage to make the gravity turn immediately and experience almost no gravity loss. Coupled with its engines' higher efficiency due to operation in a vacuum, the stage will require lighter, compact, and low-thrust engines. The first stage may also have some hybrid engines on board to attain initial speeds rapidly and reduce hydrogen tankage, hence the dry mass of the stage. The advantage of these hybrid engines is that they are simpler and cheaper than hydrolox engines and are lighter once their solid propellant is consumed. So, reducing the hydrogen requirement with these engines is advantageous, especially at the initial stages of each stage's flight.
As you may have guessed, with all these advantages over the first and second stages, their cost will be considerably lower than conventional rockets. This allows them to be expended without worrying about the cost. Even though aluminum is more expensive than liquid methane, it is still less expensive than liquid hydrogen. More importantly, the total cost of the rocket is drastically reduced due to a less expensive hydrolox engine requirement and the much smaller tankage and tooling requirements allowed by the high density of aluminum.
Unlike classical hydrolox rockets with strapped boosters, my proposed hybrid rocket has a considerably low dry mass, costs much less, and exhausts no hazardous gases. It can be classified as a green rocket similar to methalox systems, as its plume emits only non-toxic steam, carbon dioxide, and inert alumina particles, completely eliminating the acid rain and chlorine emissions of traditional solid boosters. Although aluminum production carries an upstream industrial energy footprint, sourcing metal extruded using renewable or hydro-power renders the architecture environmentally clean across both its supply chain and operations.
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