Replacing the Current Shaft Seal
Purpose of Article III
In this article, the replacement for purge seals is identified and presented. As it turns out, we don’t need to keep the reactants separate. We can let the fuel and oxidizer interact, but disable their reactivity. This would lead us to an alternative which is simpler, and therefore cheaper than the status quo.
As identified in Article II, we need an alternative to Inert Gas Seals to drag down the part number of Turbo-pumps.
In order to propose a fitting alternative to these seals, we must take a look at their fundamental duties, and assumptions while being produced.
- Seals separate the reactive gasses on the two opposite chambers of a Turbo-pump. Why?
- Because if the two gasses mix together, then a reaction will occur, leading to an explosion in the Turbo-pump, breaking the engine. Why?
- Because the molecular structures of the reactants (fuel & oxidizer) ensure reactions when molecules collide. Why?
- Because substances react when they collide with a certain amount of kinetic energy, in the right orientation.
Based on the the thought process above, it is understood that if we deprive the molecules of kinetic energy, then the substances will not react when in contact, proving the seal useless. If we can pull down the kinetic energy of these reacting particles below a certain point (in a simple way) then we can substitute the Inert Gas Seal (with ~200 parts) with the new proposed solution to lessen a Turbo-pump costs.
The Chemistry Behind Reducing Kinetic Energy
Temperature increases and decreases dependent on the amount of kinetic energy a substance’s particles contain. So if particles have a high kinetic energy, then the temperatures will also be high. Low kinetic energy means lower temperatures. So reducing a particle’s kinetic energy is equivalent to making the environment colder. If we make a substance cold enough, it will not be reactive.
But by how much? Just because something is “cold” doesn’t mean that it is non-reactive. It has to be beneath a specific level. This level depends on the substances that have to react. Specific to the SpaceX Raptor Engine, the reacting substances would be Methane (CH4) and Oxygen (O2). The ignition temperature of Methane is 600 degrees celsius, and the burning (combustion) temperature of Methane is 1950 degrees celsius. Based off of these two numbers, it means that if we keep the parts of the Turbo-pump that can potentially have reactions below 600 degrees celsius, then we will prevent reactions of any sort.
Low Kinetic Energy In Practice
To cool the substances within the chambers of the Turbo-pump as effectively as possible, we need to have a high surface area between the reactants and the cooling liquid oxygen. Here, it is possible to take inspiration from the regenerative cooling systems used in the rocket nozzle.
Liquid oxygen barely above absolute zero is exposed to the bell nozzle of the rocket engine via small pipes on its surface called capillaries. The high surface areas of the capillaries ensure that the heat is absorbed through the liquid oxygen flowing through the pipes. This cools the bell nozzle significantly enough for it to not melt.
Applying this to Turbo-pumps, we would pass liquid oxygen (-183 degrees celsius) through high-surface area capillary pipes near the point of interaction between the fuel and the oxidizer to prevent reactions of any sort at that point. This would work since -183 degrees celsius is much lower than the required activation energy equivalent to 600 degrees celsius, disabling the reactants from reacting at that singular point which we choose.
Constant Oxygen Flow
For there to be a constant exchange of temperature between the reactants and the liquid oxygen (LOX), we need a certain amount of flow in the capillaries in the Turbo-pumps. As -183 degrees cold LOX absorbs heat from the reactants, it will warm up. We need the flow to take that now warmed-up oxygen and replace it with fresh, minimally warm oxygen. For this to be done, we would need a single valve ensuring one-way flow from the oxygen tank.
The Big Picture
All of the components above working together would result in a functioning system as follows:
- Oxygen flows through the main pipe (M)
- A singular valve ensures that the oxygen only flows through (V)
- The main pipe splits up into many smaller capillary pipes to increase surface area, and therefore heat exchange (C)
- Capillary pipes decrease kinetic energy in points of reaction (R)
- Warmed-up oxygen leaves the turbo-pump (E)
- The warm oxygen can be piped into the reaction chamber, or used for other purposes. The effects of the usage of the output shouldn’t be very significant since compared to the amount of oxygen used as propellant, the cooling oxygen is of very small quantity. (E)
We can prevent the reaction of the reactants inside of the turbo-pump by suppressing the energy and not letting the materials get to their required activation energy. This idea of preventing reaction rather than preventing interaction can save a bunch of parts and complexity for the engine, making it cheaper.