Rocketry on thomaspaulin.me

Rocket Engine or Rocket Motor? Which is correct?

Tue, 20 Jul 2021 00:00:00 +0000

Historically the word “motor” was used when referring to solid fueled rockets, and “engines” for liquid fueled. Unfortunately, this rule breaks down for hybrid rockets where the fuel can be solid, or the oxidiser can be solid. That being said, most refer to hybrid rockets as motors.

These days the two words are used interchangeably. For example, the Wikipedia page for ullage motors uses a combination of “motor” and “engine”.

Which should you use? In lieu of a more specific name the following guideline would serve you well:

If there’s a solid component, use “motor”, otherwise use “engine”, as “rocket engine” is the all-encompassing term.

As for the words of “motor” vs “engine” themselves, MIT has an interesting page on the etymology of both words.

The Role Of Ullage Rockets in Restarting a Primary Rocket Engine

Tue, 20 Jul 2021 00:00:00 +0000

This is a Saturn V. You probably recognise it, and its engines, but do you recognise these?

The Saturn V’s S-IVB stage with its ullage rockets highlighted

They are ullage rockets¹. While unknown to many, they are crucial to the success of a mission.

The term “ullage” is one that comes from wine making where it refers to the empty space of a container.

The ullage of a beer barrel

This applies to rockets too because they are containers filled with liquid (propellants) and in their case, the ullage space is the void in the forward end of the propellant tanks.

The propellant tanks of a Saturn V booster’s stages. Notice they are filled with liquid and have ullage

Ullage is important because it provides room for the thermal expansion of gases. It also forms a space for the dissolved gases to accumulate.

These dissolved gases include “ullage bubbles” which are bubbles of gas which form in the propellant, in the absence of gravity. In the best case, they form small bubbles in the plumbing which cause sputtering much like a car when a combustion engine has air bubbles in its fuel line. This will degrade the rocket’s performance, but it can be corrected for with mid course correction manoeuvres. It’s also possible they block the propellant outlets entirely meaning the engine(s) cannot operate. But then the worst of all these scenarios: they could lead to “combustion instability” resulting in “catastrophic self-disassembly” (A.K.A. “rapid unplanned disassembly”) of the vehicle itself.

As you can tell, these ullage bubbles are not desirable but having room to create ullage is not sufficient by itself. We also need some form of (forward) acceleration to accumulate the dissolved gases and remove the bubbles. Any form of forward acceleration will do and this is exactly what the ullage rockets provide.

By accelerating the vehicle forward, the denser liquids settle to the aft of the tank while the lighter gases accumulate in the ullage. Thus, mitigating issues of the bubbles blocking the propellant outlets, or accumulating in the plumbing. Allowing for smooth sailing.

The ullage before and after forward acceleration is applied to the vessel

Next time you see a multi-staged rocket keep an eye out for these unsung heroes. Who knows? You may just spot one.

Ullage motors are specifically solid fueled. The Saturn V uses a mixture of solid and liquid fueled ullage rockets ↩︎

Oxygen Need Not Be Present

Fri, 25 Jun 2021 00:00:00 +0000

Why Rockets Don’t Need Oxygen

As many articles and videos throughout the internet explain, a rocket moves in accordance with Newton’s third law of motion by ejecting matter, in one direction so that the reaction force produces motion in the opposite.

The object of a rocket motor is to provide thrust, it does this by ejecting a stream of (usually hot) gas at high velocity. Liquid bipropellant engines combine a fuel with an oxidiser in either a solid or liquid form, then combust the mixture to produce gases. This is exhausted and voila! You have your stream of high velocity gas.

Some sources explain this using the analogy of an air-breathing jet engine. They describe the ability of rockets to operate in the absence of air by stating that unlike jet engines, rockets “must take the oxygen with them”. While this is accurate of operational rockets (at the time of writing) this isn’t technically correct and is disproven by chlorine trifluoride. In 1948, Bert Abramson of Bell Aircraft fired chlorine trifluoride with a hydrazine fuel. In following years, Rocketdyne produced two engines which similarly used a chlorine trifluoride oxidiser. To understand how these engines were possible, and how we can fire a rocket engine without oxygen we must understand how rockets propellants produce energy.

Propellants store their energy in the bonds between their atoms. To release this energy, we require a reaction which breaks bonds, but whose products release more energy than those bonds took to break. In the case of (liquid) hydrogen and (liquid) oxygen, the energy released when forming water is greater than that required to break the hydrogen-hydrogen bonds and the oxygen-oxygen bonds, giving us a viable bipropellant.

Because of our meddling in bond breaking, we are dealing with electrons and their movements. Thus, we enter the realm of redox reactions.

One of the major players within redox chemistry is the oxidising agent, or oxidiser. This is where our rocket oxidiser gets its name. Oxidisers are any chemical species that lose their electron(s) in a reaction. No limitations regarding oxygen are made. Two examples of oxidisers are oxygen, and the chlorine trifluoride we mentioned earlier.

If any molecule which loses its electron(s) can be our oxidiser, what properties make for a good oxidiser? In reality, the decision is a series of compromises where we consider a host of factors depending on the intended use. These factors include: energy released by the reaction, reaction time, freezing point, safety, production, cost, and more. In theory, we seek an oxidising agent that releases as much energy as possible. So in the realm of idealism and hypotheticals, oxygen need not be present.

In reality, rocket chemists must make a series of compromises, and liquid oxygen is consistently above average, leading to its preferential use over other oxidisers, like chlorine trifluoride.

To learn more about rocket propellant chemistry, I recommend reading Ignition! By John D. Clark.