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The Whissile

Whisky-powered flight

The Whissile comes from a conversation we had during our first year of university, the gist of which was that whisky (being essentially a mixture of 40% ethanol and 60% water) would probably be a viable rocket fuel. In fact, early rockets such as the A4/V2 and the Mercury-Redstone launch vehicle used an ethanol-water blend as fuel, albeit in different proportions. This initial idea was given a name; a portmanteau of ‘whisky’ and ‘missile’.

Thus, the Whissile was born.

This project is supported by the Lord Rootes Memorial Fund, through the University of Warwick.

December 3, 2013

H&S:Health & Safety, or Hubris & Schedule-slip

You may have noticed that we haven’t done much actual rocket work so far – that is, although we have a lot of supporting equipment, the actual rocket engine hasn’t been built yet. Really canny observers will notice that the deadline for submitting a project report to the Lord Rootes Memorial Fund (the source of our project funding so far) was last Friday.

Now that we actually fully understand this project, we’re talking to the Memorial Fund about extending the project deadline by another year, which will be plenty of time to allow us to finish the project. Turns out eight months wasn’t quite enough for a simple reason:

Rocket science is hard.

We started this project knowing relatively very little about designing rockets, so we’ve spent several months learning enough about the theory of rocket engineering to actually create a design which we are reasonably confident will work. (Actually, we’ve changed the design somewhat again since then—more on that once the updated design is done!)

More to the point, we didn’t really know enough about the Health and Safety parts of the project to reassure the Powers That Be that things aren’t likely to go wrong and blow us up. The only Health and Safety specialist on our team currently is the esteemed Safety Haggis:

The Safety Haggis

Oh, no, that type of whisky definitely isn’t safe to use as fuel. The Safety Haggis will make sure it’s… safely disposed of.

The talented Safety Haggis has many skills, but unfortunately none of these include HAZOP studies or failure-mode analysis. (They really should though, so we have organised a training day very soon to rectify this.) So, over the next couple of months, we will be working with both the Safety Haggis and the University’s Health and Safety Department to make sure that we can deal with all of the possible hazards of the project—from electrical shocks to an engine undergoing rapid unplanned disassembly during testing. It’s not the most exciting work, but it is important.

There will also soon be an updated design and the results of a more interesting test of the test stand happening soon. Watch this space.

August 15, 2013

The Prototype Engine

After many tweaks and redesigns, we’ve reached a final design for the scaled-down prototype Whissile engine which we’ll be test-firing in the next few months. It may still undergo some slight changes later, but it’ll probably still look something like this:

A cross-section of the prototype engine

This is a cross-section. If the real thing looks exactly like this then somebody’s probably gotten overenthusiastic with a band saw, and we will not go to space today. Actually we won’t go to space today either way, but that’s not the point.

In our overview of how this engine works, the explanations may get somewhat technical in places—after all, it is rocket science!—so if we’ve been unclear about anything then we’ll be happy to answer any questions in the Comments section.

1) Injector

The injector is the heart of a rocket engine. Its job is to atomise the incoming propellants—that is, it breaks streams of fuel and oxidiser into tiny droplets, much like a perfume sprayer or plant mister. The smaller these droplets are, the faster they can evaporate and burn, and so the more efficient the engine will be.

Different rocket engines often have very different injectors: for example, here are the injectors of the F1 engines on the first stage of the Saturn V, the Lunar Module Descent Engine which landed the first humans on the moon, and a small engine built by Armadillo Aerospace in 2006. For the Whissile, we are using a design called a pintle injector, which is the type used in the LMDE and in SpaceX’s Merlin engines. The fuel is sprayed through tiny holes around the round end of the post, where it meets the oxidiser flowing along the outside of the post and forms a cone-shaped mist of droplets.

Using a pintle injector results in an engine which is much less likely to suffer from combustion instability—a particularly insidious problem often encountered when developing rockets, in which tiny fluctuations in the pressure of the combusting gas rapidly build up into much larger vibrations capable of damaging or destroying the engine, like a wineglass being shattered by an opera singer. The pintle injector prevents this by essentially dividing the combustion chamber into two sections, each of which has different acoustic properties, so any resonance in one section is damped by the other section. Each section also contains fast-moving currents of circulating gas, which “smear out” pressure fluctuations—that is, any disturbance will have a very small effect compared to the speed at which the gas is circulating. (For a more technical discussion of the pintle injector, see TRW Pintle Engine Heritage and Performance Characteristics (pdf) and Liquid Propellant Rocket Combustion Instability, section 7.4.5.)

2a,2b,2c) Propellant and Sensor Ports

A and B are where the fuel and oxidiser (in our case, whisky and nitrous oxide respectively) are pumped into the engine. In the case of the Whissile, this uses a pressure-fed cycle—the propellants are pushed into the engine by higher-pressure nitrogen in the tanks containing them. This is by far the simplest and cheapest option, as it doesn’t require the development of complex and expensive pumps to move propellant around.

C is where a sensor will attach to monitor pressure in the combustion chamber. This will help us measure the efficiency of the engine and look out for any dangerous spikes in pressure. To prevent any of the hot reacting gases from directly contacting the pressure sensor, this hole will be filled with thick grease.

3) Combustion Chamber

The temperature of the reacting gases is likely to reach 2000° C. Although this is relatively cool compared to temperatures in most rocket engines, it’s still much higher than the melting points of most metals: for example, copper melts at around 1100°, and by 1600° steel would also melt. There are a few possible ways of dealing with this problem:

  • Make the engine out of something which can withstand these high temperatures. Unfortunately, this would limit us to either graphite, which is brittle and could crack if there is an unexpected pressure spike, or exotic metals such as tungsten or rhenium, which are very expensive.
  • Ablative cooling: we could coat the inside of the engine with a material which will gradually erode, producing a cooler layer of gas which insulates the combustion chamber walls from the heat.
  • Regenerative cooling: the fuel being pumped into the engine is first sent through small channels around the combustion chamber, carrying away heat. Effectively designing this type of cooling system requires good knowledge of the heat transfer rates in different parts of the engine. This is something which we can model, but different methods yield different results, so it is difficult to know exactly what conditions to expect without carrying out a test-fire.
  • Heatsink ‘cooling’: this approach involves making the engine out of cheaper materials, but not actively cooling it. This would mean that we would only be able to fire the engine for a few seconds at a time before it overheats.

Because of its relative simplicity and the ease of gathering useful data from such a design, we will follow the fourth approach for this first prototype. Simulations can give us some idea about the maximum safe operating time of the engine: the video below is a heat simulation showing the engine firing for 3.2 seconds and then cooling back to room temperature over the course of an hour. Comparing this with actual temperature data will give us an idea of the actual heat transfer rates we would need to design a more advanced cooling system around.

A few seconds is plenty of time to collect data on how well the engine is working. The simple design also means that we can easily swap the combustion chamber for one of a different width or length to compare performance—or, if something goes wrong, replace a combustion chamber which has broken or melted.

4) Nozzle

The nozzle’s job is essentially to convert the slow-moving high-pressure gas in the combustion chamber into fast-moving lower-pressure gas flowing out of the nozzle—for more about this process, see Wikipedia. The nozzle will be made of graphite to withstand the high temperatures here, as the heat transfer rate is highest at the throat of the nozzle.

5) End Bracket

This holds the nozzle in place, and is designed to be the weakest part of the engine. At ignition, rocket engines can sometimes encounter a problem known as a “hard start”, where too much unburned propellant builds up before igniting all at once, causing a brief pressure surge. In severe cases this can damage the engine or create dangerous shrapnel flying in every direction, so to avoid this we have designed the end bracket to detach at a much lower pressure than the point at which the combustion chamber would break. So, if there are problems at startup, the nozzle and end bracket will detach, but the entire engine will not undergo rapid unplanned disassembly, and we’ll know in advance which way any debris will fly.

Read more previous entries on our blog.

Who are we?

The Whissile team consists of four of us, all students at the University of Warwick studying various combinations of maths and physics. Although we're a group, we also each have our own specialities...

Robert Sandford
Lead Designer
James Edmondson
Lab-Monkey
James Christen
Propulsion Designer/Fuel Tester
Adam Lack
PR and Web
Safety Haggis
Chief H&S Officer

Photos