injector Archives - Castle Point Rocketry

The Hunt for the Best Igniter

March 14, 2019June 5, 2020
by Dakota

So, rockets burn fuel. That much makes sense. And that combustion of cold liquid creates a lot of hot, pressurized gas that makes the rocket go upwards. Got it. But how does that begin?

You can’t light a kitchen stove without the internal igniter sparking. In the forest, you can’t start a campfire without flint and steel. (Or lighter fluid and a barbecue lighter.) But inside a rocket engine… things are a little more complicated. Castle Point Rocketry has been upending shelves worth of books (all online, don’t worry) searching for the question on the forefront of our minds: “How do we start our engines?” And we’ve narrowed it down to three major contestants.

“The Cotton Ball”

Soak a cotton ball (or other highly-porous material) in something really flammable.
Stick said drenched cotton ball on the end of a metal stick.
Set rocket over stick, with cotton ball inside combustion chamber.
Light cotton ball on fire.
Release the fuel and LOX.

Proof of concept: It’s been done before.

Potential drawbacks include the sudden onslaught of liquid, though flammable, extinguishing the burning cotton ball.

“The Salt Crystal”

Finish researching oxidizing rock salts. Some salts, when heated, spontaneously burst into flames and release copious amounts of oxygen — which helps fuel more decomposition.
Acquire a small-ish amount of the chosen salt.
Carefully pack the salt into a small container on the end of a large stick.
Gently place the rocket over the stick, with salt container inside the engine.
Warm the container and wait for sparks, then release the fuel and LOX.

A snippet of molten oxidizing salt shooting flames.. (1:55 – 2:10.)

Potential drawbacks include the risk of salt decomposing before the igniter set-up is prepared.

“Engine-ception”

Now imagine, if you will, an engine inside an engine.

Source a suitable solid rocket motor, given its thrust/time curve.
Semi-permanently affix the motor to the bottom face of the injector.
Ignite the solid rocket motor.
Release the fuel and LOX.

Solid rocket motors produce a very well-regulated flame over a set period of time. Additionally, this set-up allows both flames (from the starter and the combustion) to travel in the same direction. By doing so, we can reduce the chance of the starter blowing out!

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Impinging Jet Test #1

February 22, 2019June 5, 2020
by Dakota

In order for our preferred combustion theory to happen correctly, we designed the injector to shoot multiple iterations of two miniature jets at one other. When the jets collide a few millimeters into the body of the engine, the liquid spreads out into a “droplet fan,” making it easier for the fuel and oxidizer to mix. (And ultimately, burn.)

The CAD was designed, the part was printed — all that remained was to verify it worked. So, three members of Castle Point Rocketry’s mechanical engineering team stayed up into the wee hours of Thursday morning to test our injector.

A five-gallon water jug, some flexible tubing, and one high-speed camera later, the team had a pretty good idea that our brain child was a go. The test differed from final implementation in five keys ways, but the fact that it didn’t — well, explode — is a good sign.

(1) The test was run on an 85% scale model printed on campus at Stevens’s PROOF Lab. (2) Our final injector will be solid metal but the test part was made of plastic shells. (3) We ran a lower operating pressure and fluid flow rate than the real part will encounter. (4) The part was not meticulously deburred before testing. Little shards of print plastic could have been partially obscuring the outlet orifices. (5) Water was used as our test fluid but the rocket will burn jet fuel in pure oxygen.

A still from a test of the model’s liquid oxygen manifold.

Over the course of the day (and night), six runs were performed over a range of water pressures. The first two, performed in a lab sink, were a public spectacle for the whole team. Monica, our resident injector design chemical engineer, spent most of that time with her head in the sink watching the impingement for defects.

Our next steps: Print a new model and find a test liquid with properties closer to that of jet fuel.

You can find a nice slow-motion video on our Facebook page (http://www.facebook.com/CPRocketry/).

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Full Circle

February 12, 2019June 5, 2020
by Dakota

One of the most competitive facets of our rocket is the engine assembly. Rather than the hundreds (and sometimes thousands!) of parts used to create a traditional rocket engine, our team has streamlined the process into just two: an injector and a nozzle.

Since they’re so integral — and so specialized — we’ve spent hours upon hours designing, modeling, and simulating these parts. And we’ve come full circle.

Phase One: Solidworks

The CAD starter kit for any Stevens engineering student, Solidworks helped us make our first few rounds of injectors. It was pretty cool. Tom spent a lot of time on it.

Injector, Mk. 1. It was bulky, but it got the job done.

Phase Two: COMSOL

Once happy with the geometry, we needed to simulate fluid flow through the manifold. What better software to use than a COMSOL, taught to grad students and marketed as a multi-physics program with a hefty computational fluid dynamics (CFD) engine. Dakota’s self-taught COMSOL regimen came to a standstill when, no matter how finely we meshed the part, physics kept saying, “Nope.”

COMSOL needed the injector flipped inside out. Crazy, right? This part seemed okay…

… But this one obviously had some problems.

They ended up both not working out. One kept growing gnarly spikes, the other had faces that wouldn’t meet up. In the end, no amount of curve smoothing could fix it.

Phase Three: Ansys Fluent

So, what now? Let’s try another CFD! Stevens also offers Ansys to its students, which comes with the handy Fluent plug-in. Abe and Dakota worked on modeling the part in early January, only realizing after returning to campus why it wouldn’t work: Even when simplifying the models by excluding symmetrical pieces, the parts were MUCH, MUCH too big for our Ansys versions to handle. (If years of math has taught me one thing, it’s that 1.1 million “cells” is larger than 512 thousand “cells.”)

Phase Four: Back to Solidworks

The most recent iteration of the injector. We’ve come quite a long way!

We were lost. What do we do? Our school’s CFD programs weren’t working. We had tried simplifying the parts to no avail. Were we just going to hope our applications of what we read in NASA journals would work? Would our rocket engine be held together by dreams and back-of-the-napkin calculations?

Of course not. In the words of Professor Aziz, who teaches courses in Modeling and Simulation at Stevens, “You started
in Solidworks. Why did you leave?”

This simulation tracks individual particles traveling through the injector manifolds. Red means high speed, blue means low speed. Good news: They don’t stop!

So we’ve come full circle. We’re back where we started. We’ve been running fluid flow simulations fairly smoothly, these last couple of weeks. Sure, we keep iterating and optimizing. But at least it’s all built-in, now.

And boy, are the simulations colorful.