Purdue Today

July 16, 2009

Research focuses on fuel injection in rockets

Steven Collicott
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Research guided by Steven Collicott and Steve Heister, professors of aeronautical and astronautical engineering, in the Aerospace Two-Phase Flow Laboratory in Neil Armstrong Hall is focused on how fuel injection responds to large pressure fluctuations in the combustion chamber of rockets.

"One of the things that endangers safe operations in rockets is large pressure fluctuation during combustion," Collicott says. "It's a very difficult phenomenon to model on a computer at realistic length scales and with so much energy transfer going on so quickly in the rocket combustor."

Collicott, Heister and graduate student Benjamin Ahn are looking at one component of the problem by focusing on how the fuel spray reacts to large pressure fluctuations.

"Others have looked at how the spray responds to small pressure changes, but this is one of the first to look at the effects of very large pressure changes on the spray," Collicott says. "We are trying to document how the fuel spray changes. Does it change a small amount, or does it collapse and reform? Our model also uses a transparent nozzle, so that we can acquire data both inside and outside the nozzle."

Changes in the fuel spray can reinforce undesired pressure fluctuations and lead to unstable combustion.

"Stable combustion is important," he says. "This leads to safe rocket designs with minimum weight. Weight is a critical issue in rockets because every pound of structure requires more thrust and more fuel to lift it off of the ground. Unstable combustion would lead to rocket motors that weigh more and produce thrust that varies over a range of values. In the extreme case, this range in operations could be from extinguished combustion to explosion."

The current portion of the experiment will wrap up in June. Collicott and Heister expect to extend the work to include more extreme conditions, or different length or time scales.

Another endeavor for Collicott includes the design and construction of a 72-foot-tall drop tower in Neil Armstrong Hall. The tower, expected to be complete in a year, will produce 2 seconds of weightless test time for experiments many times each day.

Collicott will use the tower for his own work in two-phase fluid dynamics, which involves liquids and gases together.

"If a container is all liquid or filled with all gas, then it's pretty boring and not that big of a problem," he says. "When it's a mixture of the two in the absence of gravity, then things become interesting."

Collicott will study wicking, which becomes a dominant force on liquid in space flight.

"In controlling half a ton of liquid rocket propellant in a 3-meter-long fuel tank in orbit, wicking becomes important on a large length scale," Collicott says.

Collicott has received numerous inquiries about future use of the drop tower from other universities, government agencies and international researchers.

"It's a facility that, once it's built, will be inexpensive to operate and enables this really unique class of experiments that professors at most other universities can't do," Collicott says. "I'm looking forward to seeing the impact it can have inside Purdue as well as how it attracts significant outside researchers to come in. It will bring more positive attention to the University."

Steven Collicott, professor of aeronautical and astronautical engineering, is working on the design and construction of a drop tower that will produce 2 seconds of weightless test time for experiments. (Photo by Andrew Hancock)