Fuel Slosh: An Unsolved Dilemma

Fuel slosh, the whirling of liquid fuel inside the fuel tanks of rockets and other spacecrafts, is a menace that has baffled scientists and ruined space missions for 50 years. Now, Gangadharan thinks he has a solution.

Rocket science and aerospace engineering have radically improved in the past few decades, and modern space mechanics can make much daunting tasks look so easy one tends to forget they’re talking about high end “rocket science”. With planetary flybys and extra-galactic voyages, it seems we are on a relentless mission to gather answers to all the questions regarding the secret workings of the vast universe, finally.

Approaching the miniature planet, Mercury, at a speed of 7,400 miles/hour, NASA’s Messenger spacecraft is expected to whiz by the planet for the third and the last time on Monday, the 28th of September, 2009, in order to chart the last 5-10% of the unmapped territory of this little planet hugging the sun. What secrets it will reveal, is anybody’s guess. The scientists are excited. According to Sean Solomon of the Carnegie Institute of Washington, who is the principal investigator for the Messenger spacecraft, it’s like Christmas presents under the tree. Any planetary flyby is just such an occasion of surprise and delight, for that matter.

NASA has attempted these flybys around Mercury in order to break the speed of Messenger enough for it to be captured by Mercury in 2011, which will inject the spacecraft into Mercury’s orbit permanently. This insertion would mean that the spacecraft will be permanently circling the planet, sending valuable information back to the Earth. Such orbital slinging and acrobatics are everyday manoeuvres, so to speak, for the aerospace scientists and engineers. In 2004, the Cassini spacecraft slung by Venus (twice), Earth, and Jupiter, in order to make a flyby of Saturn’s rings, which made the said planet capture it in its orbit. Similar antics were carried out by Voyager 2, which swung by Jupiter, Saturn, and Uranus, to muster enough speed in order to bullet out of the solar system. So, does that mean rocket mechanics is a piece of cake? Challenges like fuel slosh, crouching fiendishly and pouncing at all the wrong moments to thwart space missions, prove otherwise.

Fuel slosh is a result of the whirling around of liquid in circles in the fuel tanks at the microgravity of the space. Add resonance to it, and you have a big problem that can impede fuel combustion and change the direction of the spacecraft. Resonance is the collective effect of oscillating fluid that reinforces the strength and the speed of the twirling fluid, therefore accentuating the effects of such movements. In 1969, when NASA was experimenting on gravity, one of its spacecrafts, ATS-5, was misdirected into a point spin due to fuel slosh, compromising the experiment. In 2007, fuel slosh made the engine of the rocket, SpaceX Falcon 1, to shut down 180 miles up in the air before it could launch.

One solution to this menace might be the baffles. These are plates that are put in the fuel tanks to minimize the slosh. However, they increase the weight of the craft, thereby increasing its expenses, plus it doesn’t always stop the slosh from building up. What could be a way of designing slosh-resistant fuel tanks, with no extra weight, and no expensive pre-launch ground tests? Sathya Gangadharan of Embry-Riddle Aeronautical University in Daytona Beach, Fla, suggests two methods.

The first one is a simple model construction of fuel tanks that could warn engineers of potential sloshes.

The second method is the use of state of the art CFD to generate computer models of fuel tanks with sloshes to reveal the mechanics of slosh and its subsequent effects on the spacecraft.

CFD stands for “computational fluid dynamics”. This is the current rage in aerospace engineering, albeit the fact that they are expensive and not fully-automated. Gangadharan is hopeful that advances in this technology can address the problem of fuel slosh, so is Frank Bugg of NASA’s Marshall Space Flight Centre, who is impressed by the accuracy of the CFD results, and its computational ability.

So while Gangadharan with Brandon Marselle, his student, and James Sundermann of NASA suggest their two solutions in the three conferences of American Institute of Aeronautics and Astronautics, Messenger barges toward Mercury at breakneck speed in hopes of capturing a few mug shots.