Developing New Cryogenic fuel Tanks with Thermal Imaging Cameras

Dec 14 2012

Introduction
Laboratory Tests
Visualizing the Wicking Process with the FLIR SC7600
Gas-free Engine Restarts with Wicking
Conclusion
About FLIR

Introduction

In space technology, fuel plays an important role but when more fuel is added to the spacecraft, it tends to make the spacecraft heavier and as a result makes it less efficient in its propulsion. This kind of issue is constantly faced by spacecraft designers. On the other hand, present technology has provided a better solution in the form of cryogenic fuel. This fuel has an exceptional energy to mass ratio. However, the lack of gravity in deep space and the unpredictable character of cryogenic fuels make them unsuitable for on-orbit propulsion purposes. Nevertheless, researchers at ZARM, a German research facility, are using a FLIR thermal imaging camera to find a suitable solution to this issue.

Traditionally, cryogenic fuels should be stored at very low temperatures so as to maintain their liquid state. This means large quantities of this fuel can be stored in smaller tanks. Most non-cryogenic fuels tend to cause pollution, but the combination of liquid hydrogen and liquid oxygen is commonly used as it is hygienic and also offers a good energy to mass ratio. But the flip side of using cryogenic fuel is the challenge it presents in spacecraft design.

According to Ronald Mairose, Head of the Electronic Workshop at the Center of Applied Space Technology and Microgravity (ZARM), a division of the Department of Production Engineering at the University of Bremen, one major challenge is to prevent the gaseous form of cryogenic fuel from entering into the engine’s fuel outlet. This scenario can result in cavitations, which can damage the spacecraft components or cause engine failure.

On earth, gas can be prevented from entering the fuel outlet by using gravity. The cryogenic propellant in liquid form has a different density and hence will be kept in the bottom section of the tank, while the gaseous form will move in the reverse direction. When the spacecraft moves away from the earth's gravitational field and is still accelerating, the acceleration force will have a similar effect. However, when the engine is stopped in space, the lack of acceleration and gravity make it difficult to control the liquid-gas separation of cryogenic fuels. In other words, the propellant drifts around the fuel tank in both gaseous and liquid state, thus making it hard to start the engine again.

So far, two techniques have been proposed to overcome this problem. One technique is to utilize an auxiliary propulsion system to produce acceleration. This approach has already been used in the 1969 Apollo lunar landing. In this method, the force of this acceleration makes the cryogenic propellant to be positioned towards the tank outlet, enabling a gas-free restart of the engine. However, this technique restricts the quantity of fuel contained in this auxiliary propulsion system and also increases the weight of the spacecraft.

Thermal imaging helps to determine the wicking front of a cryogenic liquid.

The second technique is to use porous media, which resembles like a woven mesh made of stainless steel, in a Propellant Management Device (PMD). The PMDs are generally employed to store some quantity of the liquid propellant at the tank outlet. This set-up enables liquids to move via the porous media, but prevents gases from passing, thus ensuring a gas-free restart of the engine. A 'wicking' effect keeps the liquid in the porous media. Mairose informed that this effect is a common principle for numerous liquids and compared this phenomenon to the dipping of sugar cube in coffee, wherein the sugar cube sucks the coffee, since the capillary force between the sugar and coffee is greater when compared to the force of gravity. This technique can work ideally provided the porous media is sufficiently wet, because if components are dry then gas can easily pass through and reach the engine.

The porous media will obviously dry out if evaporation occurs and hence this technique is presently being utilized with liquid fuels that evaporate either gradually or not at all. Evaporation is commonly seen in cryogenic liquids and their wicking behavior has still not been identified. So evaporation that occurs as a result of heat transfer or local temperature variation might be more powerful than the wicking effect.

In this test setup the electromotor is used to dip the porous media in the liquid nitrogen. The high precision scale measures the change in weight and the FLIR thermal imaging camera provides information about the speed with which the liquid nitrogen travels up the porous media.

Laboratory Tests

A recent study performed using a FLIR thermal imaging camera has revealed novel insights. The researchers at ZARM employed liquid nitrogen for their experiment as it has similar physical properties like liquid oxygen and also provided a safe option in a lab set-up. Ming Zhang, a research assistant at ZARM, explained that in this experiment, a porous stainless steel mesh or glass frit is immersed in a Dewar flask containing liquid nitrogen. If the liquid nitrogen is able to travel up this porous media, similar to the way coffee travels up a sugar cube, then the wicking behavior of cryogenic liquids can be proved.

A FLIR SC7600 thermal imaging camera has been installed at the ZARM research facility that played a vital role in this study. Zhang informed that initially the team utilized visible light cameras which made it difficult to view the nitrogen movement in the porous media. Also, these cameras showed only the superficial progress of the liquid nitrogen, while the progress within the porous media was not revealed.

This sequence of thermal images shows the wicking test result for the porous glass frit.

This sequence of thermal images shows the wicking test result for the woven mesh made of stainless steel.

Visualizing the Wicking Process with the FLIR SC7600

With the help of the FLIR SC7600 thermal imaging camera, the liquid nitrogen travelling up the porous media was clearly demonstrated in the thermal image. Moreover, this liquid nitrogen travelling within the porous media influenced the surface temperature of the porous media and enabled the thermal imaging camera to precisely reveal how far the liquid nitrogen has travelled up inside the porous media. The FLIR SC7600 thermal imaging camera recorded the thermal data, which revealed that the liquid nitrogen indeed travelled up the porous media. Earlier, it remained a mystery whether cryogenic liquids would exhibit the wicking behavior until these tests were performed.

Gas-free Engine Restarts with Wicking

The data from the thermal imaging camera was merged with the readings obtained from a high precision scale that calculates the increase in weight of the porous sample while the liquid nitrogen travels up the porous media. The weight and distance of the liquid nitrogen travelling up the porous media have demonstrated the actual wicking properties of the cryogenic liquids.

Conclusion

Using this latest data, spacecraft designers can now develop new PMDs for cryogenic fuels. The capillarity force will now make the cryogenic fuels to wick in porous media. This would eliminate dry outs and assure a gas-free restart of the engine. In fact, the engines can be restarted multiple times without using an auxiliary propulsion system, concluded Zhang.

About FLIR

FLIR was founded in 1978, originally providing infrared imaging systems that were installed on vehicles for use in conducting energy audits. Later, we expanded our focus to other applications and markets for our technology, in particular, designing and selling stabilized thermal imaging systems for aircraft used by law enforcement. We have since grown substantially due to increasing demand for infrared products across a growing number of markets combined with the execution of a series of acquisitions. Today we are one of the world leaders in the design, manufacture and marketing of thermal imaging and stabilized camera systems for a wide variety of applications in the commercial, industrial and government markets, internationally as well as domestically.

Our Thermography business primarily consists of the design and manufacture of hand-held thermal imaging systems that can detect and measure minute temperature differences, which are useful for a wide variety of industrial and commercial applications. Uses for our Thermography products include high-end predictive and preventative maintenance, research and development, test and measurement, leak detection and scientific analysis. A growing distribution network has enabled us to penetrate existing and emerging markets and applications worldwide.