Impact Protection

To mitigate the impact risk from orbital debris and meteoroids, it is becoming increasingly commonplace for engineers to consider enhancing the design of an unmanned spacecraft by increasing the shielding potential of its structure. This may, for example, be through the addition of layers of high-strength materials such as Nextel and Kevlar to MLI-covered surfaces, or by modifying the design of honeycomb sandwich panels. QinetiQ has been involved in evaluating a number of novel shielding techniques, including a double honeycomb sandwich panel.

Another protection strategy is to consider the layout of equipment on a spacecraft. This works on the principle that a spacecraft may be able to survive a degree of penetrative damage if it is configured in such a way that mission-critical items are protected from the most vulnerable regions. By judiciously arranging equipment inside a spacecraft, it is possible to enhance its impact survivability without incurring a significant mass penalty. There is, however, a lack of good data to quantify how different types of spacecraft equipment respond to impacts. QinetiQ is participating in impact test programmes to address this issue.

Risk evaluation and survivability enhancement

The number of debris objects larger than 1 mm is estimated to be in the tens of millions. With relative velocities of up to ~15 km/s in low Earth orbit, objects of this size have enough impact energy to severely disrupt a spacecraft's operation or terminate the mission entirely. The SHIELD software model is used to assess the through-life mission survivability of an unmanned spacecraft against impact, and identify the optimum debris protection strategy.

A complete 3D representation of a spacecraft is constructed and 'flown' through the orbital debris environment to obtain an accurate picture of the distribution of particle impacts over the mission life. Ballistic limit equations are then used to determine which particles penetrate the spacecraft structure.

SHIELD calculation of the distribution of penetrations on a spacecraft

By tracing the penetrative impactors inside a spacecraft, the software determines which internal equipment are damaged and how. Finally, the overall consequences for the mission are assessed by employing a technique based on Fault Tree Analysis. The result is a prediction of the debris-induced through-life probability of failure of a spacecraft. To illustrate this, recent studies of a typical Earth observation satellite operating in an 800 km sun-synchronous orbit (a high debris population region) have indicated failure probabilities of the order of 5 - 10%, which is significant.

SHIELD view of equipment inside a spacecraft

By coupling this risk information with data on the costs of a given protection strategy, the software provides an objective measure of the survivability of a satellite. Radically different protection solutions can therefore be compared and the most cost-effective selected. To assist an engineer in focusing on a 'good' solution, the software also contains a genetic algorithm search technique. This uses the principles of biological evolutionary theory to automatically search for the optimum debris protection solution for a satellite. Generally, the best strategy is found through a combination of structural enhancement of a spacecraft and some rearrangement of its equipment, especially critical items.