Space on Earth: transferability of aerospace products to defence applications
Graham Mackrell, Managing Director of Harmonic Drive UK, delves into the challenges of space and looks at transferable characteristics.
The last decade has seen a growing pressure on defence equipment to perform in increasingly challenging environments. Many designers in the sector are now looking to the final frontier to provide the answers and, as a result, equipment originally designed for aerospace is now being used in defence applications.
In the 1950s toastmaster Herbert Prochnow said that, "a visitor from Mars could easily pick out the civilized nations. They have the best implements of war." I doubt he anticipated that those weapons would be informed by design innovations created to enable space travel itself.
Even had he done so, I don’t think he could have imagined the mind-boggling amounts of money involved. It costs, on average, $50,000 to send a kilogram of anything into space. It may seem expensive, but then interplanetary travel is not cheap. NASA's Mars Exploration Rover Mission sent two 185kg rovers, Spirit and Opportunity, to the Red Planet in 2003, at a total mission cost of over $820m. Having proven that Mars' now weathered landscape was once home to water and wind and, after racking up over five years of service, Spirit became stuck in sand.
The subsequent harsh Martian winter took its toll and after continuous failed communication attempts, Spirit was laid to rest in 2009. Opportunity, now in its 10th year of service, has weathered everything from dust storms and mountainous terrain, to craters and extreme temperature fluctuations.
So what has enabled the rovers to maintain their performance in space? The gears on the vehicle provide high precision movement of core areas of the operational and feedback systems on the rovers to overcome the various challenges.
The first challenge is communication. For contact with Earth the rovers used a High Gain Antenna Gimbal (HGAG). This consisted of low gain (LGA), high gain (HGA) and ultra high frequency (UHF) antennas. The LGA is omni-directional, useful for when the rover's orientation is unknown, the UHF is only usable when the orbiting Odyssey spacecraft is directly overhead and so the HGA is used predominantly, set up on a two-axis gimbal.
It necessary that the gearing system provide smooth, zero backlash, repeatable movements with absolute accuracy. Low weight and a small volume are essential but at the same time a high torsional stiffness and torque ratio is needed to cope with the parasitic losses of the gear train whilst articulating on a 40-degree slope.
A harmonic drive, referred to mechanically as a strainwave gear, fulfils this role perfectly. It differs from conventional planetary gear in that the gear teeth form the outside edge of a flex spline, leaving the central area to be bored into a hollow shaft to allow data cables and other services to pass through, whilst allowing continuous rotation. This prevents cable wrap and is essential for both the HGAG and the Pancam Mast Assembly (PMA) a high resolution panoramic camera which can rotate through 360 degrees.
Here on earth, this new found continuous rotation has been favoured for military vehicles and weapon stations on ships. In places where space comes at a premium and minimising volume is the order of the day, being able to not only save space on cabling but also retain a high torque ratio is vital.
When it comes to target acquisition and rangefinders, defence applications rely on the camera and optical components of the system. Traditional gearing has always suffered from backlash and marginal accuracy issues. Even marginal accuracy and backlash characteristics in a targeting system, amplified over a distance of many miles, can mean the difference between success and failure.
The next challenge is terrain. From Earth’s sandy deserts, barren mountains and impact craters, to ancient volcanoes, valleys and polar ice caps, experience of the Martian surface certainly delivers. To navigate this terrain, an equally reliable steering and drive system is mandatory. To overcome this challenge, the rovers implemented wheel and steering drive actuators.
To avoid the vehicle being torque limited however, all four wheels were actuated with each actuator being capable of applying half of the rover's 185kg weight in thrust. As well as this, it is important that the steering actuator is able to maintain position at a 45-degree angle whilst the drive actuator is operating.
The use of these remotely operated vehicles (ROVs) is becoming increasingly popular on Earth too in the form of bomb disposal units. These ROVs are being used in desert environments not too dissimilar from the Martian surface and characteristics such as smooth motion, low weight, and high reduction ratios are equally as important.
It is clear that equipment designed for the harshest use in space, is world-leading when used on Earth in areas such as military and defence applications. There is however a caveat, that of research and cost. A drive to reduce costs and time to market has resulted in increasing pressure on original equipment manufacturers (OEMs) to deliver.
There has been a trend over the last few years with OEMs increasingly ordering commercial off-the-shelf (COTS) products to lower the cost. Whilst this lowers the cost it also means that products are not always specified to exact requirements. This problem has given rise to modified off-the-shelf (MOTS) products. Here, the lower cost of the COTS product can be combined with the customisability of a bespoke product.
The benefit of using products designed for space is that they have already undergone a rigorous process of testing and development. Finite element method (FEM) analysis allows products to be thermally, structurally and dynamically tested in an online simulator environment, many modifications can be made digitally, to save on modelling costs.
So despite the estimated $9,250,000 it cost just to put a single Mars Rover into space, and before we even consider the cost of reaching Mars, it’s worth considering the infinitely greater value of human life. If just one life is saved by an automated bomb disposal unit, the investment will be more than justified. And the first objective of defence is to save lives, something that Toastmaster Prochnow recognised in his sardonic quip.