Ground control

A highly controllable rocket system with the ability to manoeuvre planetary landers on future space missions has been demonstrated by US engineers.

Project partners Advanced Technology Associates (ATA) and Alliant Techsystems (ATK) have developed a proposed solid-fuel propellant system designed to offer the precision control needed to land NASA’s Mars Science Laboratory.

Minneapolis-based ATK is working with NASA’s Jet Propulsion Laboratory (JPL) to develop propellants for a low-cost lightweight solid-propellant control system that could be used in payload descent and spacecraft cruise phases for planetary exploration missions.

Chip Grandits, a software specialist at ATA, explained why solid fuels have not been used to date. ‘Solid motors tend to be considered dumb motors, like a firework. Intellectual property developed at ATK allows the user to programme actuators and controls to move a nozzle to regulate gas flow, and regulate pressure and thrust to make a controllable solid motor.’

Although the MSL project is well advanced and NASA will not change its hardware at this late stage, ATK chose the mission’s requirements as a good example on which to demonstrate the use of solid fuel on a landing module. NASA’s existing mission plan is to lower the MSL from the orbiter to a few metres off the Martian surface using a sky crane. A tether is deployed with the robot at the end of it, which is then dropped to the surface. The orbiting craft then flies away from the rover vehicle so as not to contaminate the area where it will gather samples.

The ATK MSL design uses four controllable solid-rocket motor modules with variable nozzles to provide thrust for braking during descent. Each module had small thrusters oriented to provide early attitude control and internal gas generator pressure regulation. ATA developed the guidance and navigation scheme used during descent and landing, the software to control the solid engine, a vehicle simulation, and integrated the hardware into the test system. The development of the guidance, navigation and control (GN&C) of the all-solid system for the MSL mission consisted of three phases: software-in-the-loop (SIL), processor-in-the-loop (PIL) and hardware-in-the-loop (HIL).

ATA’s aerospace toolkit manager Chris Scruggs said that the company’s involvement begins at the early prototyping and modelling stage. ‘We began developing separate software models for the vehicle and for the motor and control system,’ he added. ‘They were then brought together in an SIL, where each element works together to fly a simulated mission.’

ATA developed control systems to work out the way to position the horizontal thrusters for maximum agility and save fuel by not being used when just braking. Control algorithms were integrated and tested in ATA’s systems integration laboratory, which incorporates National Instruments’ Labview platform. Labview integrates with the various programming languages and data formats used in the system. National Instruments’ PXI test platform was used to integrate the modules for the SIL test, and to incorporate the controllers and real motors in the PIL and HIL stages.

NASA was able to provide overall mission requirements for the lander and a planetary model for the shape, gravitation field and atmosphere of Mars. As the solid-fuel solution would use a different trajectory, timelines and methodology to get to the surface of Mars, ATA looked for degrees of latitude that would still achieve the objectives.

‘For the MSL model we took out all the plumbing for the liquid system and replaced it with four solid motors,’ said Grandits. ‘We were left with a bottle with three valves, one of which is the main braking thruster where all the gas comes out to slow you down as you come to the surface. There are also two horizontal thrusters that control movement or exactly oppose each other to balance the pressure in the bottle.’