Book contents
- Frontmatter
- Contents
- Preface
- Acknowledgements
- List of acronyms and abbreviations
- PART I Engineering issues specific to entry probes, landers or penetrators
- 1 Mission goals and system engineering
- 2 Accommodation, launch, cruise and arrival from orbit or interplanetary trajectory
- 3 Entering atmospheres
- 4 Descent through an atmosphere
- 5 Descent to an airless body
- 6 Planetary balloons, aircraft, submarines and cryobots
- 7 Arrival at a surface
- 8 Thermal control of landers and entry probes
- 9 Power systems
- 10 Communication and tracking of entry probes
- 11 Radiation environment
- 12 Surface activities: arms, drills, moles and mobility
- 13 Structures
- 14 Contamination of spacecraft and planets
- PART II Previous atmosphere/surface vehicles and their payloads
- PART III Case studies
- Appendix Some key parameters for bodies in the Solar System
- Bibliography
- References
- Index
5 - Descent to an airless body
Published online by Cambridge University Press: 12 August 2009
- Frontmatter
- Contents
- Preface
- Acknowledgements
- List of acronyms and abbreviations
- PART I Engineering issues specific to entry probes, landers or penetrators
- 1 Mission goals and system engineering
- 2 Accommodation, launch, cruise and arrival from orbit or interplanetary trajectory
- 3 Entering atmospheres
- 4 Descent through an atmosphere
- 5 Descent to an airless body
- 6 Planetary balloons, aircraft, submarines and cryobots
- 7 Arrival at a surface
- 8 Thermal control of landers and entry probes
- 9 Power systems
- 10 Communication and tracking of entry probes
- 11 Radiation environment
- 12 Surface activities: arms, drills, moles and mobility
- 13 Structures
- 14 Contamination of spacecraft and planets
- PART II Previous atmosphere/surface vehicles and their payloads
- PART III Case studies
- Appendix Some key parameters for bodies in the Solar System
- Bibliography
- References
- Index
Summary
There are two fundamental arrival strategies – from a closed orbit (circular or otherwise) around the target body, and from a hyperbolic or near-linear trajectory directly to the surface.
Landing places some significant requirements on the thrust capability of the landing propulsion. Obviously the thrust-to-weight ratio (in that gravity field) must exceed unity if the vehicle is to be slowed down. The ΔV requirements will depend significantly on the trajectory and thrust level chosen, and can in the case of a hover, be infinite; a lower bound is given by the impulsive approximation analogous to the Hohmann transfer between coplanar orbits – first an impulse is provided to put the vehicle on a trajectory that intersects the surface, on the opposite side in the case of a descent from orbit. A second impulse can then be applied to null the velocity at the impact site.
In practice the trajectory of the vehicle, the performance of the propulsion system and the topography of the target body are inadequately known for such a strategy to be performed open-loop, except in the case of landing on very small bodies where the orbital and impact velocities are low enough that the second, arrival ΔV can be safely provided by impact forces rather than propulsively. Thus some sort of closed-loop control is needed.
Compensation for varying propulsive performance (both due to engine performance variations, especially if feed pressure may vary in blowdown mode, and due to the progressively reducing mass of the vehicle) can be achieved by monitoring the spacecraft acceleration with onboard accelerometers.
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- Planetary Landers and Entry Probes , pp. 47 - 55Publisher: Cambridge University PressPrint publication year: 2007