ESA Document - Emits - ESA

s
HMM
Assessment Study
Report: CDF-20(A)
February 2004
page 82 of 422
and further apoapsis lowering manoeuvres at the next pericentre pass.
Alternatively, it might be an option to force a low-velocity escape. Failure before
reaching a bound orbit: The manoeuvre fails while the spacecraft is still in a
hyperbolic orbit with respect to Mars. In the case regarded in section 2.7.12.1,
only the case of a complete failure was regarded. A partial failure would result in
a trajectory closer to the orbit of Mars.
• Incomplete execution of TEI: Here the case distinction made above also applies.
2.7.12.3 Conclusions
An abort cannot be always guaranteed with no further consequences:
• During the MOI and TEI manoeuvres abort is not possible
• During the first part of the transfer to Mars, abort is always possible without mission
mass increase
• During the second part of the transfer to Mars, abort is always possible but mission mass
increase is needed (either propulsion system or ERC)
• From low Mars orbit, there are two possibilities: return via Venus swing-by but mass
increase is needed or waiting for next return window
2.7.13 Aerobraking
Aerobraking is a proven technique to remove energy from an orbit, e.g., when transferring from
a highly eccentric orbit to one of low eccentricity, with minimal propellant consumption.
Aerobraking involves lowering the pericentre of the initial orbit so that it grazes the upper
atmosphere. At every perigee pass, the spacecraft loses some orbital energy to atmospheric
friction. This lowers the apocentre radius. After a number of passes, during which the pericentre
altitude must be observed and repeatedly corrected so that it does not descend too deeply into the
atmosphere, the apocentre will have reached the required altitude. Then, a manoeuvre at the
apocentre raises the pericentre and the aerobraking phase is terminated.
The use of aerobraking rather than propulsive manoeuvres for final orbit acquisition can lead
theoretically to significant savings in propellant mass. (see Mission architecture)
For this reason, a preliminary estimation was performed in this study.
2.7.13.1 Requirements and design drivers
Aerobraking is a lengthy process but it is relatively safe. The structural and thermal loads
imposed on spacecraft components are low compared to other techniques involving atmospheric
flight such as aerocapture and entry/landing. However, with the present spacecraft there were
design concerns for some of the subsystems, in particular the solar arrays. If left deployed during
aerobraking, they would provide the large surface area required to maximize the deceleration and
minimise the manoeuvre duration but they would also be particularly vulnerable to the increased
structural and thermal loads. Therefore it was necessary to perform a trade-off between the
manoeuvre duration and the solar array restrictions. The constraints are summarized in Table
2-29: