During a nominal space flight, that is, a flight without incidents, a spacecraft entering the atmosphere, in its return to Earth, will fly a guided trajectory. During the entry flight, the guidance logic of the spacecraft commands the appropriate attitude in order to meet all pre-mission established criteria on heat rate, heat load, and deceleration loads, and to meet the parachute deploy conditions on altitude and dynamic pressure at a specific geographic location.
In order to accomplish these requirements, the spacecraft must have a fully operational Guidance, Navigation and Control (GN&C) system. The navigation component of this system tells the spacecraft where it is and what its orientation (also called attitude) is at all times. The guidance component tells the spacecraft the attitude it must achieve; and the control component, comprised by the jets in the Reaction Control System (RCS), executes the commands coming from the guidance to achieve the commanded attitude.
A major software or hardware failure in any component of the GN&C system will make it impossible to fly a guided atmospheric entry. In such a case, the emergency entry system is invoked to safely fly what is called a ballistic entry. To achieve a safe ballistic entry, three control phases need to be executed sequentially: First, in case the spacecraft is tumbling, any initial rates must be canceled to prevent the spacecraft from entering the atmosphere in an adverse attitude; second, the spacecraft must be oriented such that the heat shield faces the incoming airflow (i.e., in a heat-shield-forward attitude) to counteract the heat rate buildup; and third, a bank or roll rate must be imparted to null the average lift vector in order to prevent prolonged lift down situations that would result in excessive deceleration loads on the crew.
One scenario in which a nominal guided entry would not be flown is that in which no reliable attitude or angular rate information is available from the navigation system. In that case, a backup navigation system would be invoked and an emergency ballistic entry would be flown subsequently. Flying an emergency ballistic entry with high probabilities of success requires the knowledge of attitude and attitude rate, especially if the spacecraft is tumbling due to a possible additional RCS failure.
The objective of the invention is to provide, with the use of a 3-axis accelerometer, the attitude of a blunt capsule type spacecraft with enough accuracy as to carry out a safe emergency ballistic atmospheric entry. Being able to know the attitude, hence the attitude rate, will allow the control system (be it nominal or backup, automatic or manual) to execute the three control phases required to achieve a safe emergency ballistic entry. The 3-axis accelerometer is an instrument that is present in the GN&C system of all spacecraft. However, it has never been proposed or conceived as an instrument to determine attitude.