In 2010 the NESC performed an assessment of the destabilizing impact of the former Orion Crew Exploration Vehicle (CEV) service module propellant slosh dynamics on the Ares-I crew launch vehicle flight control system (FCS). Concerns had been raised about the Ares-I boost phase stability and control because the standard frequency domain synthesis methods could not yield an FCS design with sufficient gain and phase stability robustness margins which also met the performance requirements. The assessment prompted a follow-up investigation into how NASA and industry have historically addressed regions of instability or violations of margin requirements. When stability robustness margin requirements cannot be satisfied using frequency-domain methods, alternative methods are then needed to ensure that deficient stability margins do not present a high risk of a flight control issue (e.g., loss of control) during the mission. A large body of experience has been accumulated at NASA regarding successfully flying through temporary periods of linear instability as the flight environment and vehicle dynamics undergo rapid changes. For example, the space shuttle had ascent and entry guidance, navigation, and control (GNC) stability verification issues. The Space Shuttle GNC Team identified four possible techniques for accomplishing entry FCS certification with deficient stability margins:
- Engineering Judgment: Exploit previous experience with a specific situation to declare that no additional analysis is required if a stability margin fails the requirement by only a small amount.
- Evaluation of Uncertainties: Conduct a “sanity check” to re-assess whether the uncertainties input into the analysis are realistic. In certain cases, the effects of correlated variables can be taken into account to reduce the level of uncertainties used in the analysis.
- Checking the Time to Double Amplitude: Determine if the vehicle will fly through the region of concern before the oscillations reach unacceptable amplitudes, in which case a lower margin may be acceptable.
- Use of Time-Domain Simulations: Exploit the high-fidelity non-linear time-domain models to prove that the vehicle exhibits acceptable behavior, even with programmed test inputs or other inputs to excite oscillations. Additionally, the loop gains and/or time lags can be increased in the simulation to evaluate the actual stability margins remaining.
Similar insights and lessons were obtained by the NESC’s slosh assessment GNC team in consultation with industry. The NESC found that historically some launch vehicles have been successfully flown by industry with the known threat of slosh instabilities. The NESC learned that the Atlas-II launch vehicle was safely flown with linearly unstable (as viewed from a purely linear frequency-domain perspective) slosh modes.
The primary lesson learned during this assessment was that an FCS designer should not rely exclusively on frequency-domain approaches to verify/certify stable flight. Designers should use all the tools and techniques at their disposal, including the four previously identified. The use and application of the frequency-domain synthesis and analysis tools must be balanced with time-domain performance simulation tools and possibly other considerations. The same techniques mentioned above apply generally to the analysis and evaluation of any potential instability: propellant slosh modes, flexible structure modes, or aerodynamic instabilities encountered by vehicles flying through rapidly changing aerodynamic regimes.
For more information, please refer to NESC Technical Bulletin No. 14-01 Designing for Flight Through Periods of Instability or contact Neil Dennehy (cornelius.j.dennehy@nasa.gov)