National Aeronautics and Space Administration

www.nasa.gov

Instability Cause and Consequence

Analysis and evaluation must be performed of any potential 

source of instability (e.g., propellant slosh, flexible 

structure, or aerodynamics), while flying through periods 

of rapidly changing dynamics. A large body of experience 

has been accumulated regarding successfully flying 

through not only degraded margins, but also relatively

brief periods of linearized model instability. These 

instabilities occur as the flight 

environment and vehicle dynamics 

undergo rapid changes. When 

linearized stability robustness 

margin requirements cannot be 

satisfied, alternative methods 

are then needed to ensure that 

deficient stability margins do not 

present a high risk of losing control 

during the mission.

Best Practices for Flight

Control System Design

FCS designers should consider 

employing non-linear system 

requirements that capture both 

stability and performance aspects. Occasionally, it may 

be necessary to set aside the traditional frequency 

domain gain and phase stability robustness margins in 

favor of another technique. The tried-and-true guideline 

that stability always comes before performance in the 

design process remains the same. However, since real 

flight systems behave in a non-linear manner, “stability” 

should be understood as control of the vehicle never 

being lost while simultaneously achieving attitude control 

performance requirements. 

Consider four complementary recommendations for 

certifying FCS designs with deficient stability margins:

1) Accept some Relaxed or even Negative Stability 

Margins: additional analysis may not be required if a 

stability margin fails the requirement for only a brief time. 

Seek out prior experience with similar configurations and 

conditions.

2) Evaluation of Uncertainties: reassess 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. 

3) 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 relaxed or even negative margin may be acceptable. 

4) Use of Non-Linear Time-Domain Simulations: exploit 

the complete non-linear time-domain models to prove 

that the vehicle exhibits acceptable 

behavior, even with programmed 

test inputs to excite oscillations. 

Additionally, the loop gains and/

or time lags can be adjusted in the 

simulation to evaluate the gain and 

phase stability margins remaining 

from a non-linear perspective.

Historically, some launch vehicles 

have been successfully flown 

with the known threat of slosh 

instabilities. The Atlas-II was 

successfully flown with linearly 

unstable (as viewed from a 

purely linear frequency-domain 

perspective) slosh modes. 

An FCS designer should question the application of linear 

stability requirements and not rely exclusively on the 

frequency domain approaches to verify stable flight. The 

use and application of the frequency-domain synthesis 

and analysis tools must be balanced with the non-linear 

time-domain performance simulation tools and the Time 

to Double Amplitude criteria. 

References

1. NASA/TM-2011-217183, NESC-RP-09-00602 v2.0, 

Independent Review of the Ares-I Control Sensitivity to 

Orion Service Module Tank Slosh Dynamics, Oct. 2011 

2. NASA Document Number: EAM-CEV-09-001, Shuttle 

Ascent and Entry GN&C Stability Verification, Edgar 

Medina (PA-1 Flight Dynamics Team), Jan. 23, 2009

3. NASA/SP–2010–3408, Space Shuttle Entry Digital 

Autopilot, Section 9.0 Lessons Learned, Larry McWhorter 

and Milt Reed, Feb. 2010 

For information contact the NESC at nesc.nasa.gov

Designing for Flight Through Periods of Instability 

The Orion launch abort system successfully flew through 

brief periods of instability. Known instabilities and risks 

were evaluated prior to flight using best practices.

For completeness, it is imperative that Flight Control System (FCS) designers use both complementary 

time and frequency domain techniques to address periods of instability. Use of standard frequency domain

synthesis techniques alone may not always yield an FCS design with sufficient gain and phase stability

robustness margins while simultaneously satisfying performance requirements. 

NASA Engineering and Safety Center Technical Bulletin No. 14-01