In this section we deal with the lessons associated with fundamental technical
principles. In dealing with these principles, it is not implied that there are not other technical lessons; in fact most of the remaining lessons are of a technical nature. What we are doing here is dealing with four of the very high level technical lessons necessary for successful products. The four lessons for this principle are:
7. Physics of the Problems Reigns Supreme 8. Engineering is a Logical Thought Process 9. Mathematics is the Same!
10. Fundamentals of Launch Vehicle Design Deal with Balancing Efficiencies
Lesson 7. Physics of the Problems Reigns Supreme
The Physics (Mother Nature) of the problem reigns supreme (The God of Design). Either you bow down to Her or you will fall down.
"Mother Nature does not read our paper. If we don't follow her way, she lets us fall"
Understanding the Physics is crucial.
Designing using unrealistic assumptions results in program failure.
Technologies must be fully verified before use. Verification as you fly increases risk and cost.
Independent analysis is a great approach to risk identification leading to subsequent mitigation.
The quality of the technical features determines project success.
Designing a product optimistically--in other words, thinking that you can bypass ―the physics of the problem‖--will lead to failure. Physics will always win. As a result we must develop means of enhancing ―Critical Thinking‖ in order to always fully understand the basic physics of the product. In addition as stated in the corollaries, we need to utilize other
avenues to enhance our understanding. It is standard practice to assume unrealistic or too optimistic requirements. These must be challenged and brought into the realm of reality. The tendency is to baseline new technologies before they have are matured and verified.
Technologies should be understood, matured and verified before incorporation in a product.
Developing technologies in parallel with development and manufacturing is a high risk
approach and is not prudent. Independent analysis and open inquiry are good techniques for assuring understanding of complex space systems. Lessons Learned should be brought to play in all phases of a products lifecycle. Finally ―the quality of the technical features‖
determines the product success.
The following examples will be discussed to illustrate the lesson.
• SRB Aft Skirt Failure
• Skylab Loss Of Thermal Shield / Solar Array
• SSME Turbine Blade Cracking
• SSME Duct Bellows
• SRB Reentry Acoustics
• Heating Impacts
Space Shuttle Aft Skirt Failure
The Structural Test Article structural qualification of the High Performance Motor resulted in a failure of the SRB aft skirt at a safety factor of 1.28 (versus the required 1.4) in the hold-down post region. As a result the SRB Aft Skirt flew for many flights with a waiver before installation of a fix. The problem is a very complex load path situation that occurs due to the vehicle weight and SSME thrust-induced loads bending the vehicle over the outer SRB posts. The SRB skirt angle transfers these longitudinal loads into a lateral and longitudinal load situation causing the skirt skin to bend against the hold down post where it is welded to the hold-down post as shown on Figure 7-1. [Townsend, 1998]
• The SSME thrust bends the vehicle over the 2 SRB Holddown posts, putting a compressive load into the skirt.
• The skirt flare angle creates a horizontal test failure at a safety factor of 1.28 vs. the required 1.4
Post to Skirt Weld Horizontal Load SRB Aft Skirt Failure
Figure 7-1. SRB Aft Skirt Failure at the Holdown Post Weld Area
Figure 7-2 shows the magnitude and distribution of the stress introduced by the load.
The figure below is a linear analysis that indicates the peaking of the stress but not its real amplitude. The actual stress of the weld was highly nonlinear and requires a nonlinear
analysis and special materials characterization to understand the issue. As a result of the test failure of the SRB aft skirt, the Space Shuttle flew the skirt under waiver through flight 86.
Flying with a waiver required constant monitoring of the loads on each flight, special
inspections and special analysis in order to fly the system safely. This required approximately 5 equivalent full time engineers to accomplish. At that juncture in the program a fix was
instituted, gaining back the margins and removing the waiver.
Figure 7-2. SRB Aft Skirt Stress Distribution
X-33 Shortfalls
X-33 was a scaled model technology demonstrator for a single-stage to orbit launch vehicle scaled at 40% of anticipated full size SSTO. The goal of the project was
demonstration of the critical technologies -- mass fraction, metallic TPS, integral shape
composite fuel tanks, aerospike main engines, coupled ascent and reentry aerodynamics and control systems, and simple low cost operations. Demonstration of TPS needed trajectories that reached a Mach number of at least 18. The mass fraction requirement needed to be demonstrated at approximately 0.9 in order to say you could extrapolate to full scale.
Major shortfalls were the following:
1. Weight growth precluded reaching orbit / Weight growth limited Mach number needed to verify TPS and didn't demonstrate acceptable mass fraction.
This shortcoming had to do with large mass growth to solve tank problems, aerodynamic problems, and was limiting the achievable Mach number to 12 or less. This was hampering the ability to verify the TPS. The same mass growth was pushing the mass fraction to around 0.85 making it nearly impossible to extrapolate the results to full scale.
2. Missed coupling of aerodynamics and control (Uncovered in wind tunnel testing).
During wind tunnel testing it was found that the ascent and reentry aerodynamic
characteristics were in conflict requiring a reorientation and resizing of the canted fins. It took 1,000 hours of wind tunnel testing to reach a compromised solution that ended up with some undesirable reentry and landing response and higher loads and performance loss during ascent.
3. Composite integral fuel tank failure
The integral composite fuel tank failed during verification testing. See Lesson18 for details.
This failure ended up with the cancellation of the X-33 program.
Gravity Probe A - Redshift
If a spinning object has internal energy dissipation, it will orient itself so that the spin is about the axis of maximum moment of inertia (e.g., a long cylindrical object will flip into a flat spin). The initial design of the GP-A spacecraft was spun about the axis of minimum moment of inertia, and it had internal fluid (ammonia for the thermal system) that would slosh. The problem was recognized before launch, and the internal components were repositioned to make the moment of inertia maximum about the spin axis. The mission was successful.
Figure 7-3 illustrates the phenomenon where the figure on the left is spinning about its
minimum axis, while the figure on the right is spinning about the maximum axis and is stable.
.
Figure 7-3. Model Illustrating Effects of Spinning About Minimum and Maximum Axes of Moment of Inertia.
Aerodynamic Venting Failures
There have been at least three similar venting incidents in the history of space flight.
Two resulted in the loss of the vehicles and one crippled the payload. Those lost were the Atlas-Able-Pioneer (1959) and the Atlas-Centaur (1966). The one crippled was the Saturn Skylab (1973). The similarity in these incidents was that each had a shroud that came off during the transonic flight regime. In the first two incidences, the understanding of the flow physics was not known. Had it been known, the shrouds would have been designed so that they would have been under crush loads; however that was not the case. They were
unknowingly designed so that during transonic flight the shroud load was a burst load that resulted in failure. These failures could have been prevented had the shrouds been
adequately vented.
In the case of the Saturn Skylab an auxiliary tunnel was not adequately vented. The venting analysis was predicated on the assumption that the tunnel would be completely sealed at the aft end, but the aft end as manufactured was not sealed. The openings in the aft end were a result of lack of ―technical integration.‖ The fact is this critical sealing
requirement had not been communicated between aerodynamics, structural design, and manufacturing personnel, see [Lundin, 1973]. Furthermore, ―system engineering‖ was not adequate. There was no dedicated project engineer and that resulted in lack of effective integration.