Design of the UAMLESC corpus

On the Saturn V / Apollo vehicle, the usual trade study was done to determine potential benefits of using load relief control. Analysis that modeled the vehicle as a rigid body showed that there was sufficient reduction of basic aerodynamic loads to recommend using load relief control. However, when structural flexibility was included in the analysis, it was found that adding load relief control significantly increased the loads on the upper one-third of the vehicle, due to accentuated bending response to gusts. This detrimental effect on the upper third of the vehicle outweighed the beneficial effect of rigid-body load relief on the middle third of the vehicle, resulting in a decision to not use load relief on Saturn V. [Ryan, R.

1986, Geissler, E.D. 1970]. This example also illustrates the importance of employing sufficient fidelity in trade studies so as to avoid an erroneous conclusion.

Saturn used analog filtering technology in the flight control computer. Today‘s digital filtering technology would probably enable more effective compensating for flexibility

components of the accelerometer signal, so as to take advantage of load relief control without the detrimental effects.

Operations versus Vehicle Performance

There is a tradeoff between maximum performance and maximum operational efficiency. If we design to optimize payload performance or other measures of physical performance of a vehicle, it is likely to be highly tuned and sensitive to parameter variations and perturbations. Such vehicles are not robust and require much effort and cost to operate

safely. Historically, we have designed for performance, then dealt with the risks and consequences of a performance-based design through operational procedures. This approach comes at a high cost in operations.

We should balance the vehicle performance to achieve robustness, reducing the risk and therefore the operational complexities.

Solid Rocket Booster Water Impact / Recovery

The decision to recover and reuse parts of the Shuttle Solid Rocket Boosters is an example of a different type of trade and balance activity. The overall decision involved many technical and cost factors, and eventually was based on a probabilistic prediction of the attrition rate. Ascent payload delivery capability was affected by the mass of the recovery system including parachutes. Reusing the SRB‘s entailed development of recovery methodology, on-board systems, recovery vessels and infrastructure, refurbishment and inspection process, etc. [Nevins, 1975]

One aspect of this development was the prediction of what damage can be expected upon water impact. There are several events related to water impact that produce major loads on the SRB, as illustrated on Figures 6-7 and 6-8: (1) the initial splashdown causes large loads on the nozzle and aft skirt, (2) the air cavity created by the splashdown collapses and the water slams the aft part of the case, (3) maximum depth penetration creates

hydrostatic loads, (4) the SRB buoys up vertically, then slaps down horizontally, creating side-loads on the forward part of the case. Any of these events has the potential to damage the hardware.

Figure 6-7. Significant Loading Events for SRB Water Impact

Figure 6-8. Typical Initial Water Impact Dynamic Events

There was a large amount of scale-model drop testing done during the development of the recovery system, along with some full-scale testing. There were trade-offs for parachute sizing and assessment of sea state probabilities, along with economic analyses to assess the cost benefit of recovering and reusing the hardware. The conclusion reached was that

booster recovery and reuse would be cost effective if there were no more than 2% attrition of the hardware due to recovery damage. So there was a probabilistic balance-point, or target, for the recovery system design. The initial desire was to not modify the ascent SRB design for any recovery loading events, but to accept whatever attrition would occur. However, because the actual cavity-collapse loads were consistently higher than predicted, a design modification was made to strengthen the aft segment. While one pair of SRB‘s were lost because of a parachute triggering device malfunction, there have so far been no SRB‘s lost because of the probabilistically-defined variables such as sea states, parachute deployment conditions, etc. A major advantage of recovering the SRB‘s is the ability to inspect the recovered hardware and determine its post-use condition. This has proved important in determining actual margins (such as for thermal protection) and revealing incipient problems.

[McCool, 1991]

Vehicle Reliability Versus Engine Reliability

Another act of balancing has to do with the trade of vehicle reliability versus engine reliability and the number of engines. Here we are trading engine reliability for vehicle reliability particularly in man-rated systems where we want engine out capability for abort. If each engine has the same reliability regardless of its size, the vehicle reliability is highest if the vehicle uses a single engine. But a single engine would imply high thrust, and high thrust engines tend to be more unreliable. Smaller engines have higher individual reliability, but the need for multiple small engines can reduce the overall vehicle reliability. This is illustrated by

on Figure 6-9. Reliability numbers in the figure are for illustration only and do not represent actual systems. As an additional consideration, a vehicle with multiple engines may be designed to allow engine-out capability as a safety measure for crew survivability or mission continuance. Airlines have dealt with this issue extensively and had difficulty for a number of years getting certification for twin engine versus four engine planes. They had to show a gain in engine reliability and the ability to fly on one engine to sell the approach.

Systems Reliability vs. Engine Reliability

Systems Reliability vs. Engine Reliability

Generally, the smaller the number of engines on a vehicle, the less likely an engine failure will be experienced during a mission.

But, lowering the number of engines requires higher thrust engines, which can reduce the individual engine reliability. This entails a tradeoff.

Designing for capability to safely abort (or even complete the mission) after experiencing an engine failure can significantly increase crew safety / mission success.

Example:

Figure 6-9. Systems Reliability versus Number of Engines

X-33 Aerodynamics and Controllability

Balancing the X-33 single stage to orbit launch vehicle was a major challenge. X-33, being a single stage to orbit vehicle, had to balance between the ascent and reentry

aerodynamic characteristics. Ascent controllability and loads had to be balanced with reentry and landing controllability. Solving this balancing act took 1,000 hours of wind tunnel tests in the MSFC tunnel. There was an approximate 9 months hit in the vehicle development

schedule required to solve this design set of trades. [David, 2001]

The above examples have illustrated the multi-dimensional nature of balancing that is required for the design process. There is balancing among all aspects of the system:

subsystems, design functions, disciplines, performance, the –ilities, flight phases, life-cycle costs, and more.

A key message from Lesson 6:

Balancing Required Among All Aspects of System:

- Subsystems