• Space Station Node Gusset
Saturn V Sloshing Computer Program
During Saturn V development, propellant sloshing was a major concern. Tools for analyzing the interaction of the vehicle, the vehicle control system and propellant sloshing were in a state of development. While laboriously checking out the program to compute sloshing dynamics using computer priority, we kept making program changes due to errors found, which required new punched cards and card assembly. The computer operator finally asked Robert Ryan, ―If you know the answer why are we running the program?‖ He replied,
―If we didn‘t know the answer we should not be running the program.‖ In other words we need to know the physics of the problem so that we can continuously check the computer program.
Computer programs only give you what you ask them to. This check is made using simplified equations and back of the envelope calculations in terms of the basic physics of the problem.
Helmut Horn, our early German supervisor, would make you go to the blackboard and show a simple model of the more complex model being discussed. He said that if you couldn‘t
explain it in simple terms you did not understand it. He wanted you to use that same model to check out the computer results or even someone else‘s work or presentation.
ISS Node Load Paths and Common Berthing Mechanism Fretting
The ISS Node (Figure 8-1) had two major problems during development. The first had to do with the berthing ports gusset yielding during proof testing. The second was the galling of the common berthing mechanism during berthing simulations.
Figure 8-1. ISS Node Showing Berthing Ports and Gussets
The gusset yielding was found in the proof pressure testing and was a classic load paths problem created by trying to save weight and not understanding the primary load paths.
The hoop stress was all dumped into the two gussets installed on the radial sides of the berthing port. Understanding load paths is critical to the design of structures. Pugh has
written articles and has sections in his books on designing for load paths. [Pugh, 1991] Load paths are one of the basic physics of the design that must be adequately understood and designed for.
The berthing mechanism was tested extensively on the 5 degree of freedom motion simulator at MSFC‘s computational laboratory. During test the mechanism would gall due to the unpredictability of the contact angle. This was a result of the fact that the berthing was accomplished using the remote manipulator system (RMS) which had no unique position to bring in the second body because of the multiple joint angle possibilities. This contact angle depended on how the RMS captured the second body and its position relative to the station.
The problem was solved once the total motion possibilities were clearly understood (initial conditions) and it has performed flawlessly during ISS missions. [Ryan‘s working papers]
A key message from Lesson 8:
Engineering is a Logical Thought Process
Instead of Blindly Applying Processes and Codes
• Think Critically
• Understand
• Explain
Lesson 9: Mathematics is the Same!
The mathematical expressions (describing equations) for our systems’ physical processes are the same. The difference is in the dimensions (units) and boundary conditions.
The following mathematical categories illustrate the lesson:
• Algebra and Geometry
• Ordinary Differential Equations
• Partial Differential Equations
The fact that mathematics can describe the physical phenomena of nature is
astounding. In addition the fact that mathematics, using the same forms, can describe all of the various areas of physics is even more astounding. Of course algebra and geometry is basic to all; however, the use of ordinary and partial differential equations is also
foundational. Since the same form of the differential equations describe all the various disciplines then we can make analogies between the various disciplines, by just changing coefficients and units. This was the basis for the analog computer that many of us grew up
using to solve our equations. This also means that we can think in disciplines other than our specialties using this transformation. The basic message then is that we must be proficient in the use of mathematics while at the same time understanding the difference in the
phenomenon that is being observed. We can use the similarities to a great advantage but in the end ―the physics of the problem rules‖.
A key message from Lesson 9:
Learn the mathematics. It is the foundation of all analysis and modeling.
Lesson 10: Fundamentals of Launch Vehicle Design
Challenges of launch vehicle performance The fundamentals of launch vehicle design are:
Propulsion system efficiency
Structural (non-propellant mass) efficiency Managing the losses
The design of a launch vehicle is concerned with getting a payload to orbit in an effective and efficient manner. To design and operate a space launch vehicle one needs to understand the physical basics of the system. The essence of this lesson is the elements of that fundamental understanding: propulsion system efficiency, structural efficiency and managing the system losses. Overcoming gravity makes this a very challenging job as discussed in Lesson 3. To accomplish this task we must have highly efficient structures and propulsion systems and manage the losses in order to have a balanced system. The next section will discuss these fundamentals.
Figure 10-1 goes into more detail of the elements of the fundamentals of launch vehicle design. The discussion in Lesson 3 illustrated the complexity and challenge of space flight. Figure 10-1 summarizes the basic characteristics of the challenge. Emphasized is the fact that the technical, propulsion and structural efficiencies and managing the losses must be balanced and traded with the programmatics of cost, schedule and the -ilities. This
balancing between the programmatics and the technical is a major challenge in that what we do to reduce cost and schedule greatly affects the technical. The process is further
complicated by the fact that the technical requires that the design get all the performance possible in order to overcome gravity etc. Figure 10-1 depicts representative but not complete lists of the characteristics of the three fundamentals of launch vehicle design. Notice that for mass efficiency the three elements of mass fraction, packaging, thermal protection system are some of the things you can manage to get mass efficiency. Each though has many subsets; for example, mass fraction contains structural configuration, materials, loads etc.
Propulsion systems deal with at least the engine cycle, Isp and engine thrust to weight ratio.
Again each of these would have many sub parts. Managing the losses includes the generally known parts such as gravity losses and expands into environments, etc. A big effort is
dealing with uncertainties and interactions. Uncertainties must be clearly defined starting with an initial set. These uncertainties are then burned down as the lifecycle is moved towards operations. Interactions are very complicated and must be analyzed and controlled or the losses get very high. Interactions not understood are one of the major causes of problems in any launch vehicle system. The classic ones such as flutter, whirl, pogo etc. must be
designed out of the system.