The interface with Dr. Callaham lasted several months. He had no hang-ups with the applicability of the MCD criteria to payload design. We agreed that the initial and most important task was to acquire data and analyses of payloads that have been designed to the minimum weight/maximum performance criteria; that is, designed to decreasing levels of sophistication that weigh more and cost less.
I advised Dr. Callaham of the work of TRW in which an operating satellite had been redesigned to progressively lower levels of sophistication,1 and that I was not given the unexpected results beyond a verbal description and a casual look at the summary curves. Dr. Callaham contacted the person who did this work but he too was unable to get a copy of it. I found this to be most regrettable because I had worked with the TRW individual and knew that he fully understood the MCD criteria and the methods of design. However he did obtain two interesting reports, one prepared by Rand and the other by Boeing, copies of which he sent to me for comment. He further obtained some study results from Lockheed and personally performed a singular point analysis, all of which is discussed in OTA's final report.2
OTA labeled lower cost, higher weight payloads "Fatsats." In their report, OTA examined three other cost-cutting techniques, two of which did not directly involve the application of the MCD criteria and are not discussed further. The third, called "Lightsats," should respond to the application of the MCD criteria to the same degree as Fatsats.
The following discussion includes the information I sent to OTA as comments to the Rand and Boeing reports and to the draft of OTA's final report. I have enlarged and revised the comments I submitted, improvements I hope, as a result of experience gained in the intervening years and increased time available to me now.
I evaluated the Rand report3 as misleading except for the fact that the author permitted payload weight to increase, and assumed cost to vary inversely and exponentially with weight, thereby acknowledging one of the precepts of the MCD criteria. He correctly identified the optimum payload to occur when payload plus launch vehicle costs are at a minimum. The analysis, however, was purely parametric. Values were not assigned to the exponents in both the launch and payload cost equations. Under many assumptions, which I found difficult to evaluate in terms of reality, he concluded that lower cost launch vehicles would not provide significant savings in the cost of space operations.
OTA extended the Rand analysis by providing values to the exponents that define launch and payload costs as a function of payload weight. The costs of the Delta, Titan and Space Shuttle were used to determine the value of the exponent in the launch cost equation. However, the negative exponent in the equation for payload cost as a function of payload weight was arbitrarily assumed close to one. The latter assumption assured that only small cost savings would result in applying the MCD criteria to payload design; moreover, the optimum payload weight was shown to be about three times heavier than the minimum weight design.4
The Boeing report5 was based on
weights
and costs estimated from preliminary designs of a "typical" payload.
The
payload included a small propulsion or "kick" stage.6
The results of Eder's study is shown in Figure
1. The estimated cost was reduced 30% at a 15% weight increase. The
author prudently did not carry the analysis much beyond a payload
weight
increase of 100%.
Neither OTA nor I concurred with Eder in his calling the study payload "typical." We felt that each of the vast number of payload types and missions would have a different cost-weight relationship, and that some of the relationships may differ sharply from that shown in Figure 1. Nevertheless, from personal experience with the design of space structures of lesser sophistication, I felt that the general shape of the curve was correct for most payloads. Specifically, I felt there would be an important, initial large drop in payload cost for a small increase in weight. This contention is supported by relevant data presented in the initial pages of the OTA report that refer to TRW's payload design experience.
Also plotted in Figure 1 is the personal model that is used in the studies that follow. It differs slightly from Eder's analytically derived curve by the assumption that payload costs are cut in half at a 20% increase in weight. The studies are intended to illustrate some of the overall cost consequences of the application of the MCD criteria to payload design. Specifically, the work answers the question: Under what conditions is the optimum cost of the payload related to the cost of the launch vehicle? In other words, when can payload costs be further reduced by lower launch costs?... Certainly in the boundary case when launch costs approach zero, the payload could consist of "unpackaged" laboratory-type components adequately protected against launch and space environments.
Insight to the answer to this question is contained in Figures 2 and 3. Two classes of payloads are considered: payloads that are few in number such that the nonrecurring costs are significant, and payloads that are fabricated in the hundreds, typical of direct-access satellite systems. It is assumed that all minimum weight/maximum performance payload designs weigh 10,000 pounds. OTA launch costs versus payload weight, current at the time the report was prepared (1990) are shown as a continuous function (Curve A).7 Lower cost launch costs are assumed at one-third of "current" launch costs (Curve B).
Figure
2 shows what the optimization of low production payloads might look
like. The minimum weight cost of the payload, which includes a small
propulsion
stage that places the satellite in orbit, is assumed at $50,000 per
pound,
fully fueled.8 Curve C represents the
Fatsat
payload cost-weight relationship under the assumption made in Figure 1.
In this example no additional saving in payload cost is incurred by the
lower launch costs. The optimal payload weight is the same at 22,000
pounds
for both launch costs.
Some of the data provided by Lockheed are also plotted in Figure 2. These are estimates of the weight growths and cost reductions of redesigned versions of the Orbiting Astronomical Observatory (OAO) and the Synchronous Equatorial Orbiter (SEO).9 Because of the ambiguous conditions under which these estimates were made, the data are considered to serve only to show that both satellites responded to approximately the same Fatsat cost-weight relationship: the more expensive satellite experienced a larger decrease in cost.
Figure 3 depicts what the optimization of high production payloads might look like. Two minimum weight payload costs are considered: one at $15,000 per pound (Curve C) and the other at $7,500 per pound (Curve D). The immediate observation that may be made from viewing the payload plus launch costs is that optimal (saddle) points appear when the payload cost approaches launch costs. This means that optimal cost payloads may not require large increases in payload weight.
Of additional significance is that in both cases, reducing launch costs decrease total (payload and launch vehicle) costs above that gained by the reduction of launch costs. This point is exhibited in the following table. All costs are in millions of dollars, and payload weights are in thousands of pounds. Scaled, not computed values are given.
| Curve | Payload Weight | Launch Cost | Payload Cost | Total Cost | |
|---|---|---|---|---|---|
| C+A | 14,500 | 59 | 55 | 114 | |
| C+B | 14,500 | 20 | 55 | 75 | |
| C+B | 22,000 | 27.5 | 34.5 | 62 | |
| Maximum Savings
Among Fatsats: |
31.5 | 20.5 | 52 | ||
| D+A | 13,200 | 54 | 33 | 87 | |
| D+B | 13,200 | 18 | 33 | 51 | |
| D+B | 20,000 | 25.5 | 19.5 | 45 | |
| Maximum Savings
Among Fatsats: |
28.5 | 13.5 | 42 | ||
The reductions in total cost from $75 million to $62 million (Curve C) and from $51 million to $45 million (Curve D) are the additional reductions that are realized by lower launch costs.
Of course, launch costs are not continuous, and the method of treating discrete launch vehicles is discussed in the Column previously referenced. Note that the application of the MCD criteria results in larger payloads and launch vehicles. Further note that for a small cost penalty, optimal payloads and launch vehicles may be appreciably less in size.
It is hoped that these examples, although future payloads will have somewhat different cost-weight characteristics, will spur the development of a family of MCD/SLVs10 as well as assist in the design of low-cost payloads.
| Does
anyone wish to contribute to the design of Fatsats by reporting work
performed
since 1990?
[no discussions were submitted for this question] |
 |
Final Column: A Relevant Happening.
