OTA's Final Report on "Big Dumb Boosters" Continued

Page 11. Engines

With respect to the scalability of pressure-fed engines, I should like to point out that the configuration of TRW's Lunar Excursion Module Descent Engine (LEMDE) that was proposed for all stages of the Big Dumb Booster had exhibited a scaling factor of 250. The operational 10,000 pound thrust LEMDE engine was throttlable to 1000 pounds of thrust while the same engine configuration was satisfactorily tested at Edwards AFB at a 250,000 pound thrust level. Therefore a further scale-up by a factor of 5 to 10 to the multi-million pound thrust levels appeared quite reasonable. Thus the further development of pressure-fed engines of the thrusts required by a large Big Dumb Booster does not appear to be a "major uncertainty" as expressed in the OTA report.

The question of whether to use single or multiple engines in each stage should be resolved by conducting a cost-weight-reliability tradeoff analysis. The major inputs would be:

• The anticipated production quantity of SLV's.
• Engine development costs (Costs increase exponentially with thrust.)
• Recurring costs of small to large thrust engines.
• Recurring costs and weights of a single engine and multiple engine installations. (Recurring costs and weights of multiple engine installations are likely to be higher because of the added thrust structure, propellant lines, and trapped propellants.)
• Engine and vehicle reliabilities. (Small changes in reliability convert to very large values of cost. If all engines have the same reliability, a single engine would show a higher vehicle reliability, and may likely obviate the need for engine-out capability.)
• Limited development funds. (Although the analysis may point to single engine configurations in each stage, a start-up company, for instance, with limited funds for development may be forced to choose multiple engine configurations.)

Page 13. Propellant Tanks

Contrary to OTA's belief that inspection costs of weldments of moderate strength steel propellant tanks would be inescapably high, Boeing had demonstrated total tank fabrication costs, including inspection costs, of only several dollars per pound¹.

Since the time the NASA-Langley representative at the OTA sponsored workshop² showed that propellant tanks fabricated from composite materials had the potential for reducing the size, weight and cost of the Big Dumb Booster, extensive development of composite tanks has taken place. Consequently, discussion of moderate strength steel tanks has become superfluous. Although composite tanks have higher costs than steel in dollars per pound, their use results in SLV's of lower cost in dollars per pound of payload. OTA recognized, in this example, that the MCD criteria calls for using "appropriate technology" rather than commercial or lowest cost technology.

Page 14. Propellants

The Big Dumb Booster proposed using the same propellants, Nitrogen Tetroxide and Unsymmetrical Dimethyl Hydrazine, that were being used by the Titan III launch vehicle. At that time the propellants were inexpensive and their deleterious affects on the environment were overlooked. They also were storable at ambient (rather than cryogenic) temperatures, making them compatible with low-cost steels. Besides, these propellants made use of the existing propellant-handling ground facilities.

Use of large quantities of these propellants has been banned. The selection of propellants for a current MCD/SLV involves a rather extensive survey and a cost-weight-reliability analysis. Composite tanks appear usable even at cryogenic temperatures; however, some composite tanks may require liners that are compatible with the propellant or are part of the fabrication process. Several companies who are currently applying the MCD criteria and are using composite tanks appear to have made satisfactory propellant selections.

Page 15. Avionics

These are the only subsystems that have a history of decreasing weight and cost and increasing reliability with time.

Page 15. Launcher Reliability

The reliability argument here revolved about a single issue. I claimed that an MCD/SLV, having fewer parts and generally less sophistication, was inherently more reliable. Moreover, in many cases, critical components did not require redundancies, or it was less costly to add weight to a single component to attain its desired reliability. This is in contrast to employing redundancy to minimum weight components that are designed close to the edge of failure.

Certainly very complex, minimum weight systems and components can be made exceedingly reliable. It is just a matter of spending the necessary amounts of time and money.

Page 17. Favorable Studies

I did not have the opportunity to review the reports of the studies cited. It was reported that by applying the MCD criteria, the cost of placing payloads in orbit was reduced by a factor of 4 and 5. However, there were monstrous increases in vehicle gross weights. My past designs showed weight increases of 15 to 20 %. I suspected that their designs did not take advantage of cost and weight savings made possible by using easily fabricated materials. Boeing's "Double-Bubble" design of an MVD/SLV^B was a good example of a radical vehicle configuration that took advantage of the design possibilities the MCD criteria could provide.

Page 17. Unfavorable Studies

Nor did I review these reports. Both studies cited reported increased SLV cost. As noted previously³, these may be the studies where hardware costs were based on historical minimum weight hardware data rather than estimated costs of minimum cost hardware.

Page 19. Critique

The preceding comment also applies here. I cannot comment on the NASA Liquid Rocket Booster study because I have not been exposed to the details of this work.

Page 22. Institutional Obstacles

Here OTA did justice to the Big Dumb Booster by indicating that:

• The Aerospace Corporation management was not enthusiastic about pursuing the Big Dumb Booster because its development would not make full use of their capabilities.
• Vested interests preferred the high costs associated with advancing technology and with the traditional approach to booster design. These interests often pointed to the spin-off benefits of advancements in technology even though several investigators have shown that it would have been cheaper to develop the spin-offs directly.

I wholeheartedly agree with the comment that "It is sometimes difficult to dislodge an incumbent." It has been particularly true in this case. No compelling force wanted to "rock the boat." Hopefully with the onset of the global economy and the widespread intent on keeping our national budget balanced, the aerospace community may be forced to adopt the MCD criteria.

OTA never looked back on "Big Dumb Boosters" in the three subsequent reports they prepared in answer to the Congressional Committees' request for an "assessment of space transportation technologies."^5,6,7 The storm created by Gregg Eaterbrook's cover story in NEWSWEEK⁸ was skillfully handled until it blew over.

In late February 1989, I was asked by the project director of the OTA studies and reports discussed in these Columns if I would work with the OTA personnel currently investigating low-cost payloads. I said I would. This work culminated in the preparation of "Affordable Spacecraft⁶." The next Column will discuss this report and some of the information exchanged with the principal analyst with the hope that this will add to the understanding of the application of the MCD criteria to payload design. 

References

1. Column dated May 15, 1997.
2. Column dated September 18, 1997.
3. Column dated October 10, 1997.
4. Column dated June 12, 1997.
5. OTA Report, "ROUND TRIP TO ORBIT Human Spaceflight Alternatives Special Report", dated August 1989.
6. OTA Report, "AFFORDABLE SPACECRAFT Design and Launch Alternatives Background Paper", dated January 1990.
7. OTA Report, "ACCESS TO SPACE The Future of U.S. Space Transportation Systems", dated April 1990.
8. Columns dated February 5 and September 18, 1997.

Notes

1. Reference 1, Column dated January 5,1998.
2. Circa 1970, reference unavailable.

What questions has the OTA report and the critique raised in your mind? [clicking on the above opens link to discussions that followed the initial posting of this column]

Next Column: Relative to OTA's report on "Affordable Spacecraft."

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