The amounts and the types of margin present in a vehicle design strongly impact the resulting transportation architecture. Two types of margin will be discussed; growth and technology margins used (and used up)2 in design and development, and margins still present in the fielded system.
Traditionally, margin is used during the early stages of program design and acquisition as a contingency to allow for weight growth and growth resulting from addressing design uncertainties during development. This type of margin should always be present with the amount being dependent on the maturity and definition of the subsystems technology and the overall design. For example, in the Access to Space study14 and the RLV studies which followed a weight margin of 15% was used on all subsystems, both vehicle and propulsion. The studies represented a new architecture (SSTO) using many unproved subsystems. Since the historical weight growth of the SSME engine was 14% and the engine had relatively low TRL’s at the time the design concept was frozen, the use of 15% appears to be "in the ballpark" for experimental programs but questionable for HRST goals. All HRST design concepts should have this kind of margin included and defined. This type of margin should be included when trajectories are run to determine performance closure. All architectural concepts must include these margins to allow true concept comparisons.
The type of margin discussed above should always be present in preliminary concept designs but is not the type of margin that will increase the probability of reaching the HRST goals - very affordable space transportation, because this type of margin is, historically, always used up simply to make the design a reality. Also, note that historically past programs were focused on performance and not economics. Indeed, the choice of an appropriate number to use for this margin is based on finding what margin was needed in past designs.
| The type of margin useful to achieving the HRST goals is margin built into the system, or subsystems, and not used merely to achieve the minimum necessary system performance. It is essentially the ability to operate the system, at least before using the margin to improve affordability, in a de-rated condition and still achieve the minimum required system performance. |
The type of margin useful to achieving the HRST goals is margin built into the system, or subsystems, and not used merely to achieve the minimum necessary system performance. It is essentially the ability to operate the system, at least before using the margin to improve affordability, in a de-rated condition and still achieve the minimum required system performance. Margin of this type is available to improve affordability and not merely to compensate for the uncertainties of development. This type of margin is considered in aeronautical transportation systems but up until now has not been adopted for aerospace transportation systems.
Four measurable criteria were identified for this type of margin: mass fraction margin, thrust (chamber pressure) margin, average specific impulse margin, and payload margin. Because each of these criteria correlated against a subset of the desired attributes, none of them individually rated very high (40/63, 44/63, 50/63, and 59/63 respectively). A single criteria measuring overall system margin was also considered and it rated very high. There are also many other subsystem margins that could have been considered, such as thermal management, fluid flow or current capacity, but they would have been too detailed in relation to the other criteria.
Mass fraction and thrust (chamber pressure) margin are directed specifically at the vehicle and the propulsion system respectively. Margin designed in for the mass fraction allows use of more robust components and structures, or higher safety factors on life limiting structure and components. This margin would also allow the use of heavier but more developed technologies in selected applications. All of these are reflected in planned and unplanned maintenance and life, which in turn affects inspections, logistics, facilities, and turnaround time, responsiveness and safety.
Thrust margin is generally related to operating rocket engines below their maximum design limit to increase operating life. This is thought to increase mean time between failures of the propulsion systems. For example, the theoretical mean time between failures of the Phase 2 SSME improves more than ten times if operated at a power level of 100% instead of 109%19. The improvement is related to decreased temperature, speed, and pressure environments within the engine and component operation away from the structural limits used when the engine was designed. Low cycle thermal fatigue limits are particularly impacted. In general, de-rating an engine significantly improves its life which allows less inspection, better predictability of failure (unusual conditions do not push internal environments beyond design limits), and less maintenance. All of which lower logistic and facility requirements and improve turnaround time. The cost of including a thrust margin is an essentially linear reduction in engine thrust / weight. Also in the area of the rocket engine start transition, the stresses traditionally exceed the steady-state conditions resulting in large operating life reductions and requirements for inspection and maintenance. Margin must be included here to cover transition by providing a softer start capability. For systems where performance is the main goal or where the mission is barely possible with expected engine weight, thrust margin may be a luxury; but for systems where operating cost is the main goal, thrust margin may instead be a necessity.
The criteria on average specific impulse margin is traditionally treated as essentially a technology uncertainty margin. It is aimed at those systems where a high specific impulse is predicted and is critical to the design, where there is a tight coupling between details of vehicle geometry and engine performance. Thus, this margin is more like the margins discussed at the beginning of this section which should be present in the preliminary design. The amount of margin should probably be varied depending on the degree of database available for the vehicle / propulsion system being studied. To the degree that the specific impulse margin is not consumed during development, it acts the same as a payload margin and could be used to avoid operations such as abort options like RTLS and TAL or other operations which increase infrastructure and add considerable operating cost.
The last of the four margins considered is a payload margin. This is essentially an overall system margin which can be traded to all subsystems if necessary. To strongly affect affordability, this margin must be maintained and not simply used as a weight growth margin during development. Besides allowing propulsion subsystem de-rating (i.e., lower thrust for the nominal payload), payload margin is usable after the system is developed for system additions targeted specifically to improve operations based on operating experience (without this margin this is not possible while still meeting the nominal payload). Additionally, payload margin produces the flexibility to respond to new demands and commercial opportunities.
In summary, any system with margins over and above the development margins will be more operable than one without margins because of subsystem operation at reduced design environments. This will in turn lower maintenance, logistics, and facilities requirements, while allowing targeted system additions to address remaining or newly discovered operability problems while still performing the required mission. The payload margin will provide flexibility to new business. The specific impulse margin will reduce operational infrastructure required for safe operation. These are the characteristics which produce affordable space transportation systems.
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Edgar Zapata, NASA Kennedy Space Center
Shuttle Process Engineering Directorate, Fluid Systems Division