RECOMMENDED READINGTo communicate the KSC perspective on next generation space transportation systems I've put together the following "TOP 10". It is based on an assortment of information, as well as KSC experience from various sources.
Edgar Zapata
THE TOP TEN - PRIORITIZED MEASURABLE CRITERIA
These criteria are derived with little modification from the work of the Space Propulsion Synergy Group (1993). Although they are in priority order they should generally be used as a group to evaluate directions (concepts, technologies to invest in, strategies).
A brief walk through the meaning of the criteria. Going from the broad view at 20,000 feet and onward down to ground level meaning. The examples provided should not be considered the only derivable solutions. They are merely possible directions that would incorporate many of the lessons of Shuttle.
RECOMMENDED READING
AIAA PAPER: Overview of the Space
Propulsion Synergy Group (SPSG) Strategic Planning Support
Efforts for Earth to Orbit Transportation by Walter F. Dankhoff
and William P. Hope, Jr., SRS Technologies, Arlington Virginia.
AIAA/SAE/ASME/ASEE 29th Joint Propulsion Conference and Exhibit,
June 28-30, 1993 / Monterey, CA.
An overview of why the SPSG formed and what they did. The benefits of a strategic, well defined top-down approach to next generation planning. Also, how the details, particular technologies and research, development and demonstration directions, may be derived once a strategy is determined.
AIAA PAPER: Payoffs for Applying QFD
Techniques in the SPSG Strategic Planning Support Effort for ETO
Transportation and Propulsion Systems by J. Bray, Martin Marietta
Manned Space Systems, New Orleans, LA. AIAA/SAE/ASME/ASEE 29th
Joint Propulsion Conference and Exhibit, June 28-30, 1993 /
Monterey, CA.
An overview of the Quality Function Deployment method. QFD approaches simply provide the much needed means of manifesting operator goals (aircraft-like operations, high reusability...) and higher level goals (routine, affordable access to space) in a design approach. Measuring up any launch system concept as it evolves against these criteria serves to indicate if the direction is on target.
DATA: KEY DRIVERS
Assorted information that quantifies where the problems are. The significance of liquid propulsion system problems and the relation of "where the problems are" to the affordability of future systems (especially non-recurring costs, operation of a system) is a key point.
STS Launch Delays: Where the delays really are. It is important to recognize the difference between the number of delays for a given problem versus the impact of that delay (in manpower or days of delay).
THE TOP TEN - PRIORITIZED MEASURABLE CRITERIA
The following Top 10 are listed with the corresponding attributes or requirements of a future system to which they are most related.
Responsiveness: Primarily ease of Supportability and Maintainability
1) Minimize the number of separate vehicle systems, subsystems, GSE and facilities.
Dependable and Reliable
2) Minimize the number of potential leakage sources.
Affordability: Primarily Recurring Cost
Dependable and Reliable
4) Minimize the number of different fluids used.
5) Minimize the number of active components required to test and function.
Dependable and Reliable
6) Maximize the use of components with demonstrated high reliability.
Responsiveness: Primarily ease of Supportability and Maintainability
7) Minimize the number of active systems required to maintain a safe vehicle.
Dependable and Reliable
8) Minimize the number of systems requiring monitoring because of hazards.
Responsiveness: Primarily ease of Supportability and Maintainability
9) Maximize automation, autonomy and BIT/BITE.
10) Minimize the number of interfaces requiring engineering control.
THE CRITERIA - A WALKDOWN
The previous criteria are in no way
complete. The need to eliminate the need for any purges on the
vehicle or ground, the need to eliminate closed compartments, the
need to increase accessibility for any repair (hopefully very
rare) are all important. As criteria these would be "reduce
the number of purges" or "reduce the mean time to
access". However, the Top 10 are likely among the most
important lessons that must be incorporated into next generation
design.
These Top 10 are intended as high level strategies. They are also spaceholders for further, more detailed, information. If possible, as this more detailed information is surfaced and organized, it may be placed at this site at some time in the future.
Responsiveness: Primarily ease of Supportability and Maintainability
1) Minimize the number of separate vehicle systems, subsystems, GSE and facilities.
This is the same as reducing the number of different propulsion systems. It is basically a statement that above all comes the need for simplicity in design.
For example, vehicle tankage and architecture. The simplest architecture will be to have all tanks at zero level. No LOX tanks forward. This eliminates long feeds and leak paths, eliminates the need for a ground pump system for LOX facilities, eliminates geysering concerns, eliminates active systems such as GHe inject, makes conditioning easier, greatly increases the start box on the propulsion and enables greater modularization, greatly simplifying all propulsion by reduced parts count (complexity) and increasing mission reliability as well as supportability.
2) Minimize the number of potential leakage sources.
This references architecture and technology. For example, long feeds being avoided would be an architectural solution (synergy with Criteria 1). A technological solution could be better, leak free joints (welding techniques, seal designs). Another technological solution would be the entire elimination of a fluid system such as using electromechanical actuators instead of fluid hydraulics, or flywheels instead of hydrazine auxiliary power units. Self contained hydraulic actuation could also score positively in this area.
Affordability: Primarily Recurring Cost
3) Minimize the number of hands-on activities required (to handle, assemble, verify functionally, checkout, service, launch, flight operate, land, secure, safe, etc...)
This is basically another "elimination" criteria. For example, if an umbilical or interface is eliminated or one concept has fewer than another (all other things being equal) then this criteria is improved upon. An automation approach could work similarly (better) versus a system with marginally fewer interfaces to assemble by hand.
4) Minimize the number of different fluids used.
This criteria could apply at various level. The count could include all the way down to precision cleaning fluids for example. However, with propulsion and related ground systems being drivers the criteria is basically about eliminating major propulsion system fluids. For example, substituting LO2/LH2 for hypergolics or flywheels for power or EMA's for hydraulics. Do not add third propellants (tripropellants go against this criteria as well as previous ones). A monopropellant, even if different than the MPS propellants, is an improvement over separate fuel and oxidizer.
At a lower level this means common propellants for MPS and OMS/RCS. The full intent is to integrate as well. Common tankage, lines and subsystems for MPS and OMS/RCS as well as thermal management and fuel cells.
5) Minimize the number of active components required to test and function.
An example here is the GHe Interseal purge required on SSME's. An oxidizer rich combustion cycle would eliminate the need for this active system. GHe Inject being eliminated through elimination of long feedlines is another example. A flare stack being substituted by an air mixer would have one active system replaced by another but the use of catalytic cumbustors could be an avenue toward elimination of an active system by replacement with a passive one.
Again, although specifically a concept or technology may be favored in one "lesson learned" over another it is important to maintain a whole system perspective and measure up possible directions against the entire gamut of criteria. Being a high score against the top 10 indicates a concept could easily outweigh lack of benefit in other less important criteria.
6) Maximize the use of components with demonstrated high reliability.
Basically this is about rigorous certification, also called qualification. A high reliability gained through the application of manpower is not the intent here. For example, SSME components have a high mission reliability but low support reliability (also called dependability; any and all cases requiring support, especially on the ground.) This means lack of supportability. This means lack of affordability.
Low cycle fatigue (on/off) and high cycle fatigue (duration) testing of designs with an iterative focus on reuse (many cycles and high endurance between any action, maintenance, repair or otherwise) is required for next generation systems affordability.
For example, valves for opening and closing as well as sealing would be tested for next generation systems well beyond what is required for mission reliability. Design and iteration of designs would focus on the turnaround activity being eliminated by having had qualification to failure as well as to extremes of operation. A secondary benefit is better understanding of the degree of conservatism (or not) required in the usage of a system.
Responsiveness: Primarily ease of Supportability and Maintainability
7) Minimize the number of active systems required to maintain a safe vehicle.
An example here is the elimination of purges which is required for closed compartments. Attempts should be made to minimize the enclosed spaces. The GSE required to support these systems as well as the interfaces that these require are another point that is improved when the purges are eliminated.
8) Minimize the number of systems requiring monitoring because of hazards.
Similar to the prior criteria. A hazardous gas detection system is required for closed compartments such as an orbiter aft or an external tank intertank. Also reference criteria number 2.
Responsiveness: Primarily ease of Supportability and Maintainability
9) Maximize automation, autonomy and BIT/BITE.
This is basically a manpower issue. Launch control centers or mission control centers with hundreds of people for a handful of flights per year correspond to a low productivity. Harvesting commercial off the shelf (COTS) computational capabilities and matching with a correspondingly robust, reliable and dependable design could eliminate the current manpower intensive paradigm.
Another example is in criticality. Hazardous gas detection will have to advance toward hazardous gas "location". Current troubleshooting and detection is manpower intensive and time consuming. For instance, a leak is known to exist but can only be located via manpower intensive techniques. This is an opportunity for automation. A rapid "switch on/off" that indicates health of fluid systems and also indicates precisely where (if any) problems exist is required.
Responsiveness: Primarily ease of Supportability and Maintainability
10) Minimize the number of interfaces requiring engineering control.
This refers not only to physical interfaces (a T-O interface for example) but to the organizational interfaces that design interfaces engender. Facility to vehicle interfaces for electrical, fluid, propellant and gases must be minimized.
A facility example is the need for a GOX vent arm for Shuttle. Driven by a fragile vehicle (tile) and thus the need to avoid ice an extra interface was created. A robust vehicle thermal protection system and structure is desirable. Metallic and zero waterproofing materials would be an avenue of research, development and demonstration. Another alternative, a vent all the way down to zero level would possibly integrate with a T-0 umbilical but would not eliminate the interface. A GH2 vent is another example. Free venting would, in effect, eliminate the need for a vent arm. However, other functions would also have to taken to zero level that otherwise would have been on a vent arm. Proof of safety would be a highly desirable concept to develop and demonstrate. (i.e. why SSTO has immediate benefits - less interfaces such as for ET or SRB or to the ground).
OTHER CONSIDERATIONS
Other key characteristics to incorporate into a next generation launch system include robustness, and reusability (real, not partial). A thermal protection system such as Shuttle is extremely manpower intensive (averages of 50,000 manhours per orbiter flow). Waterproofing closes out (interferes with and stops) other work as well due to the hazardous fluids used. A robust, zero maintenance skin is a priority area for research, development and demonstration.
Other factors, primarily business related (profitability, payback, ROI, IRR, methods of design for cost, business plans, investors, payload potentials and possible markets) are beyond the scope of this compilation (for now). They are key to defining a unique mission model against which to evaluate diverse concepts (for an "apples to apples" comparison). Shuttle, though a technological wonder with great flexibility (people and payload transportation, payload deployment, repair, retrieval, science, and so forth) does not meet the expanded mission model of future systems - space transportation, and affordability, supportability, commercial viability, robustness and even higher reliability.
Return to KSC Next Gen Site
Edgar Zapata, NASA Kennedy Space Center
Shuttle Process Engineering Directorate, Fluid Systems Division
