This is the fifth in a series of articles on how to go about dramatically reducing space mission cost while maintaining a high level of mission utility. This article discusses mission design approaches to reducing cost and schedule.
One of the most important elements of reducing mission cost is considering the possibility of alternative orbits. Traditionally, the orbit is selected to provide the best mission performance with relatively little regard to cost. However, orbit selection, particularly for low Earth orbit (LEO) missions, can have a significant impact on mission cost in several ways:
- The cost of getting to orbit and maintaining the orbit over the life of the spacecraft.
- The number of satellites needed to provide the appropriate coverage.
- The cost in terms of impact on the spacecraft design.
- The potential for creating or colliding with orbital debris, which can prematurely end the life of the spacecraft.
Lower the cost of getting to orbit
The orbit for science missions is often chosen as the best orbit for that mission irrespective of cost, in part because the “cost” of an orbit tends to be intangible. This is the reason for introducing the orbit cost function, which is the ratio of the mass required in LEO, due east from the launch site, to the total spacecraft mass needed in any given operational orbit. For example, going to geosynchronous orbit (GEO) requires putting into LEO about five times the mass ultimately required in GEO. Going to the surface of the Moon requires about eight times the mass in LEO that will ultimately end up on the surface of the Moon. This implies an orbit cost function of about five for GEO and eight for the surface of the Moon.
As an example of the potential use of the orbit cost function, consider a scientific satellite for which GEO or one of the Lagrange points is the ideal location due to excessive light interference from the Earth. However, if we could get the same effect in LEO if we tripled the mass of the spacecraft by adding shields or baffles with twice the original spacecraft mass, then we could potentially be much better off. We would still be launching only a bit more than half the mass of the more traditional mission, and shields or baffles are typically much lower cost than most other spacecraft components. In addition, we’re in a very benign radiation environment and more uniform thermal environment, and we’re in a regime where it is at least possible to get at the spacecraft in the future if something goes wrong. I don’t want to suggest that all scientific spacecraft should be in LEO, but that option should be a part of the cost reduction trade for many missions.
Adjust coverage to meet current needs
Traditional Earth observation missions want to last for a decade or more and therefore need to blanket the entire Earth all the time with every sensor that will be needed in the future. (Because the spacecraft themselves are individually very expensive and effectively irreplaceable, the system as a whole tends to cost many billions.) If we are instead able to respond directly to world events, this cost can be dramatically reduced by both reducing the amount of coverage that is needed and adjusting that coverage to meet current needs. For example, at the present time, coverage of northern Africa and the Middle East is particularly important. Using a prograde repeat coverage orbit can provide five or six observing opportunities per day of this region with a single satellite versus one opportunity every other day or so with a satellite in a traditional sun-synchronous orbit.
There are several substantial advantages for LEO satellites in orbits below 400 or 500 kilometers altitude. First, we can get comparable resolution with a much smaller and, therefore, lower cost instrument. If we go from a traditional 800-kilometer altitude to 400 kilometers we can get the same resolution with an instrument that has half the aperture and half the linear dimensions. As the payload gets smaller, so will the spacecraft bus. Even if we don’t use any of our tricks for reducing small satellite cost, traditional cost models suggest that reducing linear dimensions by a factor of two will reduce volume and mass by a factor of eight (most spacecraft have about the same density) and cost by a factor of about eight as well. For active payloads, such as radar or lidar, the effect is even larger. Active payloads typically require high power, and reducing the altitude by a factor of two reduces the power required by a factor of 16.
There is a second major effect of flying low that may have an impact on overall mission cost. By flying below 400 or 500 kilometers, the spacecraft will be in a regime that has at least an order of magnitude less debris than at more traditional altitudes of 600 to 800 kilometers. This will be true in the future as well. Additional collisions or satellite breakups may dramatically increase the debris levels at higher altitudes because, once created, debris can remain in these orbits for many hundreds of years. Below 400 or 500 kilometers, debris will re-enter the atmosphere quickly and, in any case, will be swept out of orbit by the time of the next solar maximum. Satellites in this regime will not encounter large amounts of debris and cannot create a long-term debris problem. It will take more propellant to keep them at this altitude, but that is really a matter only of launch cost, since the propellant itself is cheap and most observation spacecraft will already have a propulsion system.
Use a shorter mission design life
Traditionally, we attempt to make the mission design life as long as possible so as to maximize the use of each satellite and minimize the number of satellites needed to provide continuous data. Up to a point, this makes sense. We certainly don’t want a mission life of a month or two if we’re looking for continuous, ongoing monitoring of an event or region. However, as the design life gets longer, we need to begin using redundant components and more extensive mission assurance procedures. Redundancy can be good, but it is never quite as helpful as we think it should be. Having redundant components means that we need both switches and sensors to choose between them, and these represent potential new failure modes. In addition, physical redundancy protects us only against random failures, not against design failures that will be in both the primary and redundant units. Having a shorter design life has several advantages:
- Allows us to make use of newer technology, which will typically be more capable at lower cost.
- Allows us to more directly match the spacecraft, the orbit and the mission to meet current needs. Many of today’s spacecraft were designed when global warming was essentially unknown and the biggest threat to America was the Soviet Union.
- Allows us to maintain a continuous production line. It is the production line that helps us drive down the cost of cars, airplanes and nearly all other elements of modern technology.
- Reduces risk by having another spacecraft in the pipeline in case of a launch or on-orbit failure.
- Allows us to learn from on-orbit experience and apply that knowledge much sooner. As with many elements, the key is to find the right balance by having a design life that is shorter than traditional systems but not so short as to drive up cost by needing too many spacecraft.
Use multiple sources of data
Some data are efficiently collected by satellites and some are more efficiently collected by aircraft, ground sensors or other means. Our goal is to satisfy the needs of the end user as effectively and as economically as possible. Therefore, it makes sense to define the mission in such a way that data from multiple sources can be used. This intermingling of data may be done by the end user, by the operations activity, or by some other operation. The key point is to design the space system such that it supports and enhances the potential for using multiple sources of data and keeps cost down by not duplicating data that are more economically available elsewhere.
One final note on mission design is to look at ways to reduce cost in each of the elements of the mission. If we are going to make a dramatic reduction in mission cost, it isn’t enough to simply reduce the cost of the spacecraft bus, for example. We also have to reduce the cost of the payload, the launch, the ground segment and mission operations. We need to tackle all of the pieces and, of course, be sure that they all operate together to create a mission that retains a high level of utility while strongly driving down cost.
The sixth article in this series will break from a discussion of specific subject areas and address the problem of reducing cost in larger, more traditional programs.
James R. Wertz is president of Microcosm Inc. He is co-author of “Reducing Space Mission Cost,” published in 1996, and has taught a graduate course at the University of Southern California on that topic since then. If you have questions, comments or suggestions, or simply want to discuss these issues, he can be reached at email@example.com. Information on the joint Microcosm/USC Reinventing Space Project can be found at www.smad.com/reinventingspace.html.