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when WO Rave gone from one'modsl satellitd to a
redesigned or upgraded ono. If one looks at an
actual supply/demand curve it can bo se0n that for
the sako of efsnomy, most space systems are
designod to provide a nominal capability. In geheral,
this capability is less than the peak demand
requirements. Also, as mentioned earlier, the
capability may be reduced due to system failures.
When a crisis occurs, the shflfall &tween dernand
and supply could be quite significant.
For TACSAT to fill this gap, it must possess lhrse
main characteristics; flexibility, respnsivenesb and
affordability. The first criteria for TACSAT, flei'ibility
means PRat i9 must be capable of suppoding multiple
missbn areas. These mission areas are generally
defined as surveillance, commuPications, navigation
and environmental monitoring. As 1 will discuss later
in the paper, there are several ways to design a
system that is capable of performing more than one
of these missions. If we can achieve this Ilexibility,
the number of TACSATs to be built will be
maximized, and accordingly, the unit price will be
minimized. Another element of flexibility, alludecl to
earlier, is compatibility with the existing
infrastructure. From the logistics and well as an
economic standpoint. no TACSAT specific data
receiving or processing equipment should be
required. This is true not only lor the user
ehuipment, but for the facilities required to control
and monitor the space systeiiis as well. The final
flexibility criteria is launch resiliency. Currently, the
military spaco launch strategy does not include
'launch lhrough failure". When a launch failure
occurs, a significant downtime is generally
experienced in order to troubleshoot and conduct
the analysis necessary to determine the cause of
the launch failure. Failure is rarely accepted in terms
of its statistical probability but rather. because
spacecraft and launch systems are expensive, the
financial risk attendant to the next launch warrahts a
thorough lailure investigation. The TACSAT
concept, on Ihc other hand, is premised on quick
response and low cost.
Responsiveness is the second characteristic that a
TACSAT system must possess. Responsiveness
can be achieved in two ways. Thd first is to store
sbacecralt and launch vehicles on the ground and
then launch rapidly when the need arises.
Depending on ihe location of the launch site, it may
ba possiS!o to launch Into the inclination of interest
ancl thereby obtain gerlinent data on the first orbital
pass. Another means of obtaining responsiveness
requires the capability Po reposition satellites already
in orbit. This is a particularly attractive option for
satellites in the geostationary belt.
Of all the TACSAT characteristics, the one that is
absolutely essential is affordability. If a low unit cost
can be achieved the TACSAT concept can be an
attractive option especially in this era of limited
defense spending. TACSATs can be incrementally
acquired allowing the user to purchase the capability
needed at present and then add to that capability as
the need arises.
One method of achieving affordability is to maximize
design commonality across the spectrum of
missions to be performed. In the iJltimate, one
would desire to have a single satellite design
capable of performing any mission. This, of course,
is not possible. This paper will discuss, however,
combining similar missions. A second level of
commonality would be to have a common bus and
bus subsystems such as power, attitude control,
and thermal protection. In this concept, the
payloads would be different for each mission. The
least degree of commonality would be achieved by
having a common bus with subsystems and
payloads tailored to the individual mission. Af!y
amount of commonality will result in a larger unit Suy,
theroby amortizing the RDTBE costs over a larger
base and taking advantage 0: the production
learning curve to reduce the mil cost. Second,
affordability can be achieved by using the existing
infrastructure. Operating with TACSATs should be
transparent to the user: it must use the same ground
terminals as the backbone space architecture. Also,
because TACSATs will tend to be smaller and more
proliferated than the backbone system, it will be
necessary to make these systems more
autonomous thereby reducing the need for satellite
control and the costs attendant to that function. For
those systems employing the rapid launch option,
satellite to launch vehicle integration and test will
have to be simplified to allow operation by military
personnel without contractor support. Finally, the
largest cost driver to space systems are the
requirements themselves. As mentioned earlier,
the TACSAT concept will allow the user the option
to buy only that capability that is needed; the more