(Ed. Note: When Challenger launched two SCG satellites this past June, the spacecraft were boosted up from the shuttle’s low-earth orbit by payload assist modules – standardized upper stage rockets which were attached to the satellites’ aft ends. Although these PAMs have worked well in five out of five shuttle satellite deployments, problems and controversy surround the ongoing development of more complex, large upper stages for heavier satellites and planetary spacecraft. In the following viewpoint article, originally published in a longer form in Aviation Week, SCG President Dr. Albert Wheelon offers the Group’s approach to cost-effectively propel payloads into their final orbital tracks.)
Our nation is now transitioning its space programs – both civilian and military – to the space shuttle fleet. It is gradually turning off the supply lines for traditional launch vehicles: Titan, Delta, and Atlas Centaur. However, these traditional launchers have one important advantage over the shuttle: they place spacecraft directly into a high elliptical orbit. Satellites can either remain in this high orbit or, using an apogee kick motor, transfer to the geosynchronous stationary orbit used by all commercial communications satellites. On the other hand, the shuttle carries spacecraft only to low-earth orbit, and a large additional boost must be supplied by other rocket stages to move vehicles into other, higher tracks. NASA recognized this shortcoming early in the shuttle program and encouraged the development of a few standard upper stages. For example, the space agency encouraged the Air Force to undertake the development of the inertial upper stage (IUS). In return for a regulated market franchise, NASA also induced McDonnell Douglas to develop the payload assist module PAM-D, used for our HS 376 satellites, and the PAM-A stage, planned to boost Atlas Centaur-class payloads like Ford’s Intelsat V.
PAM-D has been a reasonable success; 22 have been ordered to lift satellites from shuttle low-earth orbit into transfer orbit, and five have already flown successfully.
PAM-A is another story. Eight have been built and there are no users in sight. The IUS has been an extraordinarily costly development, and its rising launch price is rapidly discouraging its use both by commercial and military customers.
The Air Force and NASA are now embarked on the development of a new standard upper stage – the Centaur – which shows every promise of following the path of IUS.
This raises two important questions. The first is, what is wrong with these generic standard rocket upper stages? The second is, is there an alternative? Fortunately for the country, there are clear, positive answers to both of these questions.
The fundamental problem with standard upper stages is the premise on which they are based. It is assumed that such a rocket stage is a good thing to have, that a single upper stage can be used by all spacecraft, thereby spreading the nonrecurring development cost among all user programs and avoiding expensive duplication. What this argument ignores is the enormous increase in recurring costs for each program that is forced to employ a standard stage which is designed for the most demanding user.
The basic problem is that each customer must pay for all the capability needed by all customers, whether that capability is used for a particular mission or not. This situation exists for several reasons:
• A standard upper stage must be sized to the largest, heaviest spacecraft.
• Such a stage must be prepared to fly into all possible orbits.
• The upper stage’s guidance system must be extraordinarily capable to accommodate all possible requirements. (IUS has three redundant guidance systems for this reason.)
• The most valuable spacecraft sets the reliability standard for all users, and each must pay the maximum bill.
Upper Stage Inflation
These relentless pressures progressively increase the price of any standard stage. We believe that these pressures are the driving forces behind the IUS rise from the $2 million per shot promised in 1974 to the $125 million per shot now projected for the mid-80s. As the unit price increases, users shy away from employing a standard upper stage and look to other launch methods. The Air Force Satellite Data System and Navstar programs have abandoned IUS for this reason, and no commercial firm is even contemplating IUS use. This dwindling customer base further accelerates the rise in launch prices. This situation worsens with time, compelling one to ask if there is a better, cheaper solution to the problem.
The premise of integral propulsion is that each spacecraft program can supply its own post-shuttle propulsion most economically by incorporating it directly into the satellite. This has been done for 20 years by all commercial communications satellites which provide a solid rocket motor as an integral part of the spacecraft. This motor is fired at apogee and provides the thrust for transfer to synchronous orbit. The apogee impulse can also be supplied by a liquid propulsion system.
Most Air Force programs have not used integral propulsion because of their commitment to the Titan III/Transtage, or Titan 34D/IUS booster/upper stage combinations which provide the apogee boost as part of their service. (However, the FLTSATCOM and NATO satellite systems have used integral apogee propulsion.) Integral propulsion can also provide the perigee boost needed to shift a satellite from shuttle orbit to transfer orbit. This is the path that all commercial and some military users are taking.
The integral propulsion concept is simple. The shuttle itself carries a superb guidance system which can provide the satellite’s initial orientation. By spinning the satellite as it leaves the bay, the attitude reference is preserved. After a suitable separation delay, a rocket motor fires and the satellite proceeds into transfer orbit. Onboard the spacecraft is a guidance system that will position and orient the satellite for almost a decade after launch; the same spacecraft guidance system can easily control the satellite’s orientation during transfer orbit and during kick motor firing. This “integral guidance” does the job that the three inertial measurement systems of the IUS are designed to do.
Are there any limits to the application of integral propulsion? Does it place any restrictions on the design of the satellite which make it unattractive, either in terms of size or type of stabilization?
The general answer to each question is no.
The nation has a rich inventory of liquid and solid propulsion rocket elements, a result of the Apollo, space shuttle, Minuteman, PAM and IUS programs. By combining these elements appropriately, satellites have been designed whose sizes cover the spectrum of shuttle capacity.
Advocates for the new Centaur standard upper stage argue that it is needed to put very large payloads into synchronous orbit. The fact is that a version of the Hughes integral propulsion multimission bus (MMB) is under design and development that will place nearly 11,000 pounds into stationary orbit. Integral propulsion does not restrict satellite size.
Dollars and Sense
One can compare the launch costs of various spacecraft by plotting the total launch cost – shuttle charge plus upper stage – vs. the initial in-orbit weight. This is illustrated by the chart below, using civilian spacecraft examples. Military programs follow the same trend.
This plot shows some dramatic differences. The TDRS and Intelsat VI spacecraft will have the same initial weight in synchronous orbit. TDRS gets there with the IUS (government-furnished) and Intelsat VI uses its own integral propulsion.
These two similar satellites have an enormous launch cost difference: $89 million (1986 launch, 1982 dollars), which explains why no commercial user is planning to go with IUS. Most of this difference is IUS cost, but a small component is additional shuttle user charge. TDRS plus IUS takes an entire shuttle bay while Intelsat VI takes half a shuttle. Leasat also uses integral propulsion and lies on the dotted line that characterized this approach.
Also shown on this curve is Intelsat V as it would be launched by shuttle and the PAM-A standard upper stage. The large launch cost of this combination explains why the original plan to launch Intelsat V on shuttles has been dropped in favor of Atlas Centaur and Ariane, even though these are expensive rockets.
Integral Propulsion Loomed Large in I-VI Win
Another compelling example of the economics attached to integral propulsion was provided by the Intelsat VI competition in 1981. Hughes proposed an integral propulsion spacecraft. Ford subcontracted with McDonnell Douglas for a new standard upper stage. Both avoided the IUS.
The Hughes cost was $50 million less for the initial development program and resulted in a $16 million advantage in launch cost for each additional satellite placed in service. This cost difference was overriding and resulted in INTELSAT’s contracting with Hughes.
Commercial satellite users are forced to face the launch cost issue squarely – their system cost is the sum of satellites and launches. The procedure in military programs is often different. Most spacecraft program managers do not budget for launch vehicles. Launchers and upper stages are developed and budgeted for in separate, parallel organizations. Since the combined expenditures of satellite and launch meet only at much higher levels, military spacecraft programs have no incentive to reduce launch costs. Indeed, satellite managers are often discouraged from second-guessing the programs of their comrades, who have been committed for three decades to providing standard upper stages. This independence is reinforced by the industrial base that provides these standard vehicles.
Propulsion proponents submit that continued development of standard rocket stages is necessary to maintain a healthy propulsion industry. In developing a rocket stage, however, only a minor fraction of the funds is spent on propulsion elements.
Standard stages have several other negative influences which are not commonly understood. Because of their size, both the IUS and Centaur rocket stages involve dedicating an entire shuttle launch to a single mission. Using these large stages thereby undermines the efficiency of launching several satellites in one mission, as is common in the commercial arena. Further, big stages lead to large spacecraft, with many payloads, and frustrate the commonsense objectives of proliferating our military space assets for both survivability and flexibility.
Integral propulsion provides a decisively cheaper solution for space systems. It has made standard upper stage approaches like IUS and Centaur completely obsolete. The country can no longer ignore the new technology.
The old solutions represent an unwarranted tax on space systems, a tax that the nation can no longer bear in silence.