Payload mass scale economy

Question

What is the "payload mass" curve in relation to "cost to orbit?"

I mean: preparing a whole launch system for delivering a single cubesat to LEO would be an overkill, the cost to bring such a small payload up there would be excessive comparing to benefits. It's much better to bundle several smaller satellites in a single launch. On the other hand, ISS was assembled from parts on the orbit. While the shape might be unwieldy to launch as a whole, there was no fundamental reason other than economical (be it originating from technology development costs or from sheer material/fuel costs) to launch all parts in one flight.

Where's the "sweet spot" of cost/weight ratio at bringing payload to orbit? What factors create the lower and upper bounds that make both too small and too big payloads not viable? Why there's no orbital supertruck or small private-owned cubesat launchpads?

Answer 1

Some of the factors constraining launch vehicle size are the same as for any other technology-based product, namely fixed and incremental costs, the learning curve, time to market, and economies of scale.

A larger launch system will have larger fixed costs (not necessarily per unit of payload mass per launch). Gathering the funding for a larger research and development effort is more difficult, in part because the risk is higher (the payoff is likely over a longer period which increases the risk from inaccurate prediction and increases the required magnitude of the payoff by the compound interest effect).

Larger systems also have larger fixed costs with respect to manufacturing.

When a technology is relatively immature and has relatively limited demand (both from cost and lack of familiarity), fixed costs tend to dominate.

With respect to the learning curve, since a larger launch system will have fewer launches, more payload will be attempted to be launched with a lesser understanding of the system. E.g., if the first two launches each have an 90% success probability, the next four 95%, the next sixteen 98%, then to launch 20 units of payload, a four-units per launch system will expect to require 5.37 launches, losing 0.37 vehicles and 1.5 units of payload while a single-unit system will expect to require 20.69 launches losing 0.69 vehicles and 0.69 units of payload. Just counting incremental costs, if the payload value is twice the single-unit system vehicle cost, then to break even at 20 units of payload launched the heavy lift vehicle must have an incremental cost only 3.56 times that of the single-unit vehicle (11% less expensive per unit of payload capacity).

The learning curve also applies to manufacturing costs and even rocket efficiency (which can adjust payload size upward slightly or fuel costs down slightly).

Time to market interacts with funding and the learning curve. Being the first to begin making a profit (or even having a firm prospect of making a profit) reduces the cost of attracting capital (which funds additional R&D). Being the first to successfully launch will bring in more customers, increasing revenue (and profit) and increasing the number of launches. (A similar phenomenon for programming languages has been called "Worse Is Better".)

Economies of scale give a modest advantage to smaller systems beyond what would be expected from a simple model of the above factors. As volume increases, competition becomes more practical (increasing innovative pressure and opportunity [outsiders are more able/inclined to contribute] and reducing profit margins [but also risk]) and standards tend to develop which reduce production and design costs.

Another factor favoring smaller systems is that multiple types of systems become more economical to run. If an unexpected failure mode grounds a particular type of system, other systems will be available for launch.

Launch systems have some specific features that constrain the minimization of payload size. The launch itself has costs associated with use of a given area that must be cleared as an aviation hazard and the ground area cleared for launch failure safety issues. The monitoring cost of a launch is also relatively fixed per launch rather than per unit of payload.

In addition, a larger launch vehicle will tend to be somewhat more efficient due to area versus volume effects.

There are also limits on how many devices can be tracked, communicated with, and maintained in non-conflicting orbits, so there is some benefit to having a larger payload. Without in-space assembly, the size of a system that works effectively as a whole can be a limiting factor at the lower end of size.

Scheduling of launches is also a factor. With a multi-payload launch, all the cargo must be able to accommodate launch at the same time. With multiple launches, the delay of one launch would not necessarily impact the schedule of other launches. Furthermore, with more launches, there would likely be more launch sites, so localized effects like weather would have a reduced impact on launch capability.

A multi-payload system would also tend to increase the number of failure modes and might make specialization for specific components less attractive.

(The economics of space launches is one of the arguments in favor of fuel depots, despite the fact that a significant fraction of cryogenic fuels would be lost due to heating while in orbit.)

Answer 2

The incremental cost of a launch service in $/kg goes down as the vehicle size goes up. So just based on that, you'd want the largest launch vehicle you can build.

However this is balanced by the overhead costs of maintaining the launch capability. You need to maintain the launch site, the launch crews, the manufacturing capability, the supply chain for parts, etc. This drives you in the direction of smaller launch vehicles to minimize the overhead costs amortized over the launches.

Similarly, you want a higher launch rate for reliability, though that's harder to factor into the costs, since insurance is usually not paid for by the launch provider, it's paid for by the payload provider.

So ignoring for the moment what size payloads the customer needs, you do end up with a sweet spot in vehicle size based on the amount of business you have or expect to have. Where that is depends on all the factors mentioned above.

A good example is the Delta II. The Delta II had a steady stream of business driven by the launch of GPS satellites. This allowed them to provide a very cost effective and highly reliable launch service that was used by many other customers, including NASA. Mars Pathfinder, MGS, MPL, MCO, Stardust, Genesis, the MERs, Spitzer, and others were launched on the Delta II. However when the GPS launches were done, the Delta II overhead costs didn't change much (they did try to reduce them), and as a result the cost of a single launch went up dramatically. It was no longer cost effective, and NASA could no longer support maintaining the Delta II launch capability for their small number of launches. NASA is a relatively small customer in the commercial market. The Delta II was taken off the list of available launch vehicles for NASA missions. NASA science missions now use mainly EELVs, whose overhead costs are subsidized by the Air Force.

Another good example may be SLS. At a launch rate of once every two years, that vehicle seems too large to justify the overhead costs. The resulting total cost per launch (not incremental) becomes, well, astronomical. Also it is difficult to see how they would get to any level of acceptable, proven reliability with such a low launch rate. Perhaps they are imagining a world with a much higher launch rate, but it is not clear where that funding would come from, either for the launch vehicles or the payloads.