A rocket for putting a spacecraft into orbit is usually designed and optimized for a particular payload mass or a narrow range of masses. A primary payload will often weigh less than the ideal target weight for the rocket. The spare mass can be taken up with ballast but rocket operators can instead fill that extra payload capability with one or more secondary spacecraft.
The amateur radio community noticed this ballast replacement opportunity early in the Space Age. The first secondary or “piggyback” spacecraft to go to orbit was the OSCAR 1 amateur satellite in 1961, just four years after Sputnik 1. Since then, it has been common for rockets to offer rides to orbit for secondary payloads. Such ride opportunities have decreased to some degree as satellite builders have learned to optimize the mass of their spacecraft to fill a rocket’s payload capability. For example, the station-keeping fuel storage for a major communications satellite might be increased to keep the satellite working, and generating revenue, for as long as possible.
Sometimes piggyback rides are for free or for minimal charge, such as for AMSAT and university built spacecraft. However, it has become common to charge for commercial and government secondary payloads. The charges, though, are typically far less than they would be for a primary payload.
With the lower cost come several drawbacks in going to space as a secondary payload. The date and time of the launch will be determined by the rocket operator and the primary payload owner. The orbits available to the secondary will be restricted by the primary’s orbital choice. Often the propulsion systems for secondaries are limited for safety’s sake because the rocket operator doesn’t want the primary to be damaged by a mistake in a secondary satellite system.
Today, secondary payloads are generally small satellites bound for Low Earth Orbit. For a smallsat owner, launching as a secondary payload often makes the most economical sense. It can bring the cost down from millions to thousands of dollars in some cases.
Each of the launch vehicles available today for commercial secondaries comes with different restrictions on the maximum mass and size for a secondary payload. Also, the restrictions for a particular launch will depend on the primary.
Each rocket type will typically have a particular mechanism for holding and releasing the secondaries. For example, ULA uses its EELV Secondary Payload Adapter (ESPA). The adapter is 1.5 meters in diameter with a 61 cm ring structure located on top. The ESPA can support up to six secondary payloads for each launch. Each single secondary payload can be up to 181 kg (400 lbs) in mass and have a volume of 61.0 cm x 71.1 cm x 96.5 cm (24 in. x 28 in. x 38 in.).
Diagram of ULA’s ESPA
Instead of dealing directly with the launch vehicle operators, secondary satellite owners can use a third party service such as that offered by Spaceflight Inc. Such a launch service company can deal with all the operational and financial details of getting the payload installed and launched. With dozens of CubeSats going to space on some launches these days, it is obviously beneficial to the rocket owner to let someone else handle interactions with all those different satellite teams.
SpaceX is the main launch vehicle for Spaceflight Inc.’s launch services. The two companies signed a Launch Services Agreement in 2012. One of the advantages of using the launch services provided by Spaceflight Inc. is the potential to reduce the cost even more by coordinating with other smallsat owners to launch at the same time. Spaceflight has several different configuration options for its customers. The table below lists the various dimensions and masses that can be accommodated.
Standard Secondary Payload Weight and Dimensions by Spaceflight Inc.
There are several other guidelines that small satellite manufacturers need to consider if they hope to utilize the launch services of Spaceflight Inc. and SpaceX. For example the payload must inhibit wireless transmission until it is deployed. The satellite must be powered off and no electric signals should be sent until after the deployment signal is given. The satellites must also be able to maintain an interior pressure within 6.9 kPa (1 psi) of the external pressure when the satellite is mounted to the launch vehicle.
Not all launch companies have an interest in launching secondary payloads. Alan Slack of International Launch Services. for example, has stated “There’s not many dollars there, quite frankly. When we sell a satellite to a customer, we give them the whole capability. We’re not going to go put other people’s satellites on there.”
The current costs for small satellite launch vary depending on the payload’s mass and its destined orbit. For SpaceX, the price for a secondary payload to LEO is anywhere from $200,000 up to $325,000. With the small satellite industry rapidly expanding, the need for launch services for secondary payloads will most likely grow rapidly as well.