I saw Space News' An object is now orbiting alongside China’s Shijian-21 debris mitigation satellite recently about a "new object" orbiting near a relatively newly launched satellite. The article says it's currently classified as an apogee kick motor, and states the following:

Apogee kick motors usually perform a final maneuver after satellite separation so as to not pose a threat to active satellites through risk of collision.

This makes sense to me, as keeping the motor attached would mean a shorter lifetime due to the additional mass, and having stray debris wandering around an orbit seems undesirable.

This led to two questions:

  • How is this maneuver done?
  • How common is this maneuver historically and among currently-operating rockets?

For the first question, I've found a few interesting bits of information, but nothing complete. Maneuvering the kick motor like a "normal" satellite/rocket after its burn seems like an obvious answer, and Rocket Lab's kick stage appears to use this approach, but I haven't been able to find out how common this approach is or whether it's a relatively recent development. In addition, it doesn't seem feasible for something like the Star 48, since it seems much simpler technology-wise and I'm not sure how maneuvering/reignition would work for a solid motor. Are there other separation/disposal options for this or other cases?

As for the second question, I haven't been able to find anything, though I'm not sure I'm looking in the right places.

  • $\begingroup$ Huh, I've never heard of such a thing. Always assumed the separation mechanism wouldn't be worth the mass. Following this question. $\endgroup$ Nov 9, 2021 at 0:27

2 Answers 2


One example (I do not know if it is typical) was the Inertial Upper Stage (IUS) used to boost space shuttle launched payloads into their final orbits.

The multi-solid-stage IUS also had a liquid fueled reaction control system (RCS). This RCS was used to fly the IUS away from the payload after separation. Here is an example from STS-93 which launched the Chandra (aka AXAF) observatory on an IUS.

...the final IUS coast phase and included the IUS/Chandra separation event and a special final RCS burn. The special RCS burn was included to provide the combined functions of a collision/contamination avoidance maneuver (CCAM) and RCS burn-to-depletion (BTD). The CCAM/BTD was designed to preclude physical contact and to minimize contamination of Chandra by IUS SRM outgassing or RCS thrusting.

Source: Flight Results of the Chandra X-ray Observatory Inertial Upper Stage Space Mission

  1. Springs, and 2. Poorly.

Assuming a final, solid stage is separated (some don’t), the end payload separates just like it would for a liquid (assuming Western, not Soviet payload interfaces). A release mechanism splits the payload and stage interface rings along a separation line. Springs, preloaded when the stage and payload were mated on the ground, are then free to extend. These sep springs may sound puny, but with enough steel/titanium/whatever, and enough of a preloading fixture at the payload prep facility, you can get a significant spring force. Meanwhile, the fundamental principle of a coil spring is well-understood, easily sourced, and basically bombproof. In an inertial environment (no gravitational effects, drag, very little damping) this should be all you need.

Should , on paper. In the real world, the spring force just doesn’t get you anywhere close to what that stage had gotten you, so the stage and payload drift apart somewhat slowly. Based on perturbations, the dead stage can (slowly) do various things. Space missions are very keen on having the dead stage in sight, until drag pulls it away by a comfortable margin. (Empty stages have low mass for their surface area, so residual drag has a high effect, compared to the brand-new satellite which is tanked up.) Of course, if the two are orbiting high enough, drag goes to zero effectively if not literally, and that empty cask can linger for years .

For this reason, many projects opt to incorporate the final solid into the spacecraft (if it’s small enough), dedicate some payload thruster fuel for an early maneuver (handy if you need to circularize, or you just demand extreme orbital precision), or prefer a liquid as the last stage, with some sort of avoidance/disposal maneuver in the sequence. Restartable upper stages aren’t that weird and costly anymore, and are a selling point for a launch operation. Besides, a restartable upper stage can do things like circularization, extreme orbit precision, multi-burn trajectories, etc.


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