For those that watched the second launch of RocketLab's Electron launch vehicle, out of Mahia Peninsula, New Zealand on 21 January 2018; in which the vehicle successfully made it to orbit for the first time, this curious sequence of events occurred approximately four minutes into stage two burn.

Operators first called out a number of updates, before two items physically detached from the aft-end of the second stage of the vehicle.

T+6:26: "Coming up on hot swap".
T+6:32: "Hotswapping..."
T+6:36: "Eject".
T+6:38: Two items detach and fall away from the vehicle

This can be seen in this frame captured from the live broadcast of the launch on YouTube. In it, one of the items that fell away can be seen to the left, just below the Rutherford engine bell.

enter image description here

What are these two items falling away from the vehicle, and what is "hot swapping"?

  • $\begingroup$ Link for the video seems to be youtu.be/eg5234BOED8?t=1295 queued at 21:35 (T+ 00:06:36), just before the screen shot. $\endgroup$
    – uhoh
    Jan 22, 2018 at 11:34
  • $\begingroup$ totally inappropriate here.. but i love your user name "ReactingToAngularVues" $\endgroup$
    – tnt-rox
    Jan 22, 2018 at 16:22

1 Answer 1


I’ve continued to do a bit of research on this, and what we’re seeing falling away are two of the lithium-ion battery packs that power the Rutherford engine turbopumps.

The Rutherford engine is unique in that its small size allows it to utilise an electric pump-fed cycle. On the second stage in particular, there are three lithium-ion batteries, two of which—the ones that are ejected—can be seen in this diagram from RocketLab’s payload user guide as the rectangular containers on either side of the second stage engine.

enter image description here

As these two batteries become depleted of charge during flight, the third battery begins supplying power and driving the engine cycle. This process is referred to as hotswapping. Once the hot swap is completed, the two batteries are ejected to increase payload to orbit. Spaceflight101.com further states:

Electron’s second stage hosts a total of three Lithium-Polymer battery packs to power the engine during its burn. Two battery packs are jettisoned as they become exhausted during propulsive flight to shed dead weight from the vehicle and increase Electron’s payload capacity. One battery pack is carried into orbit and remains with the second stage until re-entry. With a low auto-ignition temperature of only 150°C, the second stage batteries will burn up in their entirety during re-entry while the first stage batteries are also likely to be incinerated to some extent during their sub-orbital re-entry.

The remaining batteries—13 of them— are installed and similarly provide power to the first stage’s nine engines. These are not ejected, and are designed to provide enough power for all of first stage flight.

  • 11
    $\begingroup$ Staging batteries is something I haven't even considered in Kerbal... $\endgroup$ Jan 23, 2018 at 11:55
  • 3
    $\begingroup$ Surprising how much stuff is just dumped in the ocean on these rocket launches. $\endgroup$
    – JPhi1618
    Jan 23, 2018 at 15:11
  • $\begingroup$ @JPhi1618 One day they'll kill someone. $\endgroup$
    – yo'
    Jan 23, 2018 at 16:39
  • 2
    $\begingroup$ @yo' the batteries are expected to burn up fully on reentry $\endgroup$ Jan 23, 2018 at 20:30
  • 2
    $\begingroup$ @RoelHarbers batteries actually weigh 0t in Kerbal, making it pointless; even the largest battery equals the smallest in weight. You only need to consider aero-dynamics with those :). Source: I once made a rocket where the main fuselage was made of batteries to save weight and strapped a Kerbal to the outside of it with the Rover Chair. $\endgroup$ Jun 21, 2018 at 16:31

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