# What is “propulsive passivation” and why will the SpaceX STP-2 mission do it?

The Teslarati article SpaceX’s Falcon Heavy flies a complex mission for the Air Force in launch video discusses the upcoming third Falcon Heavy mission STP-2 and quotes the caption on the linked SpaceX video:

The STP-2 mission will be among the most challenging launches in SpaceX history with four separate upper-stage engine burns, three separate deployment orbits, a final propulsive passivation maneuver and a total mission duration of over six hours. [It] will demonstrate the capabilities of the Falcon Heavy launch vehicle and provide critical data supporting certification for future National Security Space Launch (NSSL) missions. In addition, [the USAF] will use this mission as a pathfinder for the [military’s systematic utilization of flight-proven] launch vehicle boosters.

SpaceX, April 2019

The SpaceX (unlisted?) video linked in the article STP-2 Animation shows a final propulsive passivation step but does not show what happens because of it.

Question: What is "propulsive passivation" and why will the SpaceX STP-2 mission do it?

• – uhoh Apr 21 at 1:55

I believe the term 'a final propulsive passivization maneuver' is in direct contradiction to the oft referenced 'non propulsive vent' used in other spacecraft at times. A non-non-propulsive vent, so to speak.

A non propulsive vent balances orifices so no propulsion occurs, this may be important in scenarios where the upper stage has been placed in a very particular trajectory (either a graveyard parking orbit or a safe entry point) and now must passivate itself without changing this trajectory.

A propulsive passivization maneuver seems to intend to change the trajectory. Since the final DSX payload is being delivered to a medium Earth orbit, I believe the intent is to reduce the time before decay while saving on an additional engine relight - with 4 relights already built into the mission there is a lot of starting fluid used and many cycles on the engine, meaning there may be cost, mass and safety reasons to avoid another relight.

By coasting (presumably) to apoapsis and then venting residual pressure through the engine chamber and bell, they can produce a low risk controlled thrust to lower the orbit and speed decay.

• I have a hunch this is exactly the correct answer. It's a non-non-propulsive passivation maneuver. Bingo! – uhoh Apr 21 at 16:10

GTO orbits are too far to send the vehicle back to earth for re-entry and to burn off in the atmosphere. So the second stage will become another dead satellite, like many other boosters and rockets up there. For all objects that are past their usage stage, whether a 15 year old satellite or a six hours old second/third stage rocket, the (french) latest law/treaty of operating in space asks from all those vehicles to do a passivation of any liquid or electrical energy, so as they become space junk, they become a less dangerous junk. Any electrical energy in the batteries is depleted (disco lights for a night??lol) and any liquid from fuel or pressure tanks is vented. The remaining object is empty from any hazard that could see it exploding at its last orbits, which could last years

• This is a really nice, concise answer. Thanks! – uhoh May 22 at 16:07

...spent upper stages are generally passivated after their use as a launch vehicle is complete in order to minimize risks while the stage remains derelict in orbit.[5] Passivation means removing any sources of stored energy remaining on the vehicle, as by dumping fuel or discharging batteries.

• Okay that's a start, but "What is “propulsive passivation” and why will the SpaceX STP-2 mission do it?" If you can add an explanation of the specific term “propulsive passivation” and some kind of explanation why SpaceX is passivating propulsively, that would be great! – uhoh Apr 21 at 3:26
• @uhoh I guess you're reading this wrong. It's not 'passivating propulsively' but 'passivating the propulsive system', i.e. discarding all remaining propellants. In "a final propulsive passivation maneuver" the 'propulsive' connects to passivation, not to maneuver. – asdfex Apr 21 at 10:09
• @asdfex I guess I am not reading it wrong. I am reading the two word phrase as adjective, noun and I don't see any other way to read this phrase. SpaceX is a US company and can be expected to get their English correct, so I am going to assume they mean what they said, and what they said was propulsive passivation. – uhoh Apr 21 at 10:11
• @uhoh you can read it as "a final, propulsive-passivation maneuver" or as "a final, propulsive, passivation maneuver" Both are valid. – asdfex Apr 21 at 10:21
• @asdfex For the first one, that would have to be written as "a final, propulsion-passivation maneuver" to be what you want it to be. For the second one, that is exactly what I am saying it is! It is a propulsive maneuver for the purposes of passivation. fun: 1, 2, 3, 4, 5, 6 – uhoh Apr 21 at 10:23

The original text states

a final propulsive passivation maneuver

This is ambiguous as it can be read in two ways:

• "a final propulsive-passivation maneuver" - a final maneuver that does passivation of the propulsive system.
• "a final, propulsive, passivation maneuver" - a final passivation maneuver that is propulsive.

It's not obvious from the text which of the two interpretations is the intended one, but I'm very much in favor of the first one.

It should be noted that any passivation that consists of discarding propellant will be at least slightly propulsive, as is every action that expels material from a spacecraft. On the other hand, I'm very sure that this is not the intended meaning here.

• Thanks for your answer. For the record, I have respectfully disagreed (1 2) with the first interpretation. I believe that it would have to be "propulsion passivation", not "propulsIve passivation" for that to be a correct interpretation. – uhoh Apr 21 at 10:39
• Then again, I believed this was serious, so... – uhoh Apr 21 at 10:45
• But +1 because the graphics in the video (screen shot in the question) seem to show cold propellant being vented out the nozzle rather than exhaust from a started engine producing significant propulsion, That seems to argue strongly in favor of the first one (your favorite). – uhoh Apr 21 at 10:51

The video portion captioned "Propulsive Passivation" appears to answer the question by 2 noteworthy observations. First, the exhaust nozzle is venting propellant to space - the "Passivation" part. This is confirmed by the absence of any flames at the rocket's exhaust. Second, the propellant is expelled under pressure (as opposed to "free" venting) - which accounts for the "Propulsive" part. Since the word "Propulsive" by definition means driving or pushing forward, perhaps a more appropriate caption might have read: "Pressurized Passivation".

• If there was no pressure, how would venting occur? – Organic Marble May 22 at 14:52

PP is to vent the propellant from the spacecraft to prevent it exploding which could cause clouds of debris that can potentially damage other satellites. (https://en.wikipedia.org/wiki/Passivation_(spacecraft)).

It's a routine in modern rocket launches since debris issues are getting more severe and could harbor bigger disasters if there's a 'bomb' like spacecraft floating in space after the mission. STP-2 has some buffers in the fuel tank to provide contingency usages(like re-deployment if failed). The unused fuel should be passivated by burning it at a slower speed but until fully exhausted.

• Hi and welcome to Space Exploration SE. Unfortunately, your answer doesn't seem to provide anything that wouldn't be already covered by the other answers here. It'd also be best to address the second part of the question about why was a "propulsive" passivation chosen. – TooTea Jun 18 at 16:51