Self destruct mechanism on upper stages

Question

In a follow up to this question: the upper stage of the launch vehicles are usually treated as orbiting debris after the launch mission completion. In that case, are there self-destruct ordinance present in the upper stage of the launch vehicles? If so, how are they prevented from being inadvertently activated while still in orbit?

Answer 1

At the end of a mission, the upper stage is usually passivated: tanks are vented to make sure the stage doesn't explode at some point. You really don't want to add more space debris by having the stage explode.

There are also regulations for the final orbit of upper stages: they have to reenter within a set time frame or be put into a graveyard orbit.

A self-destruct mechanism (in the industry, these are called 'flight termination system' instead) is usually present, but this is only intended to be used if something goes wrong before the stage reaches orbit.

Flight termination systems use various techniques to make sure the system is only triggered when it is intended to be:

• the design has to have a predicted reliability of 0.999 at a confidence level of 95 percent
• it must have single fault tolerance
• the design must use redundancy
• system independence (the FTS must not be part of the rocket's flight control system)

For example:

A command destruct system must receive and process a valid flight termination system arm command before accepting a flight termination system destruct command.

So you need to send at least 2 messages (both properly formatted) before the FTS is triggered.

The FTS also includes the ability to switch off the FTS remotely ('automatic safing'). When the system is safed, even sending a valid destruct command won't trigger the FTS.

Any automatic safing must occur only when the flight of the launch vehicle satisfies the safing criteria for no less than two different safing parameters or conditions, such as time of flight, propellant depletion, acceleration, or altitude. The safing criteria for each different safing parameter or condition must ensure that the flight termination system on a stage or strap-on-motor can only be safed once the stage or strap-on motor attains orbit or can no longer reach a populated or other protected area.

Answer 2

Self destructs are to keep it from falling on something/somebody if something goes wrong on the trip up. It's eventual fate in space has nothing to do with this. They have just as much need of a self destruct as the first stage.

Answer 3

Your question is a little bit unclear. It can be interpreted in multiple different ways. Literally, when you ask (bold emphasis mine)

[A]re there self-destruct ordinance present in the upper stage of the launch vehicles?

The answer is: it depends. If there was self-destruct ordnance present in the upper stage of the launch vehicle when it was launched, then it will still be there, because … where would it have gone? If there was no self-destruct ordnance present in the upper stage of the launch vehicle when it was launched, then there will still be no self-destruct ordnance present in the upper stage of the launch vehicle, because … where would it have come from?

However, there is a second way to interpret your question, which is

Are upper stages of launch vehicles destroyed after use?

The answer is again, it depends. If there is enough propellant left (mostly for launches to LEO), then they will typically be de-orbited and break up and burn up in the atmosphere. (There was, however, a case where some components of a spent Falcon9 upper stage didn't fully burn up and crashed in Brazil.)

If there is not enough propellant left (typically for launches to GTO), they will be left to naturally decay on their own and re-enter at a much later date.

If the upper stage won't re-enter on its own within a specific time, it must be put into a stable orbit (called a graveyard orbit for obvious reasons). When being put into a graveyard orbit, the spacecraft will be made safe by dumping all fuel and other chemicals that could cause it to disintegrate.

The Falcon9 upper state that delivered DSCOVR to Earth-Sun Lagrange point L₁, was put into a heliocentric orbit.

A third way to interpret your question would be

Are upper stages of launch vehicles intentionally blown up in orbit?

And the answer to that is: Heck no! Space debris is dangerous enough, you'd much rather have a nice big, clearly visible chunk in a known position on a known trajectory than millions of small, invisible pieces of high-velocity shrapnel on unpredictable trajectories.