Indirect Recovery Ejection (Part 1- Old ideas)


I was reading a post over at Rocketry Planet Forum that caught my attention. The post was entitled “A Time for New Ideas” and was basically musing about alternative ejection devices for recovery. Of course with the current problems with BATF many people are looking for new ideas.
I thought at first we would look at some old ideas.
One of the first ideas put in place was a gravity switch. A gravity switch is a switch that while the rocket is in an upright position is off but as the rocket begins it’s descent gravity moves the switch in the opposite direction charging the ejection. This switch has a great number of problems and I think it is best described here. Another similar device is the mercury switch which is basically a gravity switch that uses mercury as the switching mechanism. Mercury flows easily and it solves some of the problems of the gravity switch but not all. The third switch that falls into this catagory is the accelerometer or acceleration switch. This switch assesses the acceleration of the rocket. While the rocket is accelerating the switch is open and when deceleration begins the switch is closed usually activating a timer circuit.
Richard Nakka used an air resistance switch back in the 70’s. The switch had a flap that was mounted externally on the rocket . Aerodynamic force hold the switch(flap) down as the rocket ascends. Eventually the flap raises to the on position as the spring mechanism overpowers the drag forces and causes a timer circuit to charge the ejection.
There are two other methods that need little explanation. The first is the timer switch. The timer switch must be used with great care. It is important to know how much time must elapse before the switch is closed. Premature or late closing can cause great damage to the rocket. The final method is remote control. This method can be successful if the rocketeer has a good line of sight with the rocket in order to know when to charge the ejection.

next post will be Indirect Recovery Ejection (Part 2 -Modern and new ideas)

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4 Responses to Indirect Recovery Ejection (Part 1- Old ideas)

  1. Dick Stafford says:

    Great idea for a post, I look forward to the next installment. I’ve heard of plenty of ‘newer’ approaches, but it would be fun to have them compiled in one place. One clarification – the Mercury switch was used for staging vs. ejection. The mass of the Mercury is such that it moves upward at motor burn-out and closes the switch then. I suppose this would be an issue for other similar techniques.


  2. rocketry says:

    Thanks Dick.
    I have spoken to several folks who said they used mercury switches for recovery ejection. It was used as such back in 70s. I also found this in the rocketry glossary.
    “The electrical circuits controlled by mercury switches are most commonly used to activate recovery system deployment charges and sustainer motor igniters”

  3. Dick Stafford says:

    Heh, you learn something new everyday. The only paper references I have seen (or remember seeing at least) say they were used successfully for staging and the people I’ve talked to say they didn’t work properly (reliably?) for ‘chute ejection. I guess that doen’t mean that someone didn’t get them to work.

  4. Dick Stafford says:

    Hey, is there some way you can add an RSS or atom feed to your site? I’d like to see your updates bubble up on my Blogrolling list. TIA

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