Motor Ejection and DD altimeter

[Green Arrow]

Has anyone ever flown a rocket with motor ejection and Dual Deploy?

Here is what I was thinking. Flying with a J350 and long delay motor ejection. Have ebay deploy first the drogue and then at the determined altitude deploy the main. The long delay on the motor would be extra "security" in case the main get snagged or doesn't fully deploy.

Just wondering....(I was reading about altimeter bays tonight in my HPR hand book 2 when I got the idea.) It's late and I have 4 hours before getting up. Just thought I would throw it out for discussion while i sleep and work tomorrow.

(Don't for get the PACKER game tomorrow!!! )

 

[Pantherjon]

It would have to be a pretty long delay to deploy the main...maybe use the motor eject to pop the drogue? But really, if you have the proper size vent holes and the altimeter is set up right you shouldn't have to worry about it

 

[blackjack2564]

Yes, I have done it,especially when I started doing DD. However should you use the motor eject for backup on the drogue, NOT the main.

Supplied delay on AT motors is 10 sec, Pro AMW Loki etc vary between 15- 18 sec. Way to short to be back up on most main deploy which happens after the drogue fires, and the rocket free falls to the altitude you pre-set your altimeter to fire at.
The normal set up is to break apart the rocket in the center with the drogue charge, then fire the main out the payload, which I don't know how you would do with motor ejection.

 

[KDRaven]

I plan on using motor ejction for back up on the drouge when I fire my DD camera rocket for the first time as DD configuration. I have set up a dual event timer system that allows for a merurcy switch bulb to detect motor burn out and after a set time, using the same switch, detect apogee to fire the drouge and then after a set time on the second timer fire the main chute. I do hope it does work if not well it still will be a cool video shot. Im using a bostervision cam inside of the ebay with a mirror looking down.

 

[jcsalem]

Motor ejection is good for drogue backup but not good for main backup. The problem with the latter is that if your electronics fail, the rocket will go ballistic and will probably be underground by the time the motor ejection occurs!

However, I've had good luck using motor ejection for drogue backup. It's very comforting to know that the rocket will at least avoid lawn darting if the electronics fail.

Jim

 

[jadebox]

I plan on using motor ejction for back up on the drouge when I fire my DD camera rocket for the first time as DD configuration. I have set up a dual event timer system that allows for a merurcy switch bulb to detect motor burn out and after a set time, using the same switch, detect apogee to fire the drouge and then after a set time on the second timer fire the main chute. I do hope it does work if not well it still will be a cool video shot. Im using a bostervision cam inside of the ebay with a mirror looking down.

A mercury switch won't reliably detect motor burnout and, since a rocket is in free-fall after the motor burns out, a mercury switch will not work at all for detecting apogee.

You could use a timer that starts at ignition using a g-switch or pull-string to start it. But a true altimeter is best because it can detect the actual apogee and altitude for the main 'chute.

-- Roger

 

[Micromeister]

A mercury switch won't reliably detect motor burnout and, since a rocket is in free-fall after the motor burns out, a mercury switch will not work at all for detecting apogee.

You could use a timer that starts at ignition using a g-switch or pull-string to start it. But a true altimeter is best because it can detect the actual apogee and altitude for the main 'chute.

-- Roger

Roger I hate to disgree:
But I've been using mercury switches to detect motor burnout and ignite upper stage motors for about 35 years or so...they've yet to let me down...corse these are all in MODEL rockets using BP motors. but I've yet to have an upper stage fail to start using the method.
I do agree, a model in free fall after apogee won't do much with a mercury switch as the G's increase... but as she turns over AT apogee at 0 g's it would certainly complete whatever circuuit it's connected to provided it was oriented properly in the first place.
Use of encapsulated mercury switches has declined in the last couple year, due in part to the increase in availability of decent Staging Times. However; I've seen more failures using them then seen with the old Merc-switch systems and a 9volt.
Sometimes the old tired and true methods work better then "them nu-fangled printed circuits"

 

[jadebox]

Roger I hate to disgree:
But I've been using mercury switches to detect motor burnout and ignite upper stage motors for about 35 years or so...they've yet to let me down...corse these are all in MODEL rockets using BP motors. but I've yet to have an upper stage fail to start using the method.
I do agree, a model in free fall after apogee won't do much with a mercury switch as the G's increase... but as she turns over AT apogee at 0 g's it would certainly complete whatever circuuit it's connected to provided it was oriented properly in the first place.

I'll buy that you can detect motor burnout with a mercury switch. But there is no way a mercury switch can detect apogee. While the rocket is following a ballistic course, the mercury in the switch will be weightless - experiencing 0Gs the whole time (that's the definition of "free-fall"). It won't "feel" anything special at apogee.

-- Roger

 

[Micromeister]

I'll buy that you can detect motor burnout with a mercury switch. But there is no way a mercury switch can detect apogee. While the rocket is following a ballistic course, the mercury in the switch will be weightless - experiencing 0Gs the whole time (that's the definition of "free-fall"). It won't "feel" anything special at apogee.

-- Roger

Roger:
Mercury switches "detect" Nothing.
However during flight they are anything but weightless, nor is an object "weightless" during Freefall, that object is accelerating under gravitational pull at a rate of about 3ft/sec/sec. to that objects terminal velocity within our atmosphere.
They flow with the change in orientation relative to gravitational pull as the model slows to near zero arching over at apogee, closing the circuit deploying the chute or Drogue.
Put the model on an Arching "SPEEDY" ballistic trajectory however centrifugal forces may very well be high enough to prevent the mercury from flowing to complete the circuit so there may be failures during these type "tip-off or badly weathercocking high speed flights.

 

[jadebox]

Roger:
Mercury switches "detect" Nothing. However during flight they are anything but weightless.
They flow with the change in orientation relative to gravitational pull as the model slows to near zero arching over at apogee, closing the circuit deploying the chute or Drogue.

After the motor burns out, the rocket is in free-fall. The rocket is accelerating at -1G until the parachute deploys, it reaches terminal velocity, or it hits the ground. In free-fall the rocket experiences 0Gs (1G + -1G). Therefore it is weightless. Everything inside the rocket, including the mercury switch, is accelerating exactly the same (remember what Galileo taught us?), so it is also weightless. Since everything is weightless in free-fall, there is absolutely no way for a mercury switch to detect apogee.

References:

https://en.wikipedia.org/wiki/G-force

An object in free fall (accelerating downwards at 1 g relative to the Earth) experiences a g-force of −1 g + 1 g = 0 g ("weightless")

https://www.einstein-online.info/en/spotlights/equivalence_principle/index.html

[As] long as the elevator was in free fall [...] you, everything else within the elevator and the elevator itself would all be accelerated at exactly the same rate so that, inside, no influence of gravity could be detected. Relative to the elevator, all those objects faithfully keep their relative positions (or move at a constant speed), just as they would in a gravity-free region of space. You, as the elevator's passenger, would feel weightless ...

-- Roger

 

[Reinhard]

On a ballistic trajectory, everything inside the rocket is "weightless". There are no centrifugal forces due to the rocket flying a ballistic "arc" (opposed to a cornering car, altough there are, usually small, forces resulting from the rotation of the rocket).
This principle is used in the zero-g flights of the vomit comet.
https://www.youtube.com/watch?v=MMV_a7Olf2c

However, there is one significant factor that must be considered too: drag. After burnout it decelerates (in addition to gravity) the rocket and everything inside the rocket can "feel it". A mercury switch will sense this too. Depending on the orientation of the switch it will open or close at burnout. This event can be used to trigger a timer or to ignite an upper stage.
On the zero-g flights, the engines maintain a low level of thrust, just enough to compensate the drag.

Reinhard

 

[jadebox]

The "Vomit Comet" is an interesting example.

But, the thought-experiment with the falling elevator is what I always "fall back" to (excuse the pun). It illustrates that the elevator (rocket) accelerates at the same rate as the person (mercury switch) in it. That Einstein guy was pretty smart!

-- Roger

 

[KDRaven]

I did not want to start a war over the use or scentific explnation of a mercury switch. I have been able to start a timer with one when motor burn out accured because the drag on the rocket is greater than the drag on the mercury at the time when the motor stops providing forward trust.
The idea to be able to detect apogee is when the rocket reaches its peak altitude after the coast time from the forward trust of the motor the rocket becomes almost stationary in the air as it rotates over from going nose up to going nose down and the weight of the mercury at this time will make its trassition from the bottom of the glass caspule to the top of it closing the contacts again and there for closing the circuit to ignite a ejection charge. The reason for the timer is to close a relay 3 to 5 seconds after motor burn out so when the rocket rotates from going up to going down and the mercury colses the circuit again the ignither will be in the circuit because of the closed contact of the relay. If a diagram of this circuit would be help ful I can provide one if anyone wants to see it. the file is 1.69 meg in bitmap format.

 

[jadebox]

The idea to be able to detect apogee is when the rocket reaches its peak altitude after the coast time from the forward trust of the motor the rocket becomes almost stationary in the air as it rotates over from going nose up to going nose down and the weight of the mercury at this time will make its trassition from the bottom of the glass caspule to the top of it closing the contacts again and there for closing the circuit to ignite a ejection charge.

The rocket doesn't become stationary at apogee. It continues to accelerate towards the ground. A rocket accelerates at -1G from the time the motor stops thrusting until the parachute deploys, the rocket reaches terminal velocity, or the rocket hits the ground. So there is no way for a mercury switch in a rocket to detect apogee. I'm 100%, no-doubt-about, bet-you-a-million dollars certain of this.

Einstein's falling elevator example shows why. As the elevator falls it accelerates at 9.80m/sec^2 towards the earth. A person inside the elevator is accelerated at exactly the same rate. So relative to the elevator, the person falls at the same rate. During the fall, the person floats around in the elevator and feels weightless. It's the same way with the mercury in the switch. After the motor burns out, the mercury in the switch will float all around. It'll randomly close the contacts and be useless for detecting apogee.

-- Roger

 

[Handeman]

Roger,

I've never used a mercury switch in a rocket so I could be way off on this, but I would think KD is correct. The rocket is affected by a lot more then gravity which is why the elevator analogy doesn't really apply. Drag is a very strong force at high speeds and has a bigger affect on the rocket then gravity at motor burnout. The mercury inside a glass tube of a switch is only affected by directional force (thrust and gravity) and inertia. It's the interaction of air on the rocket and inertia of the mercury that will make it work.

 

[jadebox]

Roger,

I've never used a mercury switch in a rocket so I could be way off on this, but I would think KD is correct. The rocket is affected by a lot more then gravity which is why the elevator analogy doesn't really apply. Drag is a very strong force at high speeds and has a bigger affect on the rocket then gravity at motor burnout. The mercury inside a glass tube of a switch is only affected by directional force (thrust and gravity) and inertia. It's the interaction of air on the rocket and inertia of the mercury that will make it work.

I conceded that a mercury switch could be used to detect motor burnout. But you cannot use one to detect apogee.

-- Roger

 

[Handeman]

I conceded that a mercury switch could be used to detect motor burnout. But you cannot use one to detect apogee.

-- Roger

For that I would say you are probably right. At least not consistently or exactly at apogee. It should work slightly after apogee when the rocket starts down and gets a little bit of drag. The question would be how far after apogee will it fire.

 

[jadebox]

For that I would say you are probably right. At least not consistently or exactly at apogee. It should work slightly after apogee when the rocket starts down and gets a little bit of drag. The question would be how far after apogee will it fire.

No, it wouldn't work at all as I explained earlier. After motor burnout, the mercury blob would float around. It wouldn't move in any specific direction. There's nothing special about apogee (or shortly after apogee) that could be detected by a mercury switch.

You seem to be thinking that the acceleration on the rocket changes when it changes direction from "up" to "down." That's not true. The rocket is in free-fall from the moment the motor stops thrusting and the acceleration due to gravity is constant.

If you don't believe me, please see the references I listed earlier:

https://en.wikipedia.org/wiki/G-force

An object in free fall (accelerating downwards at 1 g relative to the Earth) experiences a g-force of −1 g + 1 g = 0 g ("weightless")

https://www.einstein-online.info/en/s...ple/index.html

[As] long as the elevator was in free fall [...] you, everything else within the elevator and the elevator itself would all be accelerated at exactly the same rate so that, inside, no influence of gravity could be detected. Relative to the elevator, all those objects faithfully keep their relative positions (or move at a constant speed), just as they would in a gravity-free region of space. You, as the elevator's passenger, would feel weightless ...

Or consider that no one has ever successfully used a mercury switch to detect apogee. I can say that with confidence because it's impossible.

-- Roger

 

[UhClem]

I am glad someone else has decided to beat their brains out against this particular wall. I am depressed because the subject keeps returning from the dead like something in a bad zombie movie.

Gravity works on everything equally. A mercury switch will never, ever, detect apogee.

I suggest that anyone who believes that it will, look at data recorded by altimeters to see that nothing dramatic happens to the acceleration at or near apogee. At least until deployment happens. It is decreasing to zero as velocity and therefore drag is decreasing.

To get you started, here is my collection of altimeter data:

https://home.earthlink.net/~david.schultz/level3/erint/flightlog/log.html
https://home.earthlink.net/~david.schultz/level3/t22/flightlog/log.html
https://home.earthlink.net/~schultdw/atacms/log.html

Plus from the altimeter I built:
https://home.earthlink.net/~david.schultz/rnd/2004/index.html

(Note: Some of this data shows the prelaunch acceleration as 1G and some as 0G. Do not be confused. 1G is the measured value on the ground and 0G is what is measured near apogee. The 0G ground value is the result after the 1G offset is subtracted and is what is integrated to get velocity.)

 

[Handeman]

I understand and agree that a mercury switch will not detect "apogee".

What my question was, how far past apogee does a rocket have to get before the drag of air will slow the rocket more then the mercury in the switch and the switch will make.

Gravity is accelerating the rocket and the mercury downward after apogee, but the rocket is slowed by the drag of air. The mercury will move faster, thus making the switch.

How much drag does it take to get this effect and does it happen fast enough to allow ejection without zippers?

 

[jadebox]

What my question was, how far past apogee does a rocket have to get before the drag of air will slow the rocket more then the mercury in the switch and the switch will make.

The mercury in the switch will float around after the motor burns out. So it probably will close the connection shortly after burnout.

Now, assuming you could somehow keep it open during free-fall, the only time you can say it would definitely close is when the rocket reaches terminal velocity. But, the rocket would probably hit the ground before it reaches terminal velocity.

-- Roger

 

[UhClem]

What my question was, how far past apogee does a rocket have to get before the drag of air will slow the rocket more then the mercury in the switch and the switch will make.

Not even when it hits the ground.

Once the motor burns out the mercury will be forced to the front by drag. Drag will gradually decrease, but probably never decay to zero, as the rocket approaches apogee. So the mercury is always forced to the front. Past apogee drag begins to increase again and the mercury is pushed even harder to the front. There is never a point where it is pulled to the back. If by luck the rocket went straight up and then tail slid backwards far enough for drag to increase to the point where it overcame the mercury's tendency to stay put then it would work. How often does that happen?

 

[Handeman]

The mercury in the switch will float around after the motor burns out. So it probably will close the connection shortly after burnout.

Now, assuming you could somehow keep it open during free-fall, the only time you can say it would definitely close is when the rocket reaches terminal velocity. But, the rocket would probably hit the ground before it reaches terminal velocity.

Until terminal velocity, drag has little effect. The acceleration due to gravity is always vectored in the same direction (towards the earth) for both the rocket and the mercury. The directional component of the drag on the rocket changes constantly as the rocket follows its ballistic arc. Basically, all the drag does is shorten the distance the rocket travels horzontally.

-- Roger

That would be the rub, keeping the switch from closing while the rocket passes through apogee. I don't think it would have to make terminal velocity to close the switch on the way down. I would WAG (wild a$$ guess) that by 20 to 25 ft/sec on the way down, there should be enough drag to make the switch close again.

From an earlier post, KD said to use a timer to inhibit the switch after burnout until apogee. I think it might be better to just fire the ejection with the timer then to depend on the undependable mercury switch for post apogee deployment, although it if the timer inhibits the switch till near apogee, it might not matter if it fires slightly before, at, or slightly after apogee.

 

[jadebox]

Not even when it hits the ground.

Once the motor burns out the mercury will be forced to the front by drag. Drag will gradually decrease, but probably never decay to zero, as the rocket approaches apogee.

I'm doing the thought experiment and I think you're right and my explanation wasn't correct. If you were in a rocket in free-fall, you wouldn't be able to feel the force of gravity. But drag would make it like the brakes are slowly being applied. You'd always be pushed toward the front of the rocket. I doubt it would be much of a push, though, and any vibrations in the rocket would start you (the blob of mercury) bouncing around.

-- Roger

 

[Handeman]

I'm doing the thought experiment and I think you're right and my explanation wasn't correct. If you were in a rocket in free-fall, you wouldn't be able to feel the force of gravity. But drag would make it like the brakes are slowly being applied. You'd always be pushed toward the front of the rocket. I doubt it would be much of a push, though, and any vibrations in the rocket would start you (the blob of mercury) bouncing around.

-- Roger

So if you use one switch to detect burnout, fire the second stage and enable a second circuit. The second circuit would use a mercury switch to detect the second stage burnout and when it opened back up (at apogee when the drag was reduced to zero) it would fire the ejection.

Might work, but how do you make sure the switch opens at apogee and doesn't stay closed the whole time?

 

[KDRaven]

Ok I have read the folling submitted posts and here is my question. Why does a mercury bulb switch work in a heater/ A/C wall thermostat?
It works with the gravitonal pull on the mercury inside of the bulb when the bi-metalic coil of wire tilts the bulb from one location to the other sending the mercury from one set of contacts to the other set of contacts. It is being used as a N.C. and N.O. switch SPDT if my thinking is correct.

The way I am using it in a rocket is standing on end (instead of laying flat as in a theromostat), that way the mercury is sitting at the bottom of the bulb ceeping one set of contacts closed. Thus acting like a closed burn wire and keeping the timers from starting.
During liftoff the mercury is being heald to the bottom of the bulb due to accelaration trust. When the motor burns out the drag on the rocket is greater than the drag on the mercury in the bulb therfoer the mercury keeps travling up in the bulb past the N.C. contacts opening the connection thus starting the timers. one set for 3 to 5 seconds and the other set for a longer time for the main chute.

the reason the first timer is set for the short delay is to operate a relay that is connected to the N.O. contacts of the mercury switch, because if the igniter was connected directly to the N.O. contacts when the motor burnout detection would happen the droue chute would be deployed while the rocket was still coasting upward. So the timer is set to alow the mercury to settle back down to the bottom of the bulb while the rocket is still traveling upwards.

When the rocket reaches apogee, www.m-w.com/dictionary/apogee
(and since it did not leave earths gravitational pull) the rocket will lose it's forward thrust (if it was going straight up and if the nose is heaver than the tail and the fins provide proper drag for the rocket) and should almost come to a slow amount of forward travel. If the rocket should decide to rotate on its center of gravity thus acting like the wall thermostat and tiping the bulb containg the mercury, the weight of the mercury would allow it to move from one end of the bulb to the other end going form the N.C. contacts to the N.O. contacts fully closing the circuit that was closed by the relay the first timer closed after 3 to 5 seconds after motor burnout thus providing voltage and current to the drouge ingiter in turn deploying the drouge chute.

If you want a simple demonstration of this. take a glass with a little amount of water in it and hold it over your head and then tip the glass untill the bottom was up and the mouth of the glass was down. Where did the water go?

I do know if you take a bucket with some water in it and swing it around the water will stay in the bucket for as long as you are swinging it but try to stop or change directions with out spilling any of the water.

The reason why motors are timed with a delay is to alow the rocket to get to apogee or near apogee or past apogee so the velosity of the rocket is slow enough to deploy the chute with out tearing up the chute, shock cord, shroud lines, or rocket

The idea that I am trying to use is to get the drouge to be deployed as close to apogee as posable with out using a ALT or trying to figure out the time delay to set the timer to so it will go off at apogee.

What I have read about ALT's is that they work on air preasure inside and surrounding the rocket. The elcetronics that is used detects the pressure going up and then the change of pressure when the rocket starts falling back down, thus is when it will deploy the drouge chute. And also can detect when the rocket gets to a certain level of pressure to deploy the main chute.

Just because there ore other people greater in mind and intelagance than I have or will have saying that acceleration and G forces have affect on things dont mean that one can not try to do something and it might just work.

Has anyone ever tried to tell the Bee or Bumble bee that it cant fly????

I will be launching this mess of mine In March If the weather is permiting and I will see the if it will work or not. I will promiss that this will be my last post on this subject and project. So untill then I will coment on this subject no longer.

 

[Handeman]

KD

Make sure you post the results. I suspect they will be slightly different then most of us expect. Just as a picture is worth a thousand words, so a an actual demonstration. Good Luck.

A suggestions if I may.

Use a stop watch or two to time how long it is from burnout to drogue and main deployments.

I suspect the drogue deployment will be the same as the timer setting because I think the mercury will stay on the NO contacts because air drag + gravity is slowing the rocket faster then just the gravity working on the mercury.

BTW Your glass of water example doesn't quite hold water. (pun intended)

Try taking a plastic glass, fill it 1/4 full of water, and keeping the open top of the glass upright, toss the glass into the air. Most of the water stays in the bottom of the glass and won't spill until it hits the ground. What little might come out is usually a few drops or a small "string" of water. That's the same effect the mercury will have in the switch as the rocket goes through apogee.

 

[jadebox]

Just because there ore other people greater in mind and intelagance than I have or will have saying that acceleration and G forces have affect on things dont mean that one can not try to do something and it might just work.

If you're able to prove that Galileo was wrong, be sure to write it up for one of the scientific journals.

-- Roger

 

[jadebox]

II do know if you take a bucket with some water in it and swing it around the water will stay in the bucket for as long as you are swinging it but try to stop or change directions with out spilling any of the water.

In this example, the bucket (and water) are continually changing direction. The bucket and water are undergoing constant acceleration. That's why the water doesn't fall out.

Could you put a mercury switch in the bucket and rig it up so that it could detect when the bucket is at its highest point in the arc?

-- Roger

 

[KDRaven]

Well I will not know how this will turn out but I am giving it a try and if the weather is good in march I will find out. I will have on board video and I hope to have outside video also from the ground in some sort or another.
due to the aurgument I will post more after I get this thing launched.
here is a pic of the rocket and the electronics bay that will be used.
the rocket is a 2.1" dia and 82" long using a 38mm motor mount and Acme 38mm fin can off of a 8' rail. The motor I have not decided which size to use yet.