Multistage/clustering experience?

[Scrooge McDuck]

Hello all, I still have got some german Held-1000 rocket engines (C1.3-0), which I'm planning to use as multistage engines. A few weeks ago I used them on a rocket powered glider, this works.

They have no delay time before the ejection charge, so I think they can be used as booster engines for a first stage.
As second stage I have C6-3 engines (I know 3 seconds delay before ejection charge is probably a bit too short for a 2nd stage, but I've got a box full of these engines, so they must have a purpose )
Does anyone have any experience how far I can separate the 1st and 2nd stages, but still make the 2nd stage ignite? Should the top of the booster stage be placed directly against the bottom of the upper stage, or can I leave some room between?

The Held-1000 have a relatively low thrust and long burn time (5.5 seconds!), so it may not be able to get my rocket up to speed very well. Therefore I consider clustering 3 of them in the 1st stage. But then they need to be ignited at exactly the same time at launch.
Can I simply wire them parallel, and use some heavy battery for ignition? Or should I prevent the rocket from being launched so I can make sure all engines are running?
I can imagine that if one engine ignites a tiny moment earlier than the others, it may liftoff the whole stack and cause the others not to ignite at all, resulting in a dangerous launch.

Some interesting links
http://www.apogeerockets.com/education/how_to_multi-stage.asp
http://www.bookcase.com/library/faq/archive/model-rockets/ignition-tips.html

So although there is plenty of information on the internet, I wonder if any of you have some experience with this.

regards,
mcduck

 

[reverend]

Yea, I've done a couple small 2 stager's. The ejection charge is a pretty big flame even on a small motor. I'd recommend putting them as close as possible and not more than 1 inch away to make sure you get ignition in the 2nd stage.

On clustered motors you cannot wire them in parallel because of the old 'path of least resistance rule' This means you'd end up with sequential ignitions, not simultaneous ignitions.

Wiring them in series will do the trick, however you'd need to use a perfect size battery. One that will be enough to light the wires, but not so much that it makes them break right away... Once one ignitor breaks there's no more current to the other ignitors. Don't use a car battery... four D cell batteries should do the trick.

You shouldn't try to 'clamp down' the rocket to get all engines up before launch. Just make sure you have a guide stick long enough so that 1) all motors have time to ignite, and 2) you have enough velocity to maintain it's own stability.

 

[Scrooge McDuck]

Thanks for the comments

Wiring them in series will do the trick, however you'd need to use a perfect size battery. [..] Once one ignitor breaks there's no more current to the other ignitors.

People here use a parallel setup for that particular reason, I can imagine the change of failure in a series wiring will be larger because of this problem... So I'm a little bit confused what should be the best. I almost can't imagine that wiring them in series will even work, but apparently it does.

Just make sure you have a guide stick long enough so that 1) all motors have time to ignite, and 2) you have enough velocity to maintain it's own stability.

Ah ok, but do you mean that the ignitors have longer cables so they can be lifted a little while the rocket is guided but not simultaneously ignited?

About the Held-1000 engines; yes there are some different specifications on the internet about what class they are, but they actually do burn for about 5 seconds!

Durchmesser : 15,2mm
Länge 94 mm
Gesamtimpuls: 7,25 Ns
Schubdauer: 5,5 s
Verzögerung: keine
Ausstoß: ja

regards,
mcduck

 

[reverend]

I misread the engine type... Yes a C1.3 would burn for 3.8 to 7.7 seconds (7.25 Ns total impulse / 1.3 N average impulse) = 5.5 seconds

In Series (Simultaneous) - If an ignitor breaks the circuit no other ignitors will start
In Parellel (Sequential) - only one motor will start if the circuit doesn't break

For the people using Parallel, the key important thing here is that they're using a high power battery (12V car). This makes it so that the ignitors burn and break so quickly that even though you get sequential ignitions, they are all so close together it doesn't matter.

I'd just like to point out this paragraph from that page you linked to:

On the other hand, it was mentioned by several posters that series wiring is extensively used in the explosives and pyrotechnics industries because of the added reliability you get. With series wiring you can verify the complete igniter circuit and you will know if *any* igniter is improperly wired. Also, you would be able to ignite many more (fast igniting) igniters with series wiring, especially if the resistance in the igniter is high.

Usually I'd tell anyone, clustering is not as preferable as just using a bigger motor, but since you have a bunch of these things, go to town!

 

[Art]

Hey guys, the best way is NOT to use series wiring...it is to use parallel DC systems with a multi-pole switch. Multiple systems running off of a switch that is actually a cluster of switches. If you get an ignitor to fry off early that can snuff your system if it acts as a fuse.
Other advice: if you use thermite or magnesium in your ignitors, mill it finer than you normally would.

Another remark is that you are in possession of what is often called "moon burner" engines. You might do better in altitude and burn time is you use them as second stage motors. Put a nice first stage under them that gets the thing up to speed, and then the moon burner gives you that nice long burn under a much downsized second stage that sounds so far-out and cool when you hear the doppler effect.