Air Start

[Localx]

What does "Air Start" motors mean, and how is it done?

 

[Missileman]

Air starting refers to starting extra motors, usually clustered while in flight.
Say a 5 motor cluster is launched, but only the center motor is lit at launch, 2 seconds later 2 opposing outboard motors are lit, then 2 seconds after that the other 2 are lit.
Usually done with timers.

 

[Ryan S.]

basically secondary motors are lit a certian amount of time into the flight. Altimeters can also be used

 

[Rocketjunkie]

Here's a sequence pic showing the plume going from smoky to white. The central motor was a smoky M lighting 8 I white motors using Thermalite fuses.

 

[Karl]

Im just in the process of building a rocket that air-starts , ill post you some pictures when the electronics come!
-karl

 

[eugenefl]

Originally posted by Karl
Im just in the process of building a rocket that air-starts , ill post you some pictures when the electronics come!
-karl

Please do. I just received my Perfectflite MT3G timer and I'm looking for someone's build to see where the timer is mounted. I figure somewhere in an interstage coupler but I could be wrong. Also, where the heck is a timer mounted if you're doing airstarts from within a cluster mount?

 

[loopy]

Originally posted by eugenefl
where the heck is a timer mounted if you're doing airstarts from within a cluster mount?

I've seen a few rockets built with the electronics bay built into the motor mount between the centering rings and an access panel cut out of the body tube. Makes alot of sense to me - not too far to run the wires.

Loopy

 

[n3tjm]

My Magnum has the timer bay in the fin can... It is exposed till you join the fin can to the main airframe tube. . I had two airstart flights with it. The first one plowed into the deck when the Central motor broke free and shot through the rocket. The most recent one was more sucessful... J500 airstarting two H128's

 

[Micromeister]

Air starting is simply staging without the benifit of dropping the dead weight of motors casings.

 

[eugenefl]

Originally posted by Loopy
I've seen a few rockets built with the electronics bay built into the motor mount between the centering rings and an access panel cut out of the body tube. Makes alot of sense to me - not too far to run the wires.

Loopy

This makes sense, but I figured the heat from the motors would damage the electronics. I suppose the airframe would have to be a decent diameter to house even the smallest timer. Any chance of pictures?

Doug, if I understand your post correctly the fin can is removable? Any chance of pictures of your setup?

 

[Elapid]

they make ceramic ICs as well as the cheapo plastic ones...
ceramic ones can take a LOT of heat.
iirc

 

[loopy]

Originally posted by eugenefl
Any chance of pictures?

Sorry - wasn't my rocket, and I don't have any pictures. Although, if I'm not mistaken, there's a review of the Diablo on EMRR where the builder did this.

Loopy

 

[loopy]

Aha! Found it! It wasn't the BSD Diablo - it was the Binder Design Thug. Here's a link to the EMRR Review in question.

 

[n3tjm]

Originally posted by eugenefl
Doug, if I understand your post correctly the fin can is removable? Any chance of pictures of your setup?

Info about my timer bay:
https://www.rocketryforum.com/showthread.php?s=&threadid=3780

I since have built a platform running up the 54mm tube to mount my new PET2 timer on it, since it is to big for the centering ring idea. I will take a picture of that next time I have the Magnum apart.

 

[DynaSoar]

Originally posted by Micromister
Air starting is simply staging without the benifit of dropping the dead weight of motors casings.

So what IS the benefit of air starting then?

 

[hokkyokusei]

Originally posted by DynaSoar
So what IS the benefit of air starting then?

One advantage is when ignitng a large cluster. If you have a large central motor, you can use "airstart" electronics to ignite the other motors once it is confirmed that the rocket is moving.
Typically, it's done asap so it's an extreme case of airstarting.

There's no real 'advantage' of air starting, other than the cool factor. I guess you could argue that you can simulate a long burn motor with sequential short burn motors.

 

[solrules]

Originally posted by DynaSoar
So what IS the benefit of air starting then?

Bragging rights.

 

[Rocketman248]

The benefit of airstarting over clustering, besides the coolness factor, is increased altitude. With a cluster, you get a lot of liftoff velocity, but it slows down quicker. With airstarts, after the first motor burns out, you have more motors to continue the boost.

Here's a liftoff shot of a scratchbuilt 1/2 scale Doorknob on a K670GG. It then airstarted 2 I154J's.

 

[Rocketman248]

Here it is after the airstarts lit.

 

[solrules]

Originally posted by Rocketman248
The benefit of airstarting over clustering, besides the coolness factor, is increased altitude. With a cluster, you get a lot of liftoff velocity, but it slows down quicker. With airstarts, after the first motor burns out, you have more motors to continue the boost.

Right, but how I see it is that the altitude fo a rocket increases as the square of the burnout velocity. So, am I right in thinking that the altitude is not *that* much greater (although it all depends on mass of rocket, aerodynamics, etc.) using airstarts? In fact, I would be willing to bet on some types of rockets (espically low thrust/weight ratio) would get a lower altitude airstarting than clustering.

Food for thought:

Quoted from https://my.execpc.com/~culp/rockets/rckt_eqn.html#Method
Notice: the rocket goes more than twice as far after the burn as during the burn!

 

[loopy]

Originally posted by solrules
Right, but how I see it is that the altitude fo a rocket increases as the square of the burnout velocity. So, am I right in thinking that the altitude is not *that* much greater (although it all depends on mass of rocket, aerodynamics, etc.) using airstarts? In fact, I would be willing to bet on some types of rockets (espically low thrust/weight ratio) would get a lower altitude airstarting than clustering.

Food for thought:

I think it depends on the characteristics of the rocket/conditions. smaller motors will still be able to accelerate a heavier rocket if it's already moving, correct?

Loopy

 

[Elapid]

ground start, velocity = 0
air start velocity >0

see signature.

I think it depends on the characteristics of the rocket/conditions. smaller motors will still be able to accelerate a heavier rocket if it's already moving, correct?

as long as the thrust is greater than air friction and other drag forces on the rocket, it will accelerate.

 

[daveyfire]

Originally posted by Micromister
Air starting is simply staging without the benifit of dropping the dead weight of motors casings.

...or the complexity of booster recovery in HPR models...

And it looks really cool too!

 

[Karl]

Yep , the change of smell , noise , flame , smoke is way cool ! I dont think I would call staging a HPR rocket that drops each stage away 'safe' without some sort of recovery device. Even if it was to be flown on the smallest/lightest motor available.
-Karl

 

[FredA]

Beyond being cool, the reason for airstarts is altitude!
Drag is proportional to velocity.
To minimze drag, minimize velocity.
In order to go as slow as possible, you need a constant push to overcome gravity.
Since motors don't burn constantly, you burn several in succession.
If you've got the guts....delay betweeen starts to slow down as much as possible....just gotta keep it going up straight!


If you can't stage, then airstart.
Don't light those motors on the ground!
Clustering on the gound just means you didn't buy a big enough motor IMHO.

FredA