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Wps63

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How far should a rocket be from the blast deflector so the bottom of the rocket won't get burned from the back wash of the motors exhaust?
 

troj

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How far should a rocket be from the blast deflector so the bottom of the rocket won't get burned from the back wash of the motors exhaust?
I've found a couple inches works well most of the time -- a spent rocket casing used as a standoff is about perfect.

It also depends on your blast deflector -- one that's flat requires more of a standoff than one that's angled. The flat deflector tends to get a bit of a ricochet effect back up to the rocket, while the angled one, obviously does not.

Think in terms of dropping a rubber ball onto the deflector from the point of the rocket, and where the ball goes after.

Of course, most pads have flat deflectors, probably because they're easier to make.

-Kevin
 

jj94

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For pads that have the blast deflector facing straight up, I generally keep the rocket three inches off the deflector for 18mm black powder motors. I would go with 5 inches for 24mm black powder motors. And for AP reloads, I also go with around 5 inches. For anything bigger, I use a different pad that has an angled deflector, so I don't worry about that as the heat with be deflected away from the rocket.
 

Micromeister

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3" is all that's needed for any BP motor combination up to 12- C6's or 5- D12's. Can't tell you about AP... never touch the stuff.
 

JStarStar

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It also has a little bit to do with how heavy your rocket is, how fast the engine is likely to kick it off the pad.

A basic 3FNC model on a standard A-B-C6 motor (which basically have similar initial thrust spikes) will usually get going fast enough that a couple inches off the blast deflector is OK. A bigger rocket, a Big Bertha or something of that size, with a slower takeoff, is going to catch more of the backwash before it clears the launch rod (which is kind of the point of the 'majestic takeoff' -- the rocket lifts slowly out of the smoke cloud at the launch pad).

My blast deflectors have a little play around the launch rod, so I'll usually tilt the deflector plate down at an angle of maybe 20 degrees in one direction to let the exhaust blast spill off to one side rather than kick back on the rocket.

At some point I'd like to pick up a couple of stove pipe angle joints to use as blast deflectors.
 
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Chrisn

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Estes, Quest, Aerotech or other?
 

jflis

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I've found a couple inches works well most of the time -- a spent rocket casing used as a standoff is about perfect.
That's what I do. I always keep spent motor casings in my launch box. Here is a picture showing the spent casing (without nozzle) that I slide over the launch rod, to hold the model up off of the blast deflector.

This serves 2 purposes:
  1. Keep exhause from balsting back onto the model
  2. keep the launch clips from shorting out on the blast deflector

jim

DSC03232.jpeg
 

JStarStar

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Spent casings or clothespins do the trick just fine. I've also seen people use a wrap of masking tape to do it, but that gunks up the launch rod with adhesive, so I never do it myself.

One thing to remember when propping your rocket a couple of inches up off the blast deflector is that you're also shortening the length of the effective launch rod -- if the rocket is marginally stable you might not want to do that, although a difference of a couple inches isn't likely to make any big difference.

Exhaust backwash onto the base of rockets is kind of an occupational hazard, along with ejection charge soot building up inside the body tube.

One thing that helps also is to wipe the rocket down with a damp (not wet) cloth shortly after recovery. While motor exhaust smoke is still fresh, it usually can be wiped clean if your rocket has a fairly smooth paint finish. If you leave it sit for days (or months) the grit usually seeps into the paint finish and is just about impossible to get rid of.

Exhaust grit also gunks up launch rods, making them sticky -- it's usually good to give the launch rod a quick rubdown with steel wool or a alcohol-soaked cloth after every couple of launches. (Don't use alcohol to wipe off the rocket, it will attack the paint job.)
 
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