Thanks David.
On your example above,
- What is the distance from the ejection end of the booster motor to the nozzle of the sustainer motor?
- Is the motor tube a BT-50?
I ask because one would think that the calculation for sizing the vent holes should take the total trapped air volume into consideration.
Make sense?
Updated based on input in post #4 by
@David Schwantz
View attachment 491695
I will try not to mix up BOOSTER and SUSTAINER here!
I am curious where you get the P2 number “Pressure Increase Due to Booster” as I expect that to depend on differences not just of motor SIZE but also manufacturer (Estes BP motors’ ejection charges often described on this forum as “shotgun” power, presumably much more energetic than the older Quest Motors), diameter of the tubing BETWEEN the booster forward end and the sustainer nozzle (again IMO if you aren’t going minimum diameter you SHOULD extend the mount forward nearly to the nozzle of the SUSTAINER motor) and the length of the tube. For anything less than the shortest gap I expect the residual volume within the motor casing forward of the propellant would be negligible.
Where did you get the 0.018 PSI number?
also, since the model pic I posted, a WAC Corporal on a Tiny Tim booster, has a (for all practical purposes) completely open chamber between the booster and sustainer I think I have demonstrated you cannot make the holes too big.
yes, you COULD make the holes too small, but that begs the question, “who cares?” Meaning just put two Hole punch size holes in for anything other than the longest gaps (over 48”, and to my knowledge nobody but me is currently nutty enough to do this) and call it good. True, if you put the holes in the OUTSIDE (which is your only option on minimum diameter if you have any significant gap) than “too large” a hole does reduce efficiency (max velocity and altitude) by unnecessary drag. However, if your goal is maximum efficiently using black powder motors, first you don’t want to use gap staging, you want the shortest booster you can get away with and do direct staging which needs NO holes. Second, if you are going non-minimum diameter, your best bet is to use a chimney or stuffer extended motor mount to get close to the SUSTAINER nozzle (yup, checked, SUSTAINER, lol), then vent BACKWARD between rings, with holes or notches in rings as large as possible (again, limits are structural integrity of the rings, which I think means you have LOTS of leeway to go large), so again the argument seems moot, make big holes.
my 50+ inch gaps stage used BT-50 outer tube with a 24mm D12-0, I used a BT-20 chimney to duct the gas forward to an 18mm A8-3 mounted in a BT-50 tube. I used three paper punch holes.
@jqavins , I think you mentioned in a post the same idea of using a narrower chimney or stuffer than the actual motor mount, like I did above. I obviously agree with you, as it is what I routinely do. It reduces the VOLUME of dispersal of the ejection charge between motors, hopefully increasing the chance of enough hot gas reaching the SUSTAINER (yes SUSTAINER) nozzle. As well as guiding the gas nearly directly to the nozzle. When I get to gapping over 15 inches I start thinking this way.
for relatively short gaps, again IT MAY NOT MATTER, as the WAC Corporal/Tiny Tim combo works with essentially a wide open space between the motors, but I think when you get beyond say 15 inches you have to start working a bit harder. I do 15 inches of gap using 18mm motors routinely with minimum diameter with no chimney or stuffer, without problems. I punch two regular hole punch holes on each side of the forward end of the BOOSTER, just aft of where the SUSTAINER motor nests.
A caveat, when your ejection charge from one size motor is vented or ducted by a smaller diameter tube, my experience has been HEAT is heavily concentreated on the first few inches just forward of the BOOSTER (yes BOOSTER) motor. I have experienced burn through here, and the bad thing is it is internal so you can’t see it. my experience has been it can happen fast, after only 1 or 2 flights. My routine now is to put rolled up aluminum can glued in here and smear epoxy on the adapter ring to prevent burn through. So far no problems.
for anyone thinking of gapping longer than 6” or so with black powder, you have to consider a serious second problem, booster recovery. Short boosters recovery by tumble classically, I sometimes use horizontal spin up to 18”. Beyond that length (and likely anything over 6” if you DON‘T use horizontal spin) the BOOSTER is likely STILL stable AFTER SEPARATION (not a good thing) and often will come down ballistic, nose first. Not as dangerous to person or vehicles or other property as there is no nose cone, but as you can imagine not so good for the booster (and for those at club launches, earns you a probable stink eye from the LSO!)
anybody interested PM me for ideas on solving this issue for non-electronic black powder extreme long gap staging booster recovery, I have a few solutions, there are likely many more and better ones I haven’t thought of.