Gap staging and clusters

motor booster to a single motor >sustainer?

Lots of variables here, need more info. It's hard to say exactly without seeing the vehicle. You didn't mention the distance between engines but they want to vent the bt or stuffer, as the case maybe and a put a small amount of tape on the joint. Early separation is their biggest concern.

Randy

I know you know and this and it may seem obvious, but there may be some launch variables as well. Increasing power may mean they need a longer, larger diameter rod, stronger pad, etc. I only mention the obvious because it is the easiest to over look. Trust me I've done it plenty of times. ; )

Randy

Think bulkheads in two of the booster tubes. When the thin wall of remaining BP propellent starts breaking through on the booster motors the transition may become over-pressurized, separating the stages before the sustainer is ignited. Having solid bulkheads in two of the three booster motor tubes will alleviate the problem and the sustainer should still light nicely.

John

could your possibly use a transition cone from the bottom 3 motor cluster to direct all the particulate into the upper single motor nozzle and have a

1" extension body tube the same diameter as the upper stage with vent holes...to prevent blow off...

say bt60 to bt20 transition made from balsa or paper and hollowed out? with a 1" body tube on top? coated with epoxy? aluminum foil perhaps?

shockie B)

John, could you clarify this?

Why would you get an overpressurization on multiple boosters if one is venting just like one does for a single booster? [I assume since the team is "having good luck with their gap staging" that they are already venting]

I could see that the booster stage might still be under thrust from the 2 non-blown-through booster motors at the time that the sustainer is ignited (from the third), but I wouldn't think this would be problematic.

Or am I missing something?

Coating with ca will do the same with zero weight gain or build up.

Randy

My concern is that the size of the vent hole(s) required is probably an exponential function as the number of booster motors is increased. In a KISS moment, if they are having good success gap staging from a single motor, then adding additional motors to a cluster booster can be made transparent to the staging process if only one of the motors has a direct path to the sustainer.

OTOH, if that particular motor fails to ignite at launch it's gonna be a bad day in rocket city :-(

John

Thanks for explaining. At some point, I plan to do a gap-staged cluster-to-single, maybe eventually a gap-staged cluster-to-cluster, but I will go with using all boosters to ignite the sustainers in order to increase the probability of that ignition. I just HATE it when the upper stage comes in ballistic. :^)

Anyone ever tried gap-staging a single booster to a multiple sustainer? That would be sweet.

You may find something useful...........

here:

here: ( cluster article with photos & a flight report)

here:

If nothing else, it makes interesting reading for a few minutes.

Randy

Thanks Randy,

I've followed your previous threads on this rocket - very cool. What amazes me is that your gap staging at the first level (the 5 to 5) is entirely independent - you depend on all 5 motors to have burn through within a sufficient narrow "window" so that all the middle stage motors have a chance to light up. I wouldn't have thought that approach had a prayer, but seems to work. I guess the "pause" during staging is longer than the typical variation in motor-burn duration.

motor-burn duration.

The critical thing IS that timing or pause. It's only micro seconds most of the time but the pause is always there and even engines from the same pack do not burn through at the same rate. I have tried to minimize the time frame by using only new engines and from the same pack & lot. Strategically placed vent holes and a small piece of tape on the joint helps hold it long enough to allow the others to lite.

Something we learned early on with the first 5 engine cluster, is how much pressure there is from the burn through. Our original flight was with a D12-5 and 4 C6-0. The burn through from the 4 C's was enough to mimic staging, it actually looks on film like there was a strong kick, as though several engines of a second stage had ignited.

The surprising thing to me is that when flown with the same delays on all 5 engines, no matter the delay, there is more "ejection" from the burn through of 4 engines, than there is from 5 engines, with the same delay and it happens that way every time.

When we fly the 5 engine cluster version on 1 D and 4 A's, separation happens low enough that you can always count 5 separate delays. They're rapid but never one big whoosh pop. There is always some micro second time delay between them. We have 5 pops in less than 1 second but separate none the less.

Due to all of this, I would suggest they use a set up that allows the boosters to vent separate from the sustainer and use only one to one. In their case, I would NOT try to use 3 to lite 1.

Randy

That's what I was thinking when I first replied to this thread last night with the caveat that I don't believe its necessary to vent the booster(s) that are not intended to light the sustainer. A simple solid bulkhead of balsa (I'd CA the motor-facing side) will withstand the momentary pressure spike when the booster propellant breaks through, and serve as a motor block at the same time.

John

Be advised that if you block the engine tube...the pressure release from the booster motor at burnout WILL kick the motor out of the mount...even if you use a motor retaining clip. I learned this on my two-stage Fat Boy. You will want to vent the engine tubes so if you block the top...vent out the side. I use a 1/4 inch diameter hole and this stopped the motor kick problem.

John

Yep.

Randy

Sorry to chime in late, been off line for a bit.

There is no inherent problem with three boosters staged to a single sustainer. But, there are a number of events that need to be considered. Any design that depends on only one of the boosters to light the sustainer is asking for a lawn dart.

The first booster to burn thru will usually be enough to light the sustainer. But at the same time, the sustainer will remain in place longer than normal, since the first stage is still under thrust from the other boosters.

It helps to have a convergence tube that directs the burning particles towards the sustainer nozzle. That will also make the top of the booster last a bit longer.

Yes, the convergence tube does need some re-inforcing to protect it from the heat and pressure. Thin CA works well for several launches, aluminum can (not foil) works for dozens of launches. The top of the stage should also be reinforced, it will take quite a bit of heat on the outside as it will tend to follow the sustainer, not drop away immediately like a single engine booster does.

The heat is higher than normal staging, since the sustainer does remain in place until it comes up to enough thrust to overcome the remining boosters, instead of popping off as soon as it pressurizes the gap area.

The pressure will also be higher than normal at that time, and may climb much higher suddenly, if the other booster motors burn thru during the thrust buildup.

Every now and then you'll get really "lucky" and have all three burn thru at the same time, the pressure then will be very high, possibly high enough to destroy the top of the stage and push off the sustainer before it lights, Bigger vent holes might help, possibly covered with a single layer of masking tape that can burst to relieve the pressure after the initial spike (haven't tried this yet).

The other booster motors are very likely to burn thru right as the sustainer comes up to pressure, since the heat and particles in the gap/convergence area may break the clay and light the top of the other booster motors. This may also happen if one of the boosters fails to light, it may be lit at the top as the other boosters burn thru, toasting the stage. Used to be very common with early clusters like the Ranger, before good ignitors were available.

On single stage clusters you can put a strip of masking tape over the top of the boosters and usually prevent this type of problem, but it's not a good idea to have non-burning tape in the gap when you're trying to light the sustainer.

I haven't read the rules this year, would parallel staging/drop off boosters satisfy the requirements?

Bob Ellis