Cluster Gap Staging

[EXPjawa]

I realize that this post sort of rambles, so I've edited it to organize my thoughts as clearly as I could.

I'm curious about the reliability of gap staging, when the design uses clustered engines in the booster and a single engine in the sustainer. To be clear, I'm not wondering about staging cluster to cluster - I understand that would be problematic. What I'm think about is this: a booster with a pair of 24 or BP 29mm engines, which stages to a single 24mm BP engine in the main rocket. If the booster has a cluster of three engines that are all side-by-side, then it could readily ignite a single second stage, since the center engine in the cluster is directly inline behind the sustainer engine. However, if the booster had a typical cluster of two side-by-side or 3 or more engines in a grid or circle (so none are on the centerline), than no single engine is directly in line with the second stage engine. So, I'd imagine that would rule out direct staging and taping engines together. I thought, maybe, by gap-staging, the booster engines would be far enough behind the sustainer engine that the combined burn through of the booster engines would ignite the second stage. I'm sure someone here has experimented with this; any thoughts on the matter? Do BP cluster to single staged configurations would without air starting?

The base problem is that in a particular design I was toying with, the sims show that a single BP 29mm E16 can't lift the combine rocket and booster. Two, however, can. I could just scale the thing down - in fact, it would probably do well scaled down by a 1/3 or so, but where's the fun in that?

 

[Peter Olivola]

A radial cluster booster to a single engine, gap staged, should have no problem working. There is also a huge advantage in doing this. Instead of using three -0 motors, use two and one with the shortest delay that deploys a recovery device from that motor tube. Be sure to provide adequate venting to prevent premature separation and taper the space leading to the second stage motor to direct the heat/particles to that motor.

 

[rharshberger]

Ive done something similar in my SA3 Goa a 24mm D12-0 and 2x18mm motors (used for thrust and to eject the rear facing parachutes) to a D12-3 Sustainer. There is about 8 inches between booster and sustainer, with pressure relief holes about an inch below the sustainer motor. The 18mm motors dont have any impact on ignition of the sustainer, and the 24mm booster motor is in direct line with the sustainer motor.

See Emma Kristals presentation video about stagin BP motors, even an offset motor can be staged (from Naram 52 iirc).

https://www.rocketryforum.com/showthread.php?70827-SA-3-Goa-Semi-Scale-2-Stage-with-Cluster-Booster

 

[plano-doug]

I realize that this post sort of rambles, so I've edited it to organize my thoughts as clearly as I could.

I'm curious about the reliability of gap staging, when the design uses clustered engines in the booster and a single engine in the sustainer. To be clear, I'm not wondering about staging cluster to cluster - I understand that would be problematic. What I'm think about is this: a booster with a pair of 24 or BP 29mm engines, which stages to a single 24mm BP engine in the main rocket. If the booster has a cluster of three engines that are all side-by-side, then it could readily ignite a single second stage, since the center engine in the cluster is directly inline behind the sustainer engine. However, if the booster had a typical cluster of two side-by-side or 3 or more engines in a grid or circle (so none are on the centerline), than no single engine is directly in line with the second stage engine. So, I'd imagine that would rule out direct staging and taping engines together. I thought, maybe, by gap-staging, the booster engines would be far enough behind the sustainer engine that the combined burn through of the booster engines would ignite the second stage. I'm sure someone here has experimented with this; any thoughts on the matter? Do BP cluster to single staged configurations would without air starting?

The base problem is that in a particular design I was toying with, the sims show that a single BP 29mm E16 can't lift the combine rocket and booster. Two, however, can. I could just scale the thing down - in fact, it would probably do well scaled down by a 1/3 or so, but where's the fun in that?

If you want to do a clustered booster, one approach is to use a burn string. This holds the rocket on the pad until the center motor is lit. This helps ensure the sustainer lights. That is, with a cluster where the center motor lights the sustainer, the rocket could well leave the pad with the center motor out thereby dooming the flight.

Booster arrangements can vary. You can have three motors in row, or a center motor with several outboards around it. But, in both cases, there's a center motor aligned with the sustainer.

Whether you gap stage or use tape-together isn't so critical (IMO). Done right, they both work very well. I think getting the center motor lit is the most important consideration.

BTW, here's a pic of one of my clustered booster rockets. All four booster motors lit, and all three stages were recovered. More pics here including burn string and cluster info. Doug .

.

 

[plano-doug]

To be clear, I'm not wondering about staging cluster to cluster - I understand that would be problematic.

BTW, that's do-able. The trick is having a break-apart booster. That is, as each booster motor burns out, it and its fin separate from the rest of the rocket without pulling any other motors away prematurely (before they light their respective sustainers). There are more pieces to gather - it helps to have several trackers for such a flight. Here are a few I've built. Doug.

https://www.doug79.com/thridget60/
https://www.doug79.com/thridget70/
https://www.doug79.com/midget80/

.

 

[Handeman]

II thought, maybe, by gap-staging, the booster engines would be far enough behind the sustainer engine that the combined burn through of the booster engines would ignite the second stage.

One thing you have to make sure you consider it that there is NO "combined" burn through. Each booster motor will burn through at a different time and the first motor that burns through will complete the upper stage ignition and separation by itself. The problem with a clustered booster is that you don't know which motor will burn through first, unless you select motor with different burn times and put the short burn where you want it.

 

[EXPjawa]

This mostly confirms what I'd suspected - gapped appropriately, with a sort of funnel to direct the cluster toward the second stage ought to work. I had thought about using a delayed engine and recovery device in one of the cluster tubes, though not necessarily in this context. Though there's no reason I couldn't, you're right. Thanks, all, for the input.