Two Stage Question

[Feyd]

I recently crashed a rocket and I want to rebuild it as a two stager. The problem is I want to reuse the quad canted 18mm engine mount as the booster and make the sustainer a single 24 mm engine mount.

How close to the top of the booster does the 24mm engine need to be in order to stage correctly?

Would putting a tube coupler, sanded for a very loose fit work to keep the stages together?

[jharding58]

I am sure that there will be other opinions but I would imagine that having a 24mm booster motor reliably ignite four 18mm motors without a direct gas connection might be a little too much to ask. I would think that the over pressure would eject the booster before all four would ignite. I am not even sure if gas staging would get all four. Perhaps electrical ignition might be more reliable?

Tim Van Milligan has done some great research and published some FAQs on the Apogee website also.

[Feyd]

Actually, it's the other way around. The booster would be the Sunward 4-engine canted mount. What I'm thinking is the sustainer could be a 24mm single engine mount.

[tbzep]

You would probably need to use a tube to channel the boosters to the 24mm sustainer. The tube needs to be vented to allow pressure to escape and not pop the stage before ignition. Gap staging can be done over a considerable distance, but reliability is worse the farther away you get. How far are you wanting to separate the stages?

[Feyd]

That was the essence of my question... how much space would I need to get a reliable ignition? Since the booster engines are canted, the tops of the engines are all right there in the middle - which would be just below the single 24mm engine.

[blackshire]

In his "Handbook of Model Rocketry," G. Harry Stine wrote that he got 100% successful gap-staged second stage ignition with motors spaced as far as 12 inches apart. His test rockets used single first stage and single second stage motors (which I presume were 18 mm standard motors because he did not mention their size).

Four 18 mm booster motors erupting forward into the nozzle of a 24 mm motor should (especially if they blast into a "stuffer tube") provide plenty of burning propellant particles to ignite the 24 mm motor. If the interstage was completely open to the atmosphere (with a, say, 1/2" gap between the upper end of the stuffer tube and the nozzle of the 24 mm motor), it would vent the pressurized gas from the four 18 mm booster motors, to prevent the overpressure from blowing the stages apart before the burning propellant particles could ignite the 24 mm second stage motor.

Since G. Harry Stine's gap-staging worked at "inter-motor distances" of up to 12 inches, this distance would certainly work for the four 18 mm booster motors blasting upward into the larger nozzle of a 24 mm motor. It would almost certainly also work at greater distances between the motors, but only experimentation could determine the maximum reliable staging gap.

[tbzep]

That was the essence of my question... how much space would I need to get a reliable ignition? Since the booster engines are canted, the tops of the engines are all right there in the middle - which would be just below the single 24mm engine.

If it is open air staging with nothing to guide the boosters' hot gases and particles, you would want the 24mm nozzle to be at a distance where the boosters are pointed directly at it. If you use a vented stuffer tube, the distance could be just as long as it would be using a single booster.

My son and I have had 100% success with both regular gap staging and open air gap staging, but we haven't tried it with clusters.

[Mark II]

As others have said, you might consider installing a slightly funnel-shaped stuffer tube (or "chimney") in between the tops of your four boosters and the nozzle of your 24mm sustainer motor. Since the booster motors are angled and are not directly in-line with the sustainer motor, this chimney will help to "gather up" the forward jets of burning material from the booster motors and focus them directly at the upper stage motor. You will need to put a few side vents into the chimney so that the jets don't overpressurize the gap and force the stages apart before they have been able to ignite the upper motor. The vents will bleed off some of the gas pressure without diverting the flame away from its target. Even if your interstage gap is very small, installing a chimney is a good idea to focus the flame jets.

This chimney also does not necessarily need to be a permanent installation. You could roll a short cone out of a couple of layers of cardstock, punch 2 or 3 vent holes in it with a hand punch and then just friction fit it into the top of the booster, positioned correctly to span the interstage gap. After one of two flights, you could simply replace it with a new one, since it would probably get thoroughly scorched with use.

[jflis]

Not sure if it would help, but back in 2003 Jason Toft submitted a Terrier Sandhawk (http://fliskits.com/services/dom/2003/11-november/toft.htm) design to the FlisKits Design of the Month contest that had a 3 motor cluster in the booster with a single motor sustainer with a separation of 12 or so inches.

Perhaps there is some information there that could help?

[Feyd]

Thanks, everyone, for your suggestions. I'm going to look at this some more and gin up something in Rocksim to figure out options. I am planning on attaching streamers to the booster to ensure its safe recovery and I need to figure out fin sizes and shapes. I'll start posting pictures when I get started.

Thanks again!