I'll start off by saying my experience in 2-stage flights is exactly one flight, but it was a successful flight, so that makes me an expert, right?
(
PLEASE note the sarcasm here) By the second week in January I hope to be able to say 2-out-of-2, have a J394G3-to-I255RL lined-up for my second flight, was going to fly it earlier this month but the RRC3 in the sustainer threw a fit on the pad with ~20 minutes left on the waiver, so I packed things up at that point.
Anyway, my thread for my semi-custom 2-stage Osprey 75 is here.
Oh, and I think "Red Stage, Blue Stage" is a fantastic name.
Me, I'd be shooting for red motors in the booster and blue motors in the sustainer, also.
An Eggtimer Quantum, located in the sustainer, will light the sustainer.
An Eggtimer Quark, located in the nose cone, will handle deployment for the sustainer. I'm thinking I'll either use a Cable Cutter or a Chute Release on the main chute to reduce the drift on the sustainer.
If needed, I have a custom-build GPS tracker that I can also mount in the nose cone.
The booster will drag separate or separate when the sustainer motor fires. I'm going to line inside of the inter-stage coupler with thinned JB Weld to protect it from heat damage.
The booster parachute will be deployed using the booster motor. Depending on the booster motor, I will use a Chute Release on the booster parachute.
So if I got that, the sustainer has a single separation point, so you have electronics in the nose and are basically electronic backup for a motor eject sustainer (or perhaps vice-versa), with a Chute Release (or similar) doing the actual main deployment.
I've been looking at my risk factors and risk mitigation and here's what I have so far:
Risk: Quantum fails or sustainer doesn't light - Mitigation: Quark still deploys drogue and then main
Risk: Sustainer lights and Quark fails - Mitigation: Upper stage motor ejection deploys drogue - no ballistic reentry
My main concern here is whether motor eject will even be an option in the sustainer. It totally depends on the motor combos you're looking at, but in my research for my 2-stage rocket motor combos I've found that the coast phase for the sustainer is typically longer than the motor is designed for (since the motor assumes its starting from zero, but the sustainer will be going [much?] faster when the sustainer motor lights), so even at the longest (undrilled) delay the sustainer's motor could be firing before apogee. Now perhaps if you're only popping the nose and the chute remains reefed maybe that won't zipper, but it seems risky to me.
Also, depending on the altitudes (motor selection) and where the Chute Release ends up in the airframe, you might see some of what I know as 'piston effects', where the motor acceleration causes the column of air in the parachute section to shift up/down creating localized areas of higher or lower pressure, so I'd wonder if, depending on your motor choices and the altitude of first-stage burnout and second-stage ignition, if the CR might be fooled into seeing the ejection altitude inside the rocket. Worst thing that typically happens here is the main comes out at apogee, if the rocket isn't going too fast that should be fine it just might be a longer walk. If the motor eject goes off early it could be worse. I flew a CR in my 2-stage but it was in the booster, so it only saw one motor ignition/burnout. Better ventilation in the payload section (holes near the top and bottom) can help reduce/eliminate this.
kswing said:
Risk: Booster doesn't drag separate - Mitigation: Upper stage ignition will force separation
I guess a lot of (some?) people do this, I've designed both of my 2-stagers to have an ejection charge in the ISC forward bulkhead. I just didn't like the thought of the motor coming up to pressure while it's still enclosed in the ISC, so if there wasn't a drag separation the motor might over-pressurize? My 3"-to-3" rocket didn't separate until the charge fired, and it was
not a tight fit between the ISC and sustainer. My second 2-stage is 4"-to-54mm, so it seems like there will be more drag with the transition, when it was just the large Osprey 75 fins creating the drag that wasn't enough for my rocket to separate. Since you're also same size booster/sustainer I wonder if you'll see any drag separation.
kswing said:
Risk: Booster motor ejection fails - Mitigation: None - acceptable risk - low probability and booster should be mostly unstable with large fins and no nosecone
This part scares me personally, but once you've bit the bullet for an ejection charge in the ISC that means electronics in the ISC, so having that electronics handle both separation
and apogee deployment for the booster becomes pretty simple. So for me it was a no-brainer since I was going to have the electronics there anyway. I had also decided to have a tracker in both stages, largely because the experienced 2-stage flyer at our launches was always asking for help with keeping an eye on the booster so that he wouldn't lose it, I decided to fly trackers in both parts so that I'd never be wondering where the booster wound up. =)
kswing said:
I may try to configure the second channel on the Quantum to act as a tertiary backup for the main deployment.
I think you may find that this option is necessary since the motor delay may not be able to be made long enough, so it may be your only viable backup.
kswing said:
So far I'm planning to use a Blue Tube coupler to join the booster and sustainer and my design has it going about 3" or 3.5" into the bottom of the sustainer. I'm planning to have 3 centering rings in the sustainer and recess the lower centering ring by 3.5" so it won't interfere with the coupler. I'm basing this on the avionics bays I see for sale and they often seem to be 8" long (for a 4" rocket) with a 1" switch band, so, that would be a 3.5" overlap.
For the 4" kits I have (Binder Excel w/DD, RW X-celerator) the couplers have been 14" and 9", with 4" and 1" vent bands, respectively. The former is big, you glue 2 7" couplers together at the 4" vent band, and end up with 5" overlap on each end (though I had my vent band lengthened an inch so that an Additive Aerospace Mobius camera shroud would fit on the band, leaving 4.5" on each side), the X-celerator was clearly designed for a 1" vent band and 4" of overlap on each end. I've heard on this forum multiple times the recommendation that the coupler overlap always be at least one caliber, my RW Double Shot was definitely designed so that the ISC would go just over 54mm into the 54mm sustainer, when I designed my 2-stage Osprey 75 I similarly designed for a 3" mate in the 3" sustainer. Now as to whether 3.5" or 3" is 'close enough' to 4" that it works I can't say, I'd imagine it also depends on the flexibility of the tubing (and how tight of a fit between the coupler and tube) and I have absolutely no experience with Blue Tube, both of my 2-stagers are fiberglass. Keep in mind that you have one rocket pushing another through the ISC, so there's a good bit more weight (and a bigger moment arm) resting on this coupler than in your typical avionics bay/nosecone coupler, so if the sustainer wants to tilt during booster firing the ISC is the only thing keeping things straight.
