Upper stage ignition

[Steve Osmanski]

I am scratch-building a sport-scale Delta IV Heavy using BT-60 body tubes. This will put the upper stage motor (a C-class black powder engine) about 25 cm from the top end of the central booster motor (also C-class). I am thinking of using direct ignition with an inner tube (18 mm diameter) to carry the ignition gases from the booster motor to the upper stage motor.
Even with vent holes in both the inner tube and body tube to allow the hot gases to vent the trapped air, will the ignition gases retain enough energy to start the upper stage? Does anyone know what the maximum distance is for one BP motor to start another?

 

[rharshberger]

10" is considered to be a reliable distance for gap staging, however it has been done up to something like 32" or so.

 

[BABAR]

I don’t know the maximum. I have done 51 inches black powder gap staging successfully in flight. I have done 72 inches on a test stand.

Do you have a recovery device for the BOOSTER?

 

[Steve Osmanski]

The two side boosters will be strap-ons, like the real thing. There should be enough space between the inner and body tubes to pack a chute. What I'm not sure about is whether the air escaping from the inner tube (ahead of the ignition gases) will have enough force to deploy a chute.

 

[BABAR]

I have looked at pics of the Delta IV and I am struggling a bit to understand your project.

What I have is as follows:

At launch you are using a cluster, with a center motor and a booster on each side. I will call the center motor C1.

Boosters will presumably have zero delay engines, will release from the center rocket at burnout, and deploy their own parachutes.

So far so good.

Here is where I am a little foggy. Are you try to black powder stage the C1 motor to an UPPER stage motor? My fog is from looking at pictures and not seeing any fins on the upper stage (logical for the REAL Delta IV, since commercial and NASA lift vehicles are gimbal guided and don't use fins---- but something of a problem for hobby rockets------ aside from spin stabilized and fat bottom base drag rockets, we NEED fins.)

In any case, black powder staging (IGNITION) is pretty easy over the distance you are talking about. BUT, IMO you cannot use the ejection charge of C1 to BOTH light an upper stage AND deploy a chute (Apogee has a model which begs to differ with my opinion.) With no fins at all, your C1 motor section will however tumble recover which if it is light enough should be okay. If you can tuck a small streamer between your upper stage engine and the BT-60 tube that EASILY pulls out at separation, wouldn't hurt.

As I said, my confusion is, "where are you gonna put fins on the upper stage that C1 lights?"

Maybe you could post a pic?

 

[Steve Osmanski]

The fins will be (still in the planning phase for this rocket) clear acrylic, mounted on the upper stage and the central booster. These fins are indexed 45 degrees to clear them from the strap-on boosters. To provide enough fin area, each strap-on will also have a single fin at 90 degrees.

Here is a 3D view from RockSim.


Recovery of the central booster is something I'm not sure about yet. I'm planning to use a test stand to see if the air the ejection charge forces out of the inner tube will be enough to deploy a chute from the central booster. If that doesn't work a small ejection charge, loaded into the gap between the inner and body tube, activated by timer, will probably have to be used. If that's not possible I may have to give up on this idea.

I'm trying to teach myself model rocketry by re-creating the Delta IV Heavy in several different sizes. I've built and flown a minimum-diameter model using A10-3T engines in the strap-ons, A10-0T in the central booster, and A3-4T in the upper stage. The strap-ons were designed to remain attached, and recovery was by streamers in the strap-ons and the upper stage. The rocket we are discussing is the next step. Eventually, I want to do this with a high-power rocket, but that's years away.

 

[adrian]

If you have C6-3 in the side boosters and C6-0 in the central, then the upper stage, if it ignites at all, will do so before the side boosters detach. So the whole assembly of central booster plus both side boosters will detach as the upper stage ignites, then the side booster ejection charges will fire and the side boosters will separate while the whole booster assembly is on its way down.

I'd put B6-0s into the side boosters. Although it has no official ejection charge, the forward pressure at burnout is enough to blow off a loose nose cone and eject a streamer (or to blow off an entire upper stage, which is why you need to vent the gases or tape the booster and sustainer engines together to prevent the upper stage from separating before it can ignite). I have a parallel-staged model of my own which relies on this fact to detach its boosters and eject streamers while the sustainer continues on its way, though the sustainer is not staged.

As for recovery of the central booster, here's what I did for Rheinbote, the four-stage rocket which you see in my profile picture:

At the top of two of the boosters is a thinner tube through which the burning particles can pass to ignite the next stage. Venting is through the rear; the motor mount is centred by strips of balsa, not centring rings, so the gas is not confined. Wrapped round the thin tube at the top is a small Nomex streamer. This has to be fireproof because any wadding wrapped round the streamer will disappear the moment the next stage separates, leaving the streamer right in the exhaust of the next stage. (I tried it once with a mylar streamer wrapped with wadding, and switched to Nomex after seeing the crispy remains of the mylar. ) So the streamer isn't long enough to be a proper recovery system but is enough to destabilise the booster, making it tumble rather than dive down balllistically. (You have room for a longer streamer because your outer body tube is BT-60, while mine is BT-55.)

 

[BABAR]

If you have C6-3 in the side boosters and C6-0 in the central, then the upper stage, if it ignites at all, will do so before the side boosters detach. So the whole assembly of central booster plus both side boosters will detach as the upper stage ignites, then the side booster ejection charges will fire and the side boosters will separate while the whole booster assembly is on its way down.

I'd put B6-0s into the side boosters. Although it has no official ejection charge, the forward pressure at burnout is enough to blow off a loose nose cone and eject a streamer (or to blow off an entire upper stage, which is why you need to vent the gases or tape the booster and sustainer engines together to prevent the upper stage from separating before it can ignite). I have a parallel-staged model of my own which relies on this fact to detach its boosters and eject streamers while the sustainer continues on its way, though the sustainer is not staged.

As for recovery of the central booster, here's what I did for Rheinbote, the four-stage rocket which you see in my profile picture:
View attachment 364901 View attachment 364902
At the top of two of the boosters is a thinner tube through which the burning particles can pass to ignite the next stage. Venting is through the rear; the motor mount is centred by strips of balsa, not centring rings, so the gas is not confined. Wrapped round the thin tube at the top is a small Nomex streamer. This has to be fireproof because any wadding wrapped round the streamer will disappear the moment the next stage separates, leaving the streamer right in the exhaust of the next stage. (I tried it once with a mylar streamer wrapped with wadding, and switched to Nomex after seeing the crispy remains of the mylar. ) So the streamer isn't long enough to be a proper recovery system but is enough to destabilise the booster, making it tumble rather than dive down balllistically. (You have room for a longer streamer because your outer body tube is BT-60, while mine is BT-55.)

I like this idea!

Since the fins aren’t to scale anyway, sweep the angle of the trail edge of the central booster slightly forward. Your streamer will likely make the booster fall tail first, and if you allow your motor to stick out say one or two cm, the motor will take the brunt of the impact instead of the fins.

 

[Steve Osmanski]

I plan on the side boosters burning out and detaching first, and then having the central booster ignite the second stage. I'd not considered the risk of damage to the central booster recovery system from the 2nd stage motor. Sounds like some kind of delay will be needed to allow the 2nd stage to ignite and get clear before the center booster recovery system deploys. Or I use a Nomex system, like yours.