Kevin Wickart's Tech Tips #3

I came to gap staging right from the source: Harry Stine’s article in The Handbook of Model Rocketry. I was skeptical at first, but I needed to use it to accomplish the construction and flight of my 1:13 scale Astrobee 500 for NARAM-38. The third-place trophy collecting dust on my display shelves is testament to the success of gap staging. I’ve used this technique on several other models since, and now when I have a choice, I use gap staging as opposed to direct staging.

Here’s why: In my 33 years as a rocketeer I have flown numerous two- and three-stage rockets. I have had direct staging fail twice, once in competition (NARAM-40, B Altitude). Using gap staging I have built two different three-stage Astrobee 500 models, a two-stage WAC Corporal, and a two-stage dual egglofter. Each of the gap staged models has been flown twice without a single staging ignition failure.

Staging works due to the construction of the booster motors. Unlike the single stage motor, a booster motor has no delay or ejection charge. As the propellant burns through the end of the grain, the burn surface area is increased and the remaining propellant is consumed in a virtual flash. This over-pressurizes the casing, sending a shockwave of hot gas and a shower of burning propellant particles forward. It is these burning particles that enter the upper stage motor nozzle and ignite it.

The reason we have to tape the motors together for direct staging is that, fast as the shower of burning particles is, the shockwave moves at roughly the speed of sound and can blow the stages apart before the particles arrive. The tape prevents separation until the heat of upper stage ignition melts through the adhesive.

Gap staging takes an opposite approach. Rather than fighting the shockwave, the gas pressure is vented out of the space between the motors. When the upper stage motor is ignited, the sudden acceleration of the upper stage separates it from the booster.

The mainstay of gap staging is, of course, the gap (duh!). According to Stine, the gap can be as long as 16 inches with no measurable loss of staging reliability. I am assuming that this holds true only for models where a minimum-diameter tube bridges the gap between the motors. I certainly would not attempt open-air gap staging across that distance, nor try it with 18mm motors bridged with larger-diameter tubing.

If you are building with a minimum-diameter airframe, all you need do is make your booster motor longer than it needs to be for direct staging and cut a pair of exhaust ports on opposite sides of the booster tube between the forward end of the booster motor and nozzle end of the upper stage motor. I usually make the ports about ¼" in diameter, though smaller ports in greater number may be used as well. Bear in mind that since the upper stage motor will also act as the stage coupler, you will need it to protrude further from the upper stage body. I’d recommend at least 3/8" for 13mm motors, up to ¾" for 24mm motors. Take this into account when determining how long to make your booster, and make sure you coat the inside surface of the gap with epoxy to increase the life of the tube.

If you are building a larger-than-minimum diameter model, construction becomes a little trickier. You will be able to use a standard stage coupler, glued into the forward end of the booster body tube. Make the motor tube long enough to extend all the way forward and even with the end of the coupler—this will make mating the stages together much easier.

Cut vent holes in the motor tube as with the minimum-diameter model; you will also need to provide vents for the gases to escape to the outside air. You can simply cut another set of vents into the outer body tube, if you like. If the inherent drag of airframe holes is undesirable, you can cover these during flight with small pieces of cellophane tape which you have "de-stickified" by tacking them to the back of your hand once or twice. The tape will adhere to the body tube well enough to restore the streamlining, but will blow off easily when the pressure wave hits. The other option is to cut sizable gaps in the centering rings to the rear of the motor tube vents, or to use spacer strips instead of rings. This permits the gases to vent out the rear of the booster in the gap between the tubes.

Regardless of which method you use, you will also need to prevent the gases from pressurizing the space between the stages. This is easily done by mounting a centering ring against the rear of the coupler with the motor tube vent holes immediately rear of the ring.

The only concessions in construction you need to make with the upper stage are making sure the rear centering ring on the motor mount is far enough forward to allow room for the stage coupler, and mounting the rear of the motor tube even with the rear of the body tube. The standard motor protrusion will help staging reliability. If you want to use a motor hook on the upper stage, you can cut a small notch in the booster motor tube to accommodate it when the stages are mated.

NOTE: There has been some question of whether the vent holes should be cut closer to the booster motor, closer to the upper stage motor, or more toward the middle. What little experimental data there is suggests that there is no measurable difference in reliability for any configuration. Personally, I cut the vent holes as close to the upper stage motor nozzle as possible. My reasoning is that vent holes cut further to the rear may create a lower-pressure area behind the cascade of hot propellant, thus reducing their velocity and chance of igniting the upper motor. With the holes directly behind the upper motor nozzle the pressure wave continues to accelerate the particles, venting just before impact. Again, this is based on pure theory, not on experimental data.

Some scale models such as the Aerobee series use a booster that is physically separated from the upper stage by open air. I have seen several of these models flown successfully, gap-staging across the intervening space. The technique is not for novices, though it’s well worth the effort.

Construction is very much the same, with a few inherent difficulties. The booster motor tube should be extended as far forward as possible without compromising the scale qualities of the model, and the gap should be as small as possible. There is, to my knowledge, no research on "open-air gap size vs. ignition reliability," so hedge your bets on this. Just as a rough guide, I wouldn’t use a gap longer than about six inches.

Coupler fit between the booster and upper stages is critical! The staging charge blasting backward will tend to separate the stages, possibly before the upper stage ignites, and the coupler must fit snugly enough to prevent this. On the flipside, if the coupler fits too tightly, the stages won’t separate at all. This condition results in much-reduced thrust as the upper stage motor pushes back against the dead booster and crispy-fries it.

In addition, the staging blast can cause a drop in the model’s velocity visible to the naked eye, so bear this in mind when building in those stability margins. This same blast can cause burn damage to the upper stage, and in return the upper stage motor can cause some pretty impressive damage to the booster. So take measures to protect them both.

As mentioned before, the hot gases and leftover particles of burning propellant—as well as the backblast from the upper stage motor—can tear up the inside of the booster stage and blister the daylights out of any paint on the body tube (especially bad for scale models!). Unless you are building a one- or two-shot competition model, your rocket must have some protection against this internal inferno.

The standard method is to coat all surfaces exposed to the venting gases with epoxy. This is a painstaking procedure, but one which works quite well. It does not, however, do much to protect paint from heat-blistering unless you put on a thick coating.

A second method is to use a piece of bond typing paper as a temporary inner liner. The hot particles will expend their fury on the paper and largely spare your poor tube. Between flights you merely remove the old liner, replace it with a new one and fly again. This method offers a bit more protection to the paint by ablating the heat away, but does not entirely protect the tube when larger burning chunks burn clear through the liner. Also, you can’t use it to protect the outer body tube on a larger-than-minimum diameter model since there’s no way to insert it.

A good compromise method (especially for those outer body tubes) is to use epoxy to bond a layer of aluminum foil to the inside surface of the tube. The foil doesn’t burn through as easily as paper does, dissipates the heat by quickly conducting it away from the source, and if it does burn through there’s still a layer of epoxy to contend with. It is, however, more difficult to build into the model. Jim Fackert of Totally Tubular now sells heavy-walled tubes with an inner layer of foil. These are ideal for gap-staged motor tubes but not for outer body tubes (since it’s very hard to glue anything to the inner foil lining).

For comparatively short and lightweight booster stages, recovery isn’t a problem. With a spent casing in it, even a moderately long booster stage will tumble or settle into an "Alway glide*" for a safe landing. But longer boosters will tend to streamline in, creating a safety hazard; heavier boosters are a safety hazard whether they tumble or not. In these two cases, safe recovery must be established.

Booster recovery is an article unto itself, and one that I will address at a later date. Suffice to say that it is imperative that you determine the mass and stability characteristics of any booster stages, and provide for safe recovery. I was not aware until after construction was completed that the first stage of my 1:8 scale Astrobee 500 (see the November ’97 Woosh Pop) weighed in at over six ounces with an empty casing in it. I had to devise a recovery system based on the existing construction; it works, but I’m not very happy with it.

Flying a basic (i.e. no booster recovery system needed) gap-staged model is a delight. All you have to do is install the motors and igniter, insert liner paper (if you’re using it), and go! I recommend highly using some sort of lubricant—my favorite is baby powder—on the stage coupler and/or inside of the booster motor tube where it contacts the upper stage motor. This will help prevent the stages from sticking together.

For maximum reliability, choose an upper stage motor with as large a nozzle opening as possible to increase the likelihood of burning particles entering. It’s a good idea to gently ream the inside of the nozzle with a small drill bit or the business end of your launch controller safety key to remove the light coating of clay that is sometimes present from the manufacturing process.

Gap staging has opened up several new avenues of rocketry for me, and I encourage everyone to give it a try! As always, you can e-mail me with questions or comments via the links on this site.

*Alway glide: n. a recovery technique by which a long, slender rocket (generally with the nose ejected on a streamer or parachute) returns to earth in a poor—but unmistakable—glide, supported in "flight" by the body tube cross-section and its rather undersized fins. In recent years this recovery has been accepted as a qualified glide for competition, under the "self-penalizing" rule. It was named after the technique’s most avid and prominent practitioner, Peter BMB Alway.