Chapter 17. Multistage Rockets
Staging is a common concept in rocketry. Using one powerful motor to launch a large rocket has mass penalties: the rocket tube must be thick enough (and therefore heavy enough) to withstand the thrust of the large motor. By stacking multiple rockets on top of each other in stages, you can pack more power into a thinner (and lighter) tube.
The concept of staging is very simple: a multistage rocket is launched using only the motor in the first stage. When the first motor burns out, the rocket drops the weight of the entire first stage—motor, fins, and tube—leaving a smaller rocket to continue the journey. The smaller rocket fires, taking advantage of the speed and altitude provided by the first stage to fly higher and faster than it could have flown on its own. You can do this more than one time. In model rocketry, two-stage rockets are common, but three-stage rockets are not unusual, and you can use even more stages.
This chapter shows how it’s done. You will build Romulus, a two-stage rocket that can fly to altitudes of 1,460 feet, even though it is a big, impressive rocket.
The Two-Stage Romulus
You can fly Romulus with any combination of an A8-0, B6-0, or C6-0 motor in the first stage and an A8-5, B6-6, or C6-7 motor in the second stage. Table 17-1 shows some of the more common combinations and the calculated maximum altitudes. Table 17-2 lists the parts you’ll need to build Romulus.
| First stage | Second stage | Approximate altitude |
A8-0 | A8-5 | 270 ft |
A8-0 | B6-6 | 500 ft |
B6-0 | B6-6 | 680 ft |
C6-0 | A8-5 | 800 ft |
C6-0 | B6-6 | 1,040 ft |
C6-0 | C6-7 | 1,460 ft |
| Part | Description |
BT-55 body tube, 18” | |
BT-20 body tube, 2 3/4” (2) | You will need two motor mounts. The Estes Designer’s Special has precut white ones; it’s also fine to cut them from a longer length of BT-20 body tube. |
BT-55 size nose cone, 5” | Any BT-55 nose cone of balsa or plastic will do. The nose cone shown is from the Estes Designer’s Special; similar nose cones are available from many sources. |
3/32” balsa fin stock; about 4” x 12” | The balsa wood will be used for the fins. |
1/8” launch lug | You will use two pieces. Any length from 1” to 1 1/2” is fine; the photos show a 3” launch lug cut in half. |
BT-55 tube coupler | This is used to join the two stages. It needs to be 1” to 1 1/4” long. |
Engine block | This is usually a short, dark gray tube that slides into the BT-20, about 1/5” to 1/4” long. Anything of similar size that will slide into the BT-20 will work. |
BT-55 to BT-20 centering ring (4) | These are used to mount the BT-20 motor mount tubes in the BT-55 body tube. |
1/8” shock cord | You will need about 30” of shock cord. |
15” parachute | There is a nice 15” parachute in the Estes Designer’s Special, or you can make one following the instructions from Chapter 3. |
Snap swivel (2), optional | For attaching the parachute and shock cord. |
How Staging Works
The rocket motors you have used up to this point all work essentially the same way. There is a propellant charge that provides the power to push the rocket into the air. This is followed by a smoke delay that gives the rocket time to slow down. Finally, the ejection charge fires to deploy the recovery system.
The first stage is different in a two-stage rocket. It doesn’t have a smoke delay or ejection charge. Instead, just before the propellant burns out, it shoots burning particles of propellant into the empty space above the motor. This burning propellant ignites the second-stage motor. The hot gases shooting out of the base of the second-stage motor push the first stage away from the rocket, and the second stage continues on. The first stage has literally given the rocket a boost, lifting it to a higher altitude and giving it a high initial speed, so the second stage travels higher and faster than it would have if it were launched on a single motor.
The second stage of a two-stage rocket works just the same as a normal rocket, although the smoke delay is generally a bit longer because the rocket is expected to be traveling faster than usual.
Romulus is a classic two-stage rocket that uses a tube coupler to hold the stages together, so the second stage slides smoothly onto the first stage.
The thrust of the first stage is enough to keep the pieces together until the propellant burns through. That’s probably all it will take for enough burning black powder to ignite the second-stage motor, but it’s still a good idea to tape the two motors together to increase the chance that the first stage will ignite the second, and not just blow it off. A ring of tape completely around the motors does the job nicely. You will see details on how to apply the tape in the launch instructions at the end of the chapter.
Once the two stages separate, the first stage is unstable. It tumbles, which messes up the smooth air flow past the fins. That’s a good thing, as it means the first stage slows down very quickly instead of coasting to a high altitude. The first stage continues to tumble as it descends. This is a classic example of tumble recovery, where the rocket’s configuration changes so it is unstable, giving a gentle recovery without a parachute or streamer.
What about three-stage rockets? They’re not as common as two-stage rockets, but I’ve built and flown a few. There is no theoretical reason why you can’t just keep adding stages. At some point, though, the weight of the rocket or the amount of propellant will get too large. Also, bear in mind that a three-stage rocket flown with C motors is an E-class rocket, with 30 N-s of total impulse, so it needs to be launched from a distance of 30 feet, not 15 feet.
Construction
Refer back to Chapter 3 for basic construction techniques, tools, and supplies.
The first stage of Romulus is formed from a 2 3/4” length of BT-55 body tube, while the second stage is 15 1/4” long. Mark the 18” body tube 2 3/4” from one end and, using the tube coupler for support, cut the tube.
While it is not absolutely essential, it helps a lot when prepping the rocket for flight if the motor mount on the lower stage doesn’t rest right up against the motor mount for the second stage. That leaves room for the tape that holds the motors together until stage separation, without the need to maneuver the motor mounts over the tape.
Assuming you are using the 2 3/4” motor mount tubes from the Estes Designer’s Special or from a motor mount kit, cut one of the motor mounts to make room for the tape. Measure 1/2” from the end of one of the motor mounts. Using the engine block as a support, cut the tube.
If you are making your own motor mounts from a longer piece of BT-20 body tube, cut one motor mount 2 3/4” long and the other 2 1/4” long.
Referring back to the visual parts list in Figure 17-2, you see four thick paper centering rings. These are used to center the two motor mount tubes in the first- and second-stage body tubes. Figure 17-3 showed the two sheets of centering rings that come with the Estes Designer’s Special. If you bought individual centering rings, they will generally be two to a sheet, but the idea is similar. Detach the centering rings, using a hobby knife to trim any tabs holding them to the sheet.
Glue the centering rings to the motor mounts. The first two rings should go 1/4” from the ends on the shorter motor mount, which will go in the first stage of the rocket. The longer motor mount goes in the second stage. A tube coupler mounted in the first stage will hold the stages together until separation. We need plenty of room for the tube coupler to slide into the second stage, so the centering ring that will be closest to the bottom of the second stage needs to be 1” from the end of the longer motor mount tube. The centering ring at the other end should go 1/4” from the end.
Glue the engine block in the end of the second-stage motor mount tube. It should rest right up against the end of the tube, and be on the side where the centering ring is closest to the end of the tube.
Apply a generous fillet of glue where the centering rings meet the motor mount tube, but only on one side. Set the motor mounts aside to dry. There’s a lot of glue there, and drips are possible, so be sure to put wax paper or some other material that will peel away from glue easily under the motor mounts until the glue is firmly set.
Once the glue is hard enough that there is no danger of drips, turn the motor mounts over and put fillets on the other side.
Each stage has four fins. As you know, three fins will cause less drag, but there is a lot of weight near the back of this rocket. Rather than using three very large fins, the rocket uses four slightly smaller ones. This does add some drag, but the fins would not fit well on the first stage if they were much larger.
Use the fin guides from Figures 17-10 and 17-11 to cut the fins from 3/32"-thick balsa. Round the leading, trailing, and outer edges of the fins, but not the root edge. Use a sanding block and medium grit sandpaper to sand the fins so they are smooth to the touch.
The Estes Designer’s Special comes with 1/8"-diameter launch lugs that are 3” long. Cut one in half; we will use both pieces on Romulus, which is a fairly long rocket. There is nothing magical about the 1 1/2” length. If you have two 1 1/4” launch lugs or a 2"-long piece that you would like to cut in half, go ahead. Using 1” or 1 1/4” launch lugs will work fine.
Using the tube marking guide from Figure 3-25 in Chapter 3, mark the locations for the four fins and the launch lug on the second-stage body tube. Use the tube coupler to temporarily hold the tubes together while you use a door jamb to extend the marks to lines that run from the base of the first stage to about 4” above the base of the second stage. Marking the tubes together this way will help you get the fins positioned in exactly the same spot on the two stages. If you’re off slightly on one of the fins, both stages will be off by the same amount.
Extend the line for the launch lug all the way up the second-stage tube.
Glue the four fins in position. Look down from above to check the alignment of the fins. Glue one launch lug so it is 2 3/4” from the bottom of the second stage, and the other so it is 5” from the top of the second stage. This puts the launch lugs on either side of the center of gravity, holding the rocket straight as it slides up the launch rail.
Check the fins occasionally as they set. After they are dry enough to handle, but before the glue is completely dry, pull the stages apart. There are two reasons for doing this. The first is that it lets you turn the stages over and sit them gingerly on a flat surface to make sure the backs are aligned. It’s not too late to reset a fin if they are really out of whack. The other reason to pull the stages apart is to make sure no glue dripped into the crack between the stages. You don’t want to accidentally glue the stages together!
That’s a lot of glue. Rather than sitting around watching it dry, let’s do something useful.
Grab the nose cone and check it for flash (the extra plastic that squeezes into the cracks between molded parts). Remove any extra plastic from the center of the eyelet and scrape any flash from the nose cone. Refer back to Chapter 3 for details.
Fasten snap swivels to one end of the shock cord and to a 15” parachute. If you’re making a parachute from scratch, now is a good time to make it.
Once the glue dries, mount the motor mounts in the body tubes. Start with a dry test fit to make sure that all of the pieces will slide in easily and that everything fits. Be sure and check the clearance on the tube coupler—that’s the red tube in Figure 17-17. The shorter motor mount goes in the first stage. It should be flush with the back end of the body tube. The longer motor mount goes in the second stage. Be sure the end with the engine block goes in first, and the end with 1” between the end of the tube and the centering ring is on the bottom. Both stages should fit together with the centering ring and motor mounts in place.
Once you are satisfied with the fit, apply glue inside the body tube and push the motor mounts into place. Apply glue to the top of the first stage and slide the tube coupler into place. It should slide in halfway, so 3/4” of the tube coupler is glued in the first stage, and 3/4” sticks up to join to the second stage. Check the fit one last time by sliding the first stage into the second stage, but pull the stages apart immediately to prevent the two stages from getting stuck together.
Once the glue sets, apply generous fillets to the exposed joints between the centering rings and body tubes. Apply fillets to the joints between the fins and body tubes, too.
Once the fillets dry, glue in the shock cord. The rocket is now ready for final finishing.
Flying Romulus
While there are a few extra steps, prepping Romulus for flight uses the same techniques you have already practiced on Juno and other rockets.
Begin by stuffing enough recovery wadding into the second stage to fill about 2 1/2” of the tube with wadding.
Make sure the snap swivels for the shock cord and parachute are both attached to the eyelet on the nose cone. Be sure both snap swivels are fully closed.
Pack the parachute. Insert the shock cord into the body tube, then the parachute. Push the nose cone into place.
The parachute should slide in easily. The nose cone should be tight enough so it does not fall out if the rocket is turned over and shaken gently, but it should not be any tighter. Sand the nose cone or apply tape if you need to adjust the fit.
Line the motors up so the nozzles point the same way. The top of the first-stage motor—the A6-0, B6-0, or C6-0—should face the nozzle on the second-stage motor.
Apply one layer of tape completely around the motors to hold them together.
If you need to, apply more tape to the second-stage motor. Mount it in the second stage of the rocket so the motor rests against the engine block. It should be quite snug, so the motor stays put when the ejection charge fires. We want the parachute to pop out, not the motor!
If the tape holding the two motors together makes the motor too tight to fit into the rocket, tear it back so only 1/4” of tape is holding the second-stage motor to the first-stage motor. A total of 1/2” of tape—1/4” on each motor—is plenty to hold them together until the stages separate.
Repeat the process, applying tape if needed to hold the first-stage motor in place. Slide the first stage into the second stage until the body tubes meet. While you will probably display the rockets with the fins on the two stages neatly lined up, it’s better to fly it with the fins rotated 30° or so. Lining the fins up causes turbulence from the second-stage fins to hit the first-stage fins, causing a rough flight and reducing the effectiveness of the first-stage fins. Rotating the fins relative to each other fixes this issue.
The first stage should still be snug, but it can be a little looser than the second stage. There won’t be an ejection charge trying to push the first-stage motor out of the rocket.
Multistage model rockets have a lot of fin area, and tend to be a bit overstable. As you know, that can cause weathercocking in high winds. In general, you should fly Romulus and other multistage rockets on calm days. That helps prevent weathercocking, and also makes it more likely that you will find these high-flying rockets, since a calm wind won’t carry them as far as a stiff breeze!
The rocket is ready for an igniter and launch. Before you push that button, though, think about what is going to happen. Your rocket is going to shoot into the air, the stages will separate, and the second stage will blast a considerable height into the sky—an additional 250 to 1,500 feet, depending on the motors used. Everyone will be watching with excitement as the rocket climbs higher and higher until the parachute deploys.
Now quick—where’s the first stage?
As I write this, the first stage was lost on the two most recent two-stage flights at the Albuquerque Rocket Society. One of them was mine. It may not be a glamorous job, but be sure to assign someone to track the first stage. You need to recover both parts!