ConstructionTechniques

Building your own

An accurate altimeter for your rocket made from a pillbottle, latex glove, fishing line, and a rubber band? Soundsa bit like a “Junkyard Wars” project, but this instrument iseasy and cheap to build, and it works well. The original ideafor this altimeter was proposed by my two rocketeer nieces,Erin and Kristin Jewell.

As a helium balloon rises, the atmospheric pressureacting on the outside of the balloon decreases. This allowsthe helium to expand and increase the size of the balloon.The altimeter we are going to build works on this principle.

Instead of a balloon, our altimeter uses a hard-walledchamber with a balloon-type membrane stretched over anopen end. Instead of helium, ordinary air fills the chamber.When the chamber is carried aloft in a rocket, the expand-ing air inside tends to bulge the membrane out against thelessening air pressure at higher altitudes. As you will see,

You can make a accurate altimeter made from a pillbottle, latex glove, fishing line and a rubber band!

by Rick Weber

SPORT ROCKETRY JAN./FEB., 2003 11

HOMEBREWALTIMETER

MicroAlt electronicaltimeter in the author’sPML Tethys rocket. TheMicroAlt recordedapogee at 3650 feet, andour homebrew altimetershowed 3560 feet—adifference of only about2%.

MaterialsThe materials you

will need to build thealtimeter and the cali-bration chamber shouldcost you less than $20.However, you will needa good electronic altime-ter for the calibration. We used a Casioaltimeter watch purchased on eBay for$32. At the time of this writing, there wereseveral listed on eBay for under $35.

Here is a list of items you will need.

For the altimeter:1 Hard plastic pill bottle. Find

the largest size that will fit into your rocket. The size shown in this article is about 2 inches indiameter and 5 inches long.

1 Disposable latex examination glove1 Piece of 15-pound test

monofiliment fishing line1 Rubber band — 1/4 inch wide

by 4 inches around1 Hot glue gun

For the calibration chamber:1 12-inch long piece of 4-inch PVC

schedule 40 pipe1 Expansion rubber plug for 4-inch

PVC pipe (sometimes called a testplug)

1 5-inch x 5-inch piece of 1/4-inchthick clear Plexiglas or polycarbonate

our altimeter has a way of telling howmuch the membrane bulges during itsflight. We can measure this bulge to tell ushow high our rocket traveled.

The design can be applied to differentsizes and shapes of chambers. The largerthe chamber, the more accurate youraltimeter will be. But, of course the cham-ber must also be able to fit inside yourrocket. Figure 1 shows an 8-inch long by1.5-inch diameter chamber that wasrecently flown along with a PerfectFlite

12 JAN./FEB, 2003 SPORT ROCKETRY

Figure 1: Homebrew altimeter ready to mount in rocket.

1 6-foot long piece of 3/8 inch diame-ter polyethylene tubing

1 Valve with 3/8-inch compression fit-ting on both ends

1 Tube of silicone rubber cement1 Casio watch with altimeter or an

equivalent instrument

ConstructionLet’s start building the altimeter. Refer

to Figure 2. Drill a 3/16-inch diameterhole in the center of the bottom of a pillbottle. Cut a 12-inch long piece of 15pound monofilament fishing line and coat

it with a thin layer of vegetable shorten-ing.

Heat up a glue gun. While holding thefishing line in the center of the 3/16-inchhole, apply hot glue all around the fishingline and into the hole as shown in Figure2. Make sure the hot glue completely sealsthe 3/16-inch hole. After the glue hardens,you should be able to pull the fishing linethrough the hot glue blob. Do not pull itall the way out—just make sure it slidesfreely, but will stay put when you releaseit. If you inadvertently pull the line out ofits hole in the hot glue blob, you may haveto remove the glue and try again.

Take hold of the end of the fishing linethat extends from inside the bottle andpress it squarely against the hot tip of theglue gun to form a flat “button.” If thatdoesn’t work, press the end against a hotsoldering iron. If all went well, you willhave a button at the end of the line thatlooks like the one in Figure 3.

Cut a piece of latex rubber from aglove that is about twice the diameter ofyour pill bottle. Lay this piece out on a flatsurface. Put a small drop of cyanoacrylateglue on the button on the fishing line and

Figure 2: Fishing line inhot glue blob.

press it down squarely in the center of thelatex membrane. Hold it a few secondsuntil the glue sets.

Stretch the latex membrane over theopen end of the bottle so that the fishingline button is in the center. Tightly wrap arubber band around the membrane thatoverlaps the side of the bottle. See Figure4. This is a tricky process and may requireseveral tries to get it right. It is importantnot to stretch the membrane any morethan necessary to remove wrinkles. Makesure the rubber band is very tight so thatan airtight seal is formed between themembrane and the bottle.

Now, from the bottom end of the pillbottle, gently pull the fishing line through

SPORT ROCKETRY JAN./FEB, 2003 13

its hole in the hotglue blob until yousee it start to tug thecenter of the mem-brane into the bottle.Cut off the line sothat exactly 1.5inches sticks out ofthe blob. Other thangluing mount brack-ets on the bottle to fitit into your rocket,your altimeter is fin-ished.

But, how canit record altitude?

The altimeteris ready to belaunched asshown in Figure5A when themembrane is flatand the line taut.As the altimeter iscarried aloft, thehigher the rocketgoes, the morethe membranebulges. As itbulges it pulls the

fishing line throughthe blob into thebottle as shown inFigure 5B. When therocket descends, ofcourse, the bulgedeflates. However,the friction fit in theblob prevents the linefrom going back out.When you recoveryour rocket, accurate-ly measure the lengthof the line protrudingfrom the blob as

shown in Figure 5C. Find this length on acalibration graph, and you have your alti-tude.

CalibrationChamber

Of course we must first make a cali-bration graph for our altimeter, and that isthe reason a calibration vacuum chamberis needed. So, let’s build one.

Refer to Figure 6. Cut a 12-inch longpiece of 4-inch PVC pipe (schedule 40)

and square up the ends. Cut a 5-inch by 5-inch piece of 1/4-inch thick Plexiglas orpolycarbonate and glue it with siliconerubber glue centered to one end of thePVC pipe. Allow it to dry over night.

Drill a 3/8-inch diameter hole in thecenter of the side wall of the pipe. Use acoarse file to roughen up the end of a 3-foot piece of 3/8-inch polyethylene tubing.Spread silicone rubber glue around theend of this tubing and insert it into the3/8-inch diameter hole so that about a 1/4inch extends beyond the inside wall of thepipe. Form a generous fillet of siliconeglue between the tubing and the inside

Figure 4: Membranestretched over opening.

Figure 3: Button formed on fishing line.

Figure 5: A: Before flight. B: At apogee. C: After flight

and outside pipe walls. After the siliconehas cured for at least 24 hours, attach a1/4-turn shut off valve to the outer end of

the tube. Connect another 3-foot length oftubing to the other end of the valve.

Now we need a vacuum. An ordinary

home or shop vacuum cleaner will pro-duce more than enough vacuum. In fact,it will produce too much if we were todirect all of it to our vacuum chamber.Duct tape a thin piece of plywood overonly half of the opening of the vacuumcleaner’s hose opening. Drill a 3/8-inchdiameter hole in the plywood and insertthe hose from the outer port of the 1/4turn valve. When you turn on the vacuumcleaner, most of the air will enter the largeopening with only a small part coming infrom the 3/8-inch hose. Because allvacuum cleaners are different, you willhave to experiment to determine the bestway to get the right amount of vacuum.Ideally, turning on your vacuum cleanerfor 5 or 6 seconds should register about3000 feet on the altimeter watch.

Our vacuum calibration chamber isfinished. To calibrate our altimeter, followthese steps:1. Clean all the dust and shavings from

the calibration chamber and attachedtubing.

2. Open the 1/4-turn shut off valve.3. Put the altimeter watch into the cham-

ber so that you can read the altitude.4. Put the homebrew altimeter in the

chamber so that nothing will interferewith the free bulging of the mem-brane.

5. Insert and tighten the expansion plugwith a wrench to make sure it is wellsealed.

6. Read the altitude on the watchthrough the Plexiglas window andrecord the number displayed.

7. With your hand on the valve, turn onthe vacuum cleaner for a couple sec-onds, then quickly close the valve.Now, shut off the vacuum cleaner. It isimportant to close the valve as thevacuum is building up. If you let avacuum build up, then drop off beforeclosing the valve, the homebrewaltimeter will have responded to ahigher altitude than the watch dis-plays. A vacuum equal to air pressure at twoor three thousand feet can be createdwith a vacuum cleaner in only a fewseconds. So, be sure to turn it on andoff in short bursts.

8. Quickly read the altitude on the watchthrough the Plexiglas window. Youwill notice that the membrane hasbulged out.

9. The vacuum probably will not holdindefinitely, but should remain rela-tively steady for a couple minutes. It isimportant to read the altitude as soon

14 JAN/FEB., 2003 SPORT ROCKETRY

Figure 6: Vacuum calibration chamber.

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as you close the valve. If the vacuumdoesn’t hold, examine the chamber forair leaks and seal them.

10. Remove your homebrew altimeterfrom the chamber and accuratelymeasure the length of fishing line pro-truding from it. Use dial calipers if youhave them.

11. Write down this length next to thealtitude that the watch displayedwhen the chamber was under avacuum.

12. Repeat steps 2 through 11 with thevaccum cleaner turned on for differenttime periods to obtain different alti-tude readings from the watch. About10 readings should be sufficient.

13. Plot a graph of altitude vs. line lengthlike the one shown in Figure 7. Useyour calibration curve in the field todetermine how high your altimeterwent. After making your calibrationcurve, it is very important to not dis-turb the membrane and rubber bandsholding it. Stretching the rubbertighter or looser can change the cali-bration curve and will require recali-brating.Okay, let’s launch this critter!As with all altimeters we need a few

little holes in our rocket to allow airflowduring the flight, so that the altimeterresponds to the air pressure outside therocket. Mount the altimeter and gentlypull the line until it is taut and the mem-brane is flat, as shown in Figure 5A.Check the length of fishing line protrud-ing to verify that it is the same as whenyou calibrated the altimeter.

Launch and recover. Being careful notto disturb the line, measure the amount

protruding. Subtract it from the initiallength Find this difference on the calibra-tion curve and read off the correspondingaltitude. It’s that simple.

How about making one of thesealtimeters for your science fair project?Can you think of improvements and mod-ifications?

1000 2000 3000 4000 50001.5

1.3

1.1

0.9

0.7

0.5

Altitude in feet

Leng

th o

f lin

e pr

otru

ding

from

alti

met

er in

inch

esExample of a Calibration Curve

for Homebrew Altimeter

0

This is only an example. You must make your own calibration curve for your altimeter, which will look

similar to this one, but will have a different plot.

Figure 7

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around the U.S. and in front of an NARSenior (adult) member, no later thanMarch 9, 2003. They then send a formreporting the results of that flight, signedby the NAR member, to Team AmericaHeadquarters. The 100 teams that comeclosest to 1500 feet in these flights will getinvited to attend the final face-to-faceflyoff on May 10 and compete there forover $59,000 in prizes. Every team will belooking for a Senior NAR member,between now and March 9, to observetheir flight so they can try to qualify forthis. We are giving the teams the names ofNAR Senior members in their state. If oneof them contacts you, please lend a hand!

If you have any questions about theevent, please contact me atahbarber@alum.mit.edu. Please join in,and be part of the biggest event the NARhas ever run!

Team AmericaC H A L L E N G E

Involving theWhole NARby Trip Barber, NAR 4322 NAR Vice President

The NAR’s “Team AmericaRocketry Challenge” nation-wide high-school competitionhas become the largest eventthe NAR has ever run. Over825 high school teams, withover 6000 student team mem-bers, have entered theChallenge, including teamsfrom all 50 states. The eventhas been endorsed and publi-cized by NASA, the Air Force,and every major educationalorganization in the U.S. TheAdministrator of NASA willpersonally attend the finalchampionship flyoff on May10, 2003, in The Plains,Virginia (40 miles west ofDC). The NAR will get sever-al thousand new members(both students and teachers)and the best features of ourhobby will get unprecedent-ed public visibility as a result of this event.

It will take involvement by the whole membership of the NAR,across the U.S., to make this event as successful as it ought to be.Each of the 825 teams is required to build and fly a model rocketthat has two operating stages, weighs less than 1500 grams, carriestwo raw eggs as close as possible to 1500 feet (as measured by theAdept A1 electronic altimeter provided to each team), recoverssafely, and returns the eggs unbroken. Many of these teams are flyingcomplex rockets for the first time and need help from experiencedrocketeers in figuring out how to locate rocket components andbuild rockets. You can help, as long as you do not actually helpdesign or build their official contest rocket. The list of participatingschools is posted on the Team America website. Go to<http://www.aia-aerospace.org> and click on the Team AmericaRocketry Challenge link. Please read the list of registered teams andget in contact with any school near you and offer to help.

Each team also has to do a “qualification” flight in their local area

SPORT ROCKETRY JAN./FEB., 2003 17

SR mag 2 2/1/03 4:16 PM Page 31

Lee Piester “works hard for the money.”The owner of a small chain of hobby stores in south-central Arizona,

Piester works 14 to 18 hours a day, 7 days a week. Usually, he takes 3 days offeach July to go salmon fishing in Alaska, but he managed to set aside a wholeweek this year. Piester’s unswerving dedication is only one of the reasons hisHobby Bench stores have outlasted scores of other hobby shops in the Phoenixarea.

Piester is also the founder of Centuri Engineering Company, which he cre-ated and ran on paper for a college class. Centuri model rockets captured theattention and the imaginations of countless young rocketeers in the 1960s and70s. In fact, many “Born-Again Rocketeers” (BARs) cite Centuri as their favoritemodel rocket company.

But like many other greatly admired institutions, Centuri rose from humblebeginnings. Piester’s interest in rocketry was initially piqued in the mid-1950sduring the early days of the Space Race. “It was probably the beginnings of thefirst satellite program, Project Vanguard,” Piester reveals. “I did a project inhigh school, an English paper, and that was the trigger.”

18 JAN./FEB., 2003 SPORT ROCKETRY

Lee Piester “in his natural environment” at Hobby Bench #5.

Text and photos by Jay Goemmer (“Centuri Guy”)

LeePiester: An Unassuming Legend

LeePiester: An Unassuming Legend

SR mag 2 2/1/03 3:59 PM Page 18

Piester began producingrocketry components out ofnecessity while he was stillin high school. During hisjunior and senior years, hewas part of a group of ama-teur rocketeers who laterresisted the label, and tried toconvince people to use theterm “model rocketeer”instead. “We did someadvanced amateur rocketrywork, and it got very expen-sive,” says Piester. “I beganmachining parts for this groupto make a little ‘pin money,’ andto finance my own amateurrocket projects. And that’s whatbegan the commercial effort inrocketry.”

Piester graduated from WestPhoenix High School in 1956,and then attended Phoenix

College, as well asArizona StateUniversity in nearbyTempe. An IndustrialEngineering major, hisundergraduate thesis

dealt with the formation of a hypothetical busi-ness: namely, Centuri Engineering Company.“We had to form a company, the manufacturingprocesses, and show the financing—literally, runthe entire company on paper, andpresent it,” says Piester. Hisinstructors were duly impressedwith his proposal. “It was acceptedby the profs on one condition: thatI let them buy into the program.They thought it was a good enoughthesis, and made enough sense, thatthey thought it should be financedand proceeded with.”

So Piester forged ahead with hisimaginary company, bringing it intoreality. “I formed the ideas, and putthem into a catalog,” he says. “Iwrote all of the copy, and my wifeBetty, who has been a major part ofour businesses all along, did all of thecomposition on a borrowed IBMSelectric, which was the best we had.We didn’t have computers—at leastnot PCs—back then. I borrowed timeon a printing press, and we put outour first catalog, and our first ad inBoy’s Life. And I think that I was asophomore or junior in college at thatpoint.” The year was 1961.

Over the years, there’s been a greatdeal of speculation regarding the originof the name Centuri. Piester sets the

record straight. “‘Centuri’ is achopped-up portion of ‘AlphaCentauri.’ Jim Carder—my oneemployee at the time—and Bettyand I were sitting in the workshop,knocking names around.” Cardersaid, “Let’s call it ‘Alpha Centauri.’”Piester replied, “No, that’s too long.”Carder responded, “Let’s just call it‘Centauri.’”

Even in its abbreviated form, thename still wasn’t completely toPiester’s liking. “And I said, ‘Let’sreally make it original, let’s knock outone of the letters,’” Piester observes.“Anyway, that’s where ‘Centuri’came from.”

Piester’s company expandedsteadily, thanks to his market-ing savvy. “Boy’s Life magazinewas a target audience,” heconfides. “For that time, itwas just the right age group.It was very expensive. Alittle two-inch by one-column ad, even at thattime, those many yearsago, was about $400.

Very expensive, but it did payoff, and all during ourefforts throughout all ofour business career, Boy’sLife was with us, and wasone of the best advertis-ing buys. That’s whatgot us started.”

As Centuri grew,administrative dutiesand financial responsi-bilities drew Piesteraway from involve-ment at the “hands-on” level. “I love theformation process, thedeveloping of an idea,and creating a prod-uct. So the early stages, when I was more ‘hands-on,’ were the most enjoyable. In Year 5 of thebusiness (1966), I had 160 employees, and 40,000square feet of manufacturing. So the duties justbegan to lead me elsewhere.” Centuri reached itspeak in 1971, with a total of 170 employees.

During the 1960s and 70s, comic book fanscould select from only two publishers, for themost part: Marvel and DC. Model rocketeerscould also choose from two alternatives: Centuriand Estes. Piester points out what his staff did in

Left: Centuri print ad from Boy’s Life, May, 1971.

Below: Centuri print ad fromBoy’s Life, October, 1971.

SPORT ROCKETRY JAN./FEB., 2003 19

1/36 scale Centuri Mecury Redstone.

Science meets fiction:Centuri’s adaptation ofNASA’s “Sky-Lab” on

display at HobbyBench.

SR mag 2 2/1/03 3:59 PM Page 19

trying to make Centuri products differentfrom their competition. “We focused onscale kits, and true scale. Not what todayis called ‘stand-off scale,’” he says. Piesterhired staff people from the model railroad,model airplane, and plastic model indus-tries who knew how to scale a modeldown meticulously. “We did a tremendousamount of research,” he continues. “Andthen we did a great deal to get cooperationfrom the full-scale manufacturers, whichsurprisingly were very cooperative, oncethey realized who we were, and our seri-

wrote most of Centuri’s “TechnicalInformation Reports” (the TIR series), wasalso involved in product design, as wasGrant Boyd, who later formed AcmeRockets, a clearinghouse for old and dis-continued model rocketry kits and parts.

While many rocketeers consideredNiskern’s Saturn V design to be Centuri’s“top of the line” kit, another Saturn heldPiester’s attention, out of all the Centuridesigns. “Oh, there were so many,” Piester

admits. “There were literally hundreds,but of course, the big ones—actually, theSaturn 1B was my favorite.”

In contrast, many of Centuri’s designswere inspired by science fiction themes.The “Alpha Centuri Fleet”—the Vulcan,the Taurus, and the Vector-V—wereaccompanied by science fiction “storysheets,” written specifically to capture theimaginations of juvenile rocketeers. “DougMalewicki was part of it at one time,”recalls Piester. “Larry Brown did some ofthat, and there were one or two others,perhaps. But those were probably the leadpeople.”

ousness about the proj-ect.”

Piester was primarilyresponsible for designingCenturi’s model rocketkits for the first 4 or 5years the company was inbusiness. “And later, forthe next few years, itbecame a collaborativeeffort. And then after that,

I was pretty much out ofit, except for a finalreview,” he says. Otherdesigners on the Centuripayroll were KeithNiskern, who went towork at Estes after Piestersold Centuri, and LarryBrown, who became ateacher at a private boys’school in New England.Douglas Malewicki, who

Employee MichaelVillagomez shows offan original Centuri

“E.S.S. Raven”.

20 JAN./FEB., 2003 SPORT ROCKETRY

An enthusiastic customer at Hobby Bench #5looks forward to flying some R.C airplanes.

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For rocketeers looking for a biggerchallenge, Centuri’s “large-scale” modelrockets had tougher building materials—body tubes and balsa fins which werethicker, along with hard balsa nosecones—not to mention larger and morepowerful engines. “In the early 60s, theCoaster Corporation in Euless, Texas,made large F-size black powder motors.They also did a small E” reveals Piester. “Ipurchased that company, moved it toPhoenix, and continued production.”Piester changed the name of the Coastermotors to “Mini-Max,” and began sellingthem in 1968. Centuri used black powderas the propellant in its regular 1/4Athrough C model rocket or “low power”motors, as well. “However, we saw limita-tions on what black powder could do. Aswe all know, it’s a fairly low energy

source,” he admits. Piester continues, “Another individual

named Irv Wait, who had a companynamed Rocket Development Corporationin Seymour, Indiana, was perhaps the firstone to use composite propellant, and Ipurchased his company, and moved himand his company to Phoenix as well. And,with some modifications, I began the firstEnerjet line.” Piester unveiled the newEnerjet engines in the 1971 Centuri cata-log.

Centuri’s bigger “birds” foreshadowedwhat is now considered “mid power”rocketry. At that time, Enerjet products

months, we sold Centuri to the DamonCorporation.” Aside from having a newparent company, the major outcome was asizable influx of cash into Centuri.

Piester then signed a 3-year contractwith Damon, remaining as president andCEO of Centuri until 1975, when hebecame a stockbroker. As a customer ofthe Paine-Webber brokerage firm, Piester

were promoted as “high power rocketry.”But just like today, the bigger the rocket,the more expensive it was. The higherprice tags for Centuri’s large-scale modelrockets—as well as Mini-Max and Enerjetengines—meant they were somewhat outof reach for many of Centuri’s “model”rocketry consumers. “Just like in myHobby Bench business, the lower pricepoints tend to appeal to more people,”says Piester. “So in terms of unit sales, thesmall A-, B-, and C-sized rockets werealways the giants. But in terms of extend-ed dollars, Enerjet was very successful.Actually, we were glad to be doing it,because the relative safety of manufactur-ing was much greater in the composites,as opposed to the black powder manufac-turing.”

Piester sold Centuri EngineeringCompany to Boston-based DamonCorporation in 1972, just a year and a halfafter Damon purchased Estes. Centuri hadreached a point where they were sellingtheir products to dealers, and anotherlevel of distributors—what’s called the“third level of distribution”—wanted tocarry Centuri’s line. But the quantitiesrequired by the distributors were so largethat Centuri needed a tremendousamount of capital input. This would alsorequire modernizing the operation, aswell as purchasing land to expand therocket motor plant.

“In fact, I was at the Manned SpaceCenter in Houston when I wasapproached by a Damon representativewho was interested to see if I wanted tosell our company to them,” Piesterremembers. “My lawyer and I made sever-al trips back to Boston, and after 6 or 8

“In my travels forCenturi, I had visited

hundreds of hobbyshops, and seen thegood and the bad.

So I decided to startone which would

be the ideal hobby shop.

SPORT ROCKETRY JAN./FEB., 2003 21

Hobby Bench at the corner of North 43rdAvenue and West Bell Road in Glendale, AZ.

SR mag 2 2/1/03 3:59 PM Page 21

Dual-Stage Recoveryfor a 1/16 Scale V2

by Chuck Pierce

Having recently flown and lost ascratch-built 1/16 scale (4-inch diameter)V2/A4 rocket, due to winds aloft hijackingand depositing the rocket into a nastilyovergrown briar thicket, I decided that thenext time I built a 4-inch (or larger) V2, Iwould configure the rocket for dualdeployment. The opportunity to do sopresented itself when LOC/Precisionoffered a pre-release 1/16 sport scale V2for evaluation by the model rocketry com-munity.

First off, there are quite a few ways tobuild a dual deployment capability into arocket, such as building the altimeter bayinto the nose cone, into the motor mountarea, or into a center section of the air-frame. My preferred method has always

been the latter: to build amodular avionics baythat fits between the fincan and the nose cone.The big constraint forthis method of dualdeployment is real estate,as there must be availablevolume between the for-ward centering ring inthe fin can and the aftbulkhead of the avionicsbay for a drogue chutecompartment, volume inthe avionics bay for thealtimeter, and volumebetween the forwardbulkhead of the avionicsbay and the nose cone fora main chute compart-ment. For most 4-inchrockets, there is ampleinternal volume intowhich to build these nec-essary compartments. A1/16 scale V2/A4, how-ever, is another storyaltogether since the air-frame is only 10 incheslong and the remainderof the 36-inch rocketlength is nose cone andboat tail. My quest was toovercome the real estateproblem and to build a V2 that had a cen-trally located avionics compartment. Thefollowing is brief description of how Iachieved this goal.

Building the Boat Tail

The most significant feature of the V2boat tail for allowing dual deployment isthe available volume between the forwardcentering ring and the forward lip of theboat tail. I decided to limit my motorchoices to 38/360 cases, which meant thatmy motor tube needed to be approximate-ly 7 inches long. The outer diameter of theforward centering ring was turned downslightly to allow the CR to be bonded inplace approximately 4 inches from the for-ward lip of the boat tail. Into the forwardcentering ring, a loop of 1/8 inch tubularKevlar was installed, onto which theshock cord would later be attached. Sincethe boat tail is heavy, it has a negativestatic stability margin after separation,which means that a drogue chute is not

Building Recovery Systems

24 JAN./FEB., 2003 SPORT ROCKETRY

Figure 1: V2/A4 Boat Tail

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tion is an adaptation of the Smokin’Rockets’ AltiTube concept, in which theswitches and terminal blocks are builtonto the altimeter subassembly, whichthen slides into an internal tube which hasbeen permanently mounted into the air-frame of rocket. The tremendous advan-tage of this type of altimeter bay

Building theAvionics Bay

The avionics bay was built for aMissile Works RRC2 barometric altimeter,which is approximately 6 inches long. Theauthor’s preferred altimeter bay configura-

SPORT ROCKETRY JAN./FEB., 2003 25

required; therefore, the drogue compart-ment need only provide the internalvolume for the drogue shock cord. Theinternal volume in the forward end of theboat tail is spacious enough to contain 15to 20 feet of half-inch shock cord. Theboat tail is shown Figure 1.

Figure 2: AltimeterBay, View 1

Figure 3: Altimeter Bay, View 2

SR mag 2 2/1/03 4:21 PM Page 35

configuration is that it allows quick-and-easy transfers of an altimeter assemblybetween multiple rockets.

When configured to mount the RRC2,the altimeter tube measures approximate-ly 9 inches long. Therefore, in this dualdeployment configuration, the entire 10-inch airframe becomes the avionics bay.Figure 2 shows the airframe (left), altitubesubassembly (center), and altimeter sub-assembly (right), before the altitube sub-assembly has been permanently mountedinto the airframe. Figure 3 shows the samethree subassemblies, but has been rotated

avionics bay, the only available volume forthe main parachute is within thenosecone. To convert the nose cone into amain chute compartment, the back end ofthe shoulder of the nose cone must be cutoff. Since the V2 is characteristically tailheavy, due to the large fins, large boat tail,and short airframe, substantial noseweight must be added to the nose toinsure a proper static stability margin.After the weight has been bonded into thevertex of the nose cone, the remainingvolume of the nose can be dedicated tothe shock cord and main parachute.

Final AssemblyFigure 4 shows the assembled 1/16

scale V2/A4. To provide ambient pressureinto the altimeter bay, four 1/4" vent portswere drilled near the mid section of theairframe at 90-degree intervals from eachother. There was no special analysis thatwent into determining the size and quan-tity of the vent ports, other than author’sgut feel that four 1/4" ports would easilyallow for pressure equalization within theinner tube of the avionics bay.

Since the shoulder of the boat tail is soshort (about one inch long), two diametri-cally-opposed shear pins, are used to holdthe boat tail to the airframe, until thedrogue ejection charge forces the rocket toseparate at that plane. Although the shoul-der of the nose cone is much longer thanthe boat tail’s shoulder, two diametrically-opposed shear pins were also used to thehold the nose cone to the airframe, priorto main deployment. Shear pins help pro-tect the rocket from premature separationdue to the inertia differences (e.g., heavynose with a light tail), shock forces (e.g.,drogue charge and/or shock cord recoil),and internal pressure build up (e.g., highaltitude flights with an unvented internalcavity). Shear pins are the high-power

90 degrees to better show the terminalblock on the forward end of the altitubesubassembly, and to show the relativepositions of the subassemblies to eachother, once the altitube subassembly hasbeen permanently mounted inside the air-frame. Please note in Figure 3 that left endof the subassemblies will face the boat tail.Numerous holes have been drilledthrough the altimeter tube to allow thealtimeter to properly sense the externalambient pressure during flight. The alti-tube subassembly is mounted into the air-frame, such that the distance from the aftend of the airframe to the aft centeringring of the altitube subassembly is just ahair longer than the shoulder of the boattail. Since the V2 is being configured fordrogueless dual deployment, this arrange-ment allows 4 inches into which thedrogue shock cord will be placed.

Building the Main ChuteCompartment

With the airframe converted to an

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SPORT ROCKETRY JAN./FEB., 2003 27

rocketeer’s friend. The author’s shear pinsof choice are #2 nylon screws, but thereare several other options, such as styrenerod and the ubiquitous Kurt Kessler plas-tic pickle fork tines.

Flight andRecovery

An I357 motor was chosen for the firstflight of the V2 because it was one of thesmallest high-power motors in theauthor’s pitifully small motor cache and isalso one of the author’s favorite motors.

The rocket weighs 72 ounces whenloaded with the I357. Seven ounces ofnose weight gave the model a static stabil-ity margin of 1.3 calibers.

In Figure 5, the RockSim simulationpredicts that the V2 will reach an apogeeof 2853 feet, at which altitude the boat tailwill be separated from the airframe by thedrogue ejection charge. The two rockethalves will then tumble to 300 feet, thealtitude at which the altimeter will triggerthe deployment of the main parachute.RockSim predicts the touchdown velocityto be 18 mph.

On February 23, 2002, at the HARAlaunch site in Ardmore, Alabama, the V2

tion, though, special consideration shouldbe given to the amount of nose weightcontained in a rocket which will be recov-ered via droguless dual deployment. Aheavily nose-weighted rocket could pullthe drogue shock cord taut, forcing thetail section to align itself with the nosecone during descent, potentially resultingin a high-velocity deployment of the mainchute and/or entanglement of the trailingtail section into the deployed main para-chute, either of which could result in seri-ous damage to the rocket.

was launched into a clear skywith gusty 5-15 knot winds.The ascent of the V2/A4 wasnearly perfectly vertical. Perthe altimeter, apogee was2935 feet, at which altitudethe boat tail was separatedfrom the airframe. The mainchute deployed on cue at 300feet, and the rocket landedsafely within a 100 yards ofthe pad.

ConclusionDrogueless dual deploy-

ment is a useful and reliablemethod for recovering ahigh-altitude rocket in gustywinds and/or on short fields.For rockets with limited internal volumes,the volume of the drogue compartmentcan be minimized by designing the fincanto be unstable after drogue separation,which will allow the rocket to tumblefrom apogee (and, thus, does not usuallyrequire a drogue parachute to slow thedescent). By locating the altimeter bay inthe midsection of the rocket, managementof the drogue and main ejection chargewiring is much simpler than when thealtimeter bay has been built into either thenose cone or the fincan. As a word of cau-

Figure 4:Assembled V2

Figure 5: RockSim Simulation

V2 flight configurationMotor: I357 motorAltimeter: RRC2 altimeter (set for apogee deployment

of the drogue and 300' deployment of the main)Parachutes: 48 inch main parachute, no drogue chuteEjection: 1 gram of BP for the main ejection charge,

1 gram of BP for the drogue ejection chargeShock Cords: 15-foot, 1/2" diameter tubular nylon main shock

cord 15-foot, 1/2" diameter tubular nylon drogue shock cord

SR mag 2 2/1/03 4:21 PM Page 37