S1 Altitude

Event Highlights 

• Flown in three standard rounds
• Score is the single best measured flight
• Maximum of two models may be entered for the three standard rounds; stages cannot be mixed between models after checkin
• Juniors fly S1A (1/2A motors in each stage), Seniors fly S1B (staged A motors)
• 50% of the rocket's total impulse must be in the motor used in the upper stage
• S1 model requirements: minimum length = 500 mm, minimum diameter = 40mm for at least 50% of length; upper stage minimum diameter 18mm for at least 75% of length, maximum mass = 30 grams for S1A, 60 grams for S1B
• Altitude is measured by an onboard electronic altimeter provided by the contest organizers

Major Challenges 

• Reliable and straight-up staging
• Proper venting and functioning of the electronic altimeter
• Seeing the upper stage at ejection and then finding it (for S1B)
• Deployment of the required recovery streamers on both lower and upper stages

Discussion

The FAI rules do not specifically require that models in S1 must be two-staged. However, single-stage models are non-competitive at a WSMC and are never flown there. Staged designs are required in the US Team flyoffs.  Effective starting in 2019 the FAI rules now prohibit the use of piston launchers for S1.

Over the years, there has been some controversy as to whether or not a boattail is allowed on the upper stage airframe. In 2015, the rules for S1 will change to explicitly disallow a boattail on the upper stage.

The dimension rules for this event drive the airframe design. The upper stage should be as short and light as possible. The assembled airframe (both stages) should not exceed 500mm in length. The lower stage must have a section that is 40mm diameter for 50% (250mm) of the model's overall length.  Tapered sections should be used between the 18mm upper stage and the 40mm lower stage, and potentially as a boat-tail on the lower stage, to make up the remainder of the 500mm overall length.

The upper stage should use a parabolic nose cone for drag minimization.  If this has a length-to-diameter ratio of 1.5 (blunter noses than this have a higher drag coefficient), then 1.5 x 18 = 27mm of that stage's length is below 18mm in diameter, which means that in order for the upper stage to have no less than 75% of its length at 18mm, the rest of the upper stage must be no less than 3 x 27mm = 81mm in length, giving a minimum overall upper stage length of 27 + 81 = 108mm or 4.25 inches.  It may take a length of as much as 5 inches (126mm) to accommodate the altimeter, motor, and recovery device depending on design, but upper stages should be no longer than this.

With a 108mm long upper stage and a 250mm long section of the lower stage that is 40mm diameter, 142mm of airframe length remains to be distributed between tapered sections that go between the 18mm upper stage and 40mm booster stage, or as a boat-tail on the lower stage.  This is a 4.4 degree taper if it is all used in one length, and two 8.8 degree tapers if split between the two (with a boat-tail down to 18mm).  Some fliers use a single lower-stage taper to the upper stage, others use a double taper.  There is no clear evidence that either is better than the other, but it is a subject of lengthy theoretical debate.

Check out this tutorial video on S1 made (in English) by the manager of the Russian team.

Vehicle Construction

• Lightweight fiberglass or vellum (see Construction page)
• Upper stages typically use rear ejection and use very long thin mylar streamers for recovery/visibility
• The lower stage fins are made out of lightweight 1/20 or 1/16 balsa wood. Upper stage fins are .015 waferglass.
• Airframe mass of the upper stage, without motor or altimeter, should be less than 5 grams (the lighter the better); airframe mass of the lower stage should be less than 10 grams.
  
  

Recovery Systems

Each stage must have (and deploy) its own recovery device. The FAI rules do not specify the size required, but European RSO's are looking for a 1.0 inch by 12-inch streamer on both the lower stage and the upper stage. Usually a very long (6 feet or greater) narrow (0.5 inch or less) aluminized mylar streamer is attached to the end of this on the upper stage.

The lower stage uses a zero-delay booster motor with no ejection charge, so its streamer must be deployed by the action of the upper stage separating.  This can be done by attaching one end of the streamer to the base end of the lower stage, then tucking the free upper end into the joint between lower and upper stages, pinning the streamer along the exterior of the lower stage airframe for its short portion of the trajectory and allowing it to deploy at stage separation.  This approach is not used by fliers from any other country and is not well-regarded by European RSO's.  A more elegant technique (and the one used by many European fliers) is to leave a gap between the 18mm upper stage and its internal 10.2mm motor mount tube, and put a rolled streamer in this gap that is connected to the lower stage (and protected from the upper stage motor by a paper sleeve).  As the upper stage pulls away this streamer is pulled free and deploys.  A final method is to roll the streamer tightly and hold it shut on the outside of the lower stage (next to a fin) with a thread that is burned through to release the streamer when the booster motor blows through to ignite the upper stage.

The upper stage streamer is key to having the RSO see the model after ejection, in order to qualify the flight for having deployed the required recovery device.  It also helps the recovery crew find your model.  This streamer is very long aluminized mylar, generally rolled up in the space between the internal motor mount tube and the 18mm airframe for rear-ejection designs.  This material is fragile (unless it is made from the very expensive aluminized Kapton material) so the streamer must have some form of protection from the ejection charge, such as a paper wrap.

Staging Technique

US black powder booster motors produce a large amount of burning propellant and hot gas that shoots forward at the moment that the motor burns out.  This material easily lights upper stage black powder motors with reasonable-size nozzle throats, even across distances of as much as 12 inches if a "stuffer tube" (engine-diameter body tube) is used to connect the two stages.  This technique works fine for staged Estes motors at the US Team flyoffs, but it will not work with the European motors that are used at a WSMC.

Most European booster motors have a solid bulkhead at the top of the propellant grain with a tiny hole it it that permits a small "jet" of hot gas to go forward.  A thin (one grain) layer of black powder is then placed on this bulkhead as part of flight preparation.  This is used to provide a robust flame that goes up a "flash tube", a thin (3/16 inch or so inside diameter) fiberglass tube that ducts the booster jet into the tiny nozzle of the upper stage motor to ignite it. One end of the flash tube plugs into the top of the casing of the booster motor, the other end terminates 2-3 mm short of the nozzle of the upper stage motor.  Prior to flight a tiny amount of black powder is rammed into the nozzle of the upper stage motor with a tiny drill bit, to ensure that the flash tube flame has something easy to ignite that will then ignite the upper stage motor.  Use too much black powder on the booster motor, or fail to leave the little gap before the upper stage motor nozzle, and the flash tube just pops the upper stage off without ignition.

Propulsion

No US-made motor is of any use in this event at a WSMC, only European motors are competitive.  Lower stages use high-thrust (3 to 5N average thrust) 10.2mm motors with zero delay; upper stages use the lowest available thrust (preferably 1N average) 10.5mm motors with the longest available delay.  S1B upper stages need a delay of at least 8 seconds, and these have to be special-ordered.

Since altimeters are used to measure peak altitude, models should use long delays to eject well after apogee rather than trying to eject at apogee as would be required if optical tracking were used.

US Team-selection flyoffs are flown (for both S1A and S1B) with an Estes A10-0T to an Estes 1/2A3-4T.  A3-4T upper stage motors would eject long before apogee and flight altitude would be a function of delay time accuracy rather than model performance.

Altimeter

The WSMC is flown using Polish "Adrel" altimeters. These are issued to each competitor at the beginning of the first round of the event (in exchange for a 40-50 Euro deposit) and are returned after each flight for reading and re-zeroing, and at the end of the meet for refund of the deposit. These altimeters do not begin reading for 3 minutes after being powered up, so you have this long to get them into your rocket, which you do in front of the judge who issues it.

Adrel altimeters are notoriously sensitive to wind, so if your rocket has vent holes in the nose section directly at the altimeter (standard US practice), the altimeter will be highly vulnerable to inadvertent triggering. Most Europeans equalize the pressure inside their airframes by perforating the motor mount centering rings and any bulkheads inside their rockets, and venting to the rear end of the rocket.

The most recent version of the Adrel altimeter, with battery, weighs 1.8 grams and will fit easily inside the volume of the nose cone of the upper stage.

Competition Strategy

Go for broke on every flight. You have three tries to get one good flight. Unlike the duration events, it's the best single flight that counts, not the sum.

Always launch from a tower. You want maximum velocity at staging so that you get a straight and prompt separation and a purely vertical boost from the upper stage.

Fly as early in the round as possible. If you have an altimeter malfunction that results in no recorded score (happens 10-20% of the time), you are permitted to fly again in that same round if you can re-prep fast enough. These models take a lot of time to prep, between loading two motors, prepping the flash tube and two streamers, getting your altimeter before flight, and returning the altimeter after the flight. Practice fly a lot so this complex routine comes automatically to you under the pressure of the WSMC round. Practice flying with Estes motors is negative learning. You need to practice fly with the European motors that you will be using in the contest.

Use the highest average thrust booster motor and the lowest average thrust upper stage motor with a delay time of at least 8 seconds (S1B).