Model rocket fins are usually glued to the surface of an airframe. However, when higher thrust motors are used (E and
above) the increased thrust can literally rip fins off or shoot a motor up through the airframe. Instead,
"through-the-wall" (TTW) mounting refers to fins that protrude through a slot in the airframe and are glued to the motor
mount tube, one or more centering rings, and the airframe surrounding the slot. This construction technique significantly
strengthens fin joints and motor mounts.
There are three measurements necessary to create a fin tab: tab length, tab height, and tab position.
-**Tab length** is the distance from one side of the fin tab to the other. This is also the length of the slot that is
cut through the airframe, the distance between the inside edges of the outermost centering rings.
-**Tab height** is the distance from outside of the airframe to the outside of the motor mount tube. This is calculated
as follows: (BT OD - MMT OD) / 2, where BT is the airframe body tube and MMT is the motor mount tube diameters.
-**Tab position** is the distance from the root chord reference point to the fin tab reference point. *OpenRocket* features
three choices:
Relative to:
-**the chord root leading edge**– the tab position is the distance from the fin chord root leading edge to the fin tab leading edge.
-**the chord root midpoint**– the tab position is the distance from the fin chord root midpoint to the fin tab midpoint.
-**the chord root trailing edge**– the tab position is the distance from the fin chord root trailing edge to the fin tab trailing edge.
*OpenRocket* will automatically calculate fin tab dimensions, within the following constraints:
- If there are no centering rings beneath a fin, the trailing edge of the fin tab is the fin chord trailing edge and the leading edge of the fin tab is the fin chord leading edge.
- If only one centering ring is beneath a fin, the trailing edge of the fin tab is the fin chord trailing edge and the leading edge of the fin tab is the trailing edge of the centering ring.
- If two centering rings are beneath a fin, the trailing edge of the fin tab is the leading edge of the trailing centering ring and the leading edge of the fin tab is the trailing edge of the leading centering ring.
- If more than two centering rings are beneath a fin, referring to the centering rings in order from the trailing edge to the leading edge of the fin chord, the trailing edge of the fin tab is the leading edge of the first centering ring and the fin tab leading edge is the trailing edge of the second centering ring. *OpenRocket* supports only one fin tab on each fin.
Converting a simple rocket to through-the-wall design:
1. At the *OpenRocket***main window**, left-click the **File** menu, then left-click **Open example design** in the drop-down menu.
2. In the pop-up **Open example design** box, left-click the "*A simple model rocket*" selection, then left-click the **Open** button.
3. In the **Rocket design** view, double left-click the **Trapezoidal fin set** component.
4. Left-click the **Fin tabs** tab.
5. Left-click the **Calculate automatically** button.
And, a through-the-wall fin tab is automatically created between the two motor mount centering rings.
Complex rockets fall into two basic categories, a rocket that is propelled by a cluster of motors intended to be simultaneously ignited or multi-staged (massively-staged), propelled by a series of motors that successively ignite the next in line when the prior motor burns out.
:alt:From [xkcd 'what if' #24](https://what-if.xkcd.com/24/): *How many model rocket engines would it take to launch a real rocket to space?*, a 65,000 motor staged-and-clustered rocket. Recommended reading for all rocketeers.
Motor Clustering
----------------
Clustering refers to launching a rocket with more than one simultaneously-ignited rocket motor. Clustering is common in "real" aerospace programs. Familiar American examples include: the [Gemini Titan](https://en.wikipedia.org/wiki/Titan_II_GLV) - a two-motor cluster, the [Saturn V](https://en.wikipedia.org/wiki/Saturn_V#S-IC_first_stage) - a cluster of five Rocketdyne F-1 motors driving the first stage, and the [Falcon 9](https://en.wikipedia.org/wiki/Falcon_9#Launcher_versions) - a cluster of 9 Merlin motors driving the main stage.
In model and high-power rocketry, typical clusters seen are **2-motor**, always side-by-side, due to the geometry, **3-motor**, in a triangle or straight line, **4-motor**, in a square, and **5-motor**, typically arranged with one central motor surrounded by 4 in a square - though other arrangements are possible. There's nothing preventing much larger ones, but 2, 3, 4 and 5 are most-often seen.
In three- and five-motor clusters, it's not uncommon to see a larger or higher-power central motor, surrounded by smaller or weaker motors. This may be done for effect, or due to modeling constraints, or to more closely resemble its full-scale inspiration, or possibly for reasons of cost. Clustered motors may be "canted" - that is, pointed to the outside of the rocket fuselage's circumference, for effect, stability, or spin.
Designing a Rocket with Clustered Motors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
OpenRocket makes it easy to design motor clusters. To begin with, add an **Inner Tube** to your aft-most **Body Tube**. On the **Motor** tab, check the "This component is a motor mount" box. Set its inner diameter to one of the standard motor sizes, unless you have a unique need: 13, 18, 24, 29, 38, 54, 75 or 98mm. Next, click on the **Cluster** tab.
The **Cluster** tab lets you choose a common cluster configuration, and adjust it in your model. When you make an **Inner Tube** a cluster, you treat every tube in the cluster identically with each addition. If you add an **Engine block** or a **Mass component**, all of the tubes in the cluster will receive one.
First, pick a cluster configuration from the image tiles on the left side of the tab. Realize that depending upon the sizes of your motor tube and body tube, not every cluster that you can make will fit.
Next, adjust the **Tube separation**. This value controls how close the clustered motors are to each other. A value of 1 places the tubes in contact with each other. You can enter decimals like "1.25" in the separation field. In addition to potentially affecting your rocket's stability, the **Tube separation** you choose may influence the difficulty of wiring your clustered motors for ignition, and your ability to place adhesive and parts around tightly-packed tubes during construction.
The **Rotation** setting rotates your cluster around the major axis of your rocket (the Up <--> Down one). It's used to line up the motors with other decorative and structural components of your rocket. This alignment may be critical if you're creating a design that ducts eject gasses from one part of the rocket to another.
The **Split cluster** button changes this component from a clustered motor component that can be handled as a unit, to individual motor tubes, which may be positioned and edited independently of each other. Once you split the cluster, items and settings you change for each tube will not automatically be added to the other tubes in the cluster. You may want this option if you have motor mount tubes of different lengths or diameters in the cluster. *Once split, a cluster cannot be recombined*. You must re-create the cluster as a unit if you'd like to revert to that approach.
Igniting a Cluster
^^^^^^^^^^^^^^^^^^
Important to the stability of the rocket's flight is that all the motors ignite more or less simultaneously. The initial concerns here are that all the motors' igniters are wired to take a single application of voltage from the launch controller, and that the controller be able to provide adequate voltage and current to ignite all the motors.
Estes Rockets used to advise that igniter wires be twisted together in either [series or parallel configurations](https://en.wikipedia.org/wiki/Series_and_parallel_circuits). Each has its advantages: with a series connection, any burnt igniter will show an open circuit upon arming, while with a parallel connection, the launch controller can use the same voltage as always, but supply more current to ignite multiple motors at once. Today, most clusters are wired in parallel, and the rocketeer must ensure that ample current is available for launch.
Some cluster igniter wiring schemes use a **buss bar** - a short length of regular conductive wire, typically non-insulated, for ease of connecting to it as needed - as a way of bridging what can be complex connections in a tight space, into an easier connection plan. For example, you can twist one end of each igniter together in a bundle, and the other end of each to the buss bar. The launch micro-clips then connect one to the bundle, and the other to the buss bar, for a parallel connection.
A convenient tool for igniting a cluster is a **cluster whip** - a set of wires and micro-clips that allows the single pair of clips at the launch pad to be easily broken into multiple sets of clips, to attach to multiple igniters, and providing a parallel connection. The cluster whip connects to the igniters, and the launch controller's micro-clips connect to conductors on the cluster whip.
Igniting Clustered APCP motors
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
APCP (*Ammonium Perchlorate Composite Propellant*) motors typical of Aerotech, Cesaroni, and Loki, are slower to ignite than Black Powder motors (typical Estes motors). They may unpredictably "chuff", sit quiet for a moment and then ignite, or even "spit" the igniter out. Because of this difference, and the unpredictability of APCP motor ignition, it's more than a little likely that clustered APCP motors won't ignite simultaneously, if at all. When designing for an APCP cluster (if you decide to roll these dice...), take into account what will happen to the rocket if not all motors ignite before it pulls away from the pad. The safety of observers, and of your airframe hang in the balance.
Using Clustering for Body Tubes With, or Without Motors
It's possible to create imaginative designs, or mimic scale rockets of yesteryear by using OpenRocket's clustering capability. One limitation of doing so is that regular **Body tube** components have no Cluster tab. To add Clustered tubes (which are, by OpenRocket's definition always **Inner Tubes**) using OpenRocket's clustering features, you must first have a regular **Body tube**.
You can use your regular **Body tube** as strapping around your clustered tubes, as a **Nose cone** mount, as an eject gas manifold, or even create a "Vestigial" body tube. To do this, add a regular **Body tube** then set its length to something like .0001 cm. The **Body tube** will be in the hierarchy and can have **Inner tubes**, but will barely be seen in the renderings.
There are some limitations, as **Inner tubes** are not meant to be used in this way. They can't take a **Nose cone** nor some other components. They won't affect aerodynamics, even if you're trying to make them into tube-fin-like things. And whether as a unit or as a **Split cluster**, you can't convert **Inner tubes** to **Body tubes**.
Conventional Staging
--------------------
A "closed-hull" design with a separating airframe in which finned-stages holding motors are stacked up, and lower stages holding burned-out casings separate under pressure as upper stages ignite. Conventional staging is inherently limited to three stages because of the "Pisa Effect" which results in an increasing arcing trajectory with each stage.
In designing multi-staged rockets, it's important to realize that the center of mass will tend to start well toward the rear of the rocket, based on the booster stage(s) weighted with the loaded motors. As booster motors are spent and the spent stage(s) ejected, the center of mass will tend to move forward. Careful design ensures that the center of mass remains forward of the center of pressure throughout the flight. Weighting and weight redistribution can move the center of mass forward, while larger fin area tends to move the center of pressure aft. Ensure at least 1.0 airframe caliber of separation between the (forward) center of mass and (aft) center of pressure. This is a rule of thumb, not a hard-and-fast stability solution.
Rack Staging
------------
An "open-hull" design with a non-separating airframe in which motors are staked up, end-to-end, in a frame, and only the burned-out casings are ejected under pressure as higher stages ignite, stage-after-stage. So long as high average impulse lower stage motors are used to ensure adequate initial velocities, rack staging is not inherently limited because this design overcomes the "Pisa Effect."
Here's a **[2007 video demonstrating rack staging](https://sites.google.com/site/theskydartteam/projects/model-rocketry/rack-rocket-design)**.
The BPS Aerospace thrust-vectoring design uses this approach to move a new motor into position for a landing burn.
Regulatory Concerns
====================
Rocketry is subject to regulation by federal, state, and local governments, and most of the regulations that rocketeers must follow are promulgated by the National Fire Protection Association (NFPA) and the Federal Aviation Administration (FAA). The NFPA divides rockets into two major classifications, model rockets (NFPA § 1122) and high power rockets (NFPA § 1127), the difference primarily being weight and power, as follows:
-**Model Rocket**. A rocket vehicle that weighs no more than 1500 g (53 oz) with motors installed, is propelled by one or more model rocket motors having an installed total impulse of no more than 320 N-sec (71.9 lb-sec), and contains no more than a total of 125 g (4.4 oz) of propellant weight. (NFPA § 1122, subd. 3.3.7.2.)
-**High Power Rocket**. A rocket vehicle that weighs more than 1500 g (53 oz) with motors installed and is either propelled by one or more high power rocket motors or by a combination of model rocket motors having an installed total impulse of more than 320 N-sec (71.9 lb-sec). (NFPA §1127, subd. 3.3.13.1.)
Within the high power rocket classification, a subclassification for "complex" rockets is defined as a high power rocket that is multi-staged or propelled by a cluster of two or more rocket motors. (NFPA §1127, subd. 3.3.13.1.1.) And, a high power rocket launched with an installed total impulse greater than 2,560 N-sec (576 lb-sec) must have an electronically actuated recovery system. (NFPA §1127, subd. 4.10.2.)
National Association of Rocketry pursuits the goal of safe, fun and educative sport rocketry. It is the oldest and largest sport rocketry organization in the world. Visit dedicated [Wiki page](http://en.wikipedia.org/wiki/National_Association_of_Rocketry) or [NAR official website](http://www.nar.org/) for more information.
The major work of the NAR includes, but not limited to:
- Certification of Rocketry-Related products and establishment of safety codes
The NAR is a recognized authority for safety certification of consumer rocket motors and user certification of high- power rocket fliers in the U.S. It plays a major role in establishment of safety codes for the hobby used and accepted by manufacturers and public safety officials nationwide.
- Certification of experienced rocketeers
NAR issues three levels of High Power Rocketry (HPR) certificates, Level 1 (L1) through Level 3 (L3). Certificates are necessary to purchase powerful rocket motor components.
- Communication with public officials
The NAR helps in communication with local public safety officials, and government regulatory agencies such as the Department of Transportation, Federal Aviation Administration, Bureau of Alcohol Tobacco Firearms and Explosives, and Consumer Product Safety Commission.
- Other work
The NAR publishes the bimonthly color magazine Sport Rocketry (sent to each member and selected libraries and newsstands around the nation). The NAR provides a wide range of other services to its members, including: education programs; national and local competitions; grants to teachers and scholarships for student members; flight performance record recognition; liability insurance; and publication of technical literature.
