Advanced Rocket Design

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The current User's Guide is very much a work in progress, any help would be greatly appreciated!
If you'd like to contribute something, just hit the 'Edit' tab at the top.

Advanced Rocket Design


In this section, advanced design principles and concepts are discussed, with step-by-step instructions describing how to incorporate these techniques into designs created in OpenRocket. Implementing the techniques described in this section may require specialized materials and electronic devices intended for use only by experienced rocketeers.

Advanced rocket design encompasses configurations for high power rockets generally, including:

  • recovery systems
  • through-the-wall fin construction
  • electronic and dual deployment
  • complex multi-staging (such as motor racking) and motor clustering
  • roll stabilization

Additionally, you may find that using mass and center of gravity (CG) overrides will improve OpenRocket flight and recovery simulation accuracy.

Recovery Systems

Recovery Techniques

Recovery systems are intended to return a rocket safely to the ground, without harm to people or damage to the rocket or other objects. Though recovery mechanisms vary greatly, recovery systems generally include elements of one or more of these techniques:

  • featherweight
  • break-apart
  • streamer
  • parachute
  • helicopter
  • gliding

Featherweight and Break-Apart Recovery

Featherweight and break-apart recovery work by creating enough drag to ensure that the terminal velocity of the rocket is so low that it won't be damaged or do damage when it hits the ground. Featherweight designs are often minimum diameter rockets that eject the burned-out motor casing altogether, or use the ejection charge to shift the casing position rearward after motor burnout (within an extended motor hook), to induce instability and cause the rocket to tumble. Break-apart recovery, aerodynamically, does the same thing, increasing drag and inducing instability by breaking the rocket into two or more sections connected together by a shock cord. Typically, a feather weight rocket, and each section of a break-apart rocket weighs less than one ounce.

Example Featherweight Design

In earlier years, Estes sold a few rockets that featured Featherweight Recovery, meaning that a very light model could spit out its used motor casing, and land directly on the ground with no damage, rather like a badminton shuttlecock.

The Estes Astron Streak, ca 1970 - a featherweight recovery rocket

Ejecting burned-out motor casings is not allowed in NAR contests unless a streamer or parachute is attached to the ejected casing.

Example Break-Apart Design

Another approach to bringing a lightweight rocket safely to earth is to break the airframe into several aerodynamically unstable pieces, and "flutter" them back to earth.

One such design: the Unicon (for "Unified/Consolidated" - a stretch I know) appeared as a plan in the Estes Model Rocket News publication, probably about 1973-4. The body tube featured 3 pieces of launch lug, as did the nose cone. Into these lugs plugged balsa one of 3 balsa sticks, each of which held a fin, and a piece of body tube attached to the fin root and the stick. to provide a stable anchor against the body. The entire assembly was connected with pieces of heavy-duty thread.

When the ejection charge fired, the nose cone popped off, releasing the sticks with fins, which, tied to the body tube and nose cone, fluttered in the air, and slowed descent of the main airframe as it landed.

This author built one, and it worked pretty well.

Streamer and Parachute Recovery

Streamers and parachutes add drag to slow the rocket descent rate. Generally, a larger streamer is always better. But, streamer size is an example of the principle of diminishing marginal returns, eventually making a streamer larger will only add slightly increase drag (a rocket weighing more than 10 oz is beyond the effective use of a streamer). On the other hand, because of their efficiency, parachutes create more drag with less cloth than any other method, and virtually all high power rockets use parachutes.

Example Streamer Design

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NAR requires 10 square cm of streamer area per gram of mass in contest models.

Example Parachute Design

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NAR requires 5 square cm of parachute area per gram of mass in contest models.

Helicopter and Gliding Recovery

Helicopter Recovery

Helicopter recovery relies upon rigid lift-generating blades and auto-rotation to slow terminal velocity. This design technique is the most complicated of all, and requires that the entire rocket be designed around the recovery device. As important, the stresses generated by rapidly spinning blades hitting the ground effectively limits the use of this technique to low mass (model) rockets.

Gliding Recovery

A glider uses aerodynamic lift to control terminal velocity. However, because the aerodynamic requirements of vertical flight are vastly different than gliding flight, to make this transition there must be a shift in the center of gravity or the center of pressure. This transition can be made by reducing mass (ejecting the motor mount tube and weights) or changing aerodynamic signature (ejection activated fin-elevators or swing-wings). Radio and other control systems are currently being used to fly gliding recovery rockets, even high power.

Example Design

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Protecting Recovery Components

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Heat Shields

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Fire Resistant Wadding and Blankets

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Piston Ejection

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Ejection Gas Cooling

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Cooling Mesh

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Baffles

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CO2 Ejection Devices

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Tube Fins and Ring Tails

Tube Fins

A tube fin is just that, using a shorter section of body tube, adhered to the main body tube, as a fin, with or without other flat fins. Although this type of rocket is easy enough to build, creating an accurate simulation can be anything but.

The aerodynamic flight of a rocket is affected by both how far the tip of a fin is from the body of the rocket and the surface area and profile of the fin. Using a 1 inch diameter tube, 2" long as an example, the tube fin has a surface area equal to pi (22/7) times the tube diameter times its length, or about 6 square inches. But, looking at the tube fin from the side (in profile), its area is length times height (the tube diameter), or about 2 square inches. Dividing the former by the later, three "flat" fins are aerodynamically about the same as one tube fin.

So, to simulate tube fins, for each tube fin, three flat fins having the same profile as the tube fin are substituted. One tube fin equals three flat fins, two tube fins equals six flat fins, and so on. OpenRocket allows a maximum of eight fins per fin set, so six tube fins of the same kind can be represented by six fin sets (each containing three flat fins), or by three fin sets (two containing eight flat fins and one containing two flat fins). Just be aware that the Too many parallel fins warning in OpenRocket activates when more than eight total fins are created, and this warning appears in the results of every flight simulation.

How does that work in OpenRocket? First, load the Simple model rocket example:

  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.

Tubes as Fins

Keeping in mind how tube fin simulation is done as described above, the Simple model rocket example design is converted to six tube fins, each with a 45 degree slant on the leading edge, following these steps:

  1. Double left-click the Body tube component in the Rocket design view, and write down the Wall thickness of the tube.
  2. Double left-click the Trapezoidal fin set component in the Rocket design view.
  3. Change the Number of fins to 6 (six tube fins).
  4. Change the Root chord to 2 cm.
  5. Change the Tip chord to 1 cm.
  6. Change the Height to 1 cm.
  7. Change the Sweep angle to 45 degrees (the Sweep length will automatically calculate).
  8. Change the Thickness to match the fin tube Wall thickness from Step 3 (.039).
  9. Now, to create flat fin sets for each tube fin, left-click the Split fins button.
    10. At this point, six individual fin sets have been created, each fin set containing one fin.
  10. Next, double left-click Trapezoidal fin set #1, and change the Number of fins to 3. Make sure the Fin rotation is 0 degrees, then left-click the Close button.
  11. Left-click the Back view button so that you can see the fin angles for the remaining steps.
  12. Double left-click Trapezoidal fin set #2, and change the Number of fins to 3. Change the Fin rotation to 20 degrees (360 degrees divided by 18 fins), then left-click the Close button.
  13. Double left-click Trapezoidal fin set #3, and change the Number of fins to 3. Change the Fin rotation to the previous rotation plus 20 degrees (20 degrees + 20 degrees = 40 degrees), then left-click the Close button.
  14. Repeat the previous step for each of the remaining fin sets, increasing the Fin rotation by an additional 20 degrees for each set (0, then 20, then 40 then 60, and so on).
    15. At this point, each fin set contains 3 fins, 18 fins in all.
  15. Now, rotate the launch lug so that it is between fins. Double left-click the Launch lug component and change the Radial position to 10 degrees, then left-click the Close button.

The conversion is complete. Left-click the Side view button to see the completed rocket in profile.

By adjusting the Tip chord length and Sweep angle, tube fin angles from 90 degrees to virtually any other angle can be created.

Tube Around Each Fin

This section describes how to convert a simple model rocket design to flat fins with tube fins surrounding each at the trailing edge. The tube fins are the same diameter as the main body tube.

Beginning with the Simple model rocket example started above, modify the main flat fins as follows:

  1. Double left-click the Body tube component in the Rocket design view window, and write down the Outer Diameter, Inner Diameter, and Wall thickness, (2.5, 2.3, and .1 cm). Then, left-click the Close button.
  2. Double left-click the Trapezoidal fin set component in the Rocket design view window.
  3. Change the Root chord to 8 cm.
  4. Change the Tip chord to 2 cm.
  5. Change the Height to the outside diameter of the tube fin, 2.5 cm.
  6. Change the Sweep length to 6 cm (the Sweep angle will automatically calculate).

Now, the main flat fins need to be converted from trapezoid to freeform, and then notched to accept the tube fins.

  1. Left-click the Convert to freeform button.
  2. Left-click the Shape tab.
  3. On the graph of the fin, left-click on a fin line to create a new coordinate, and repeat doing so in different places until there are seven coordinate boxes.
  4. In the x/y table, change the coordinates to the following points:
    • 0 | 0
    • 6 | 2.5
    • 6 | 2.4
    • 8 | 2.4
    • 8 | 0.1
    • 6 | 0.1
    • 6 | 0.1
  5. Then reposition the fin, left-clicking the General tab, and changing the Relative position to - plus to "-2" cm. Then, left-click the Close button.

Now that the main flat fins are done, its time to add the surrounding tube fins.

  1. Left-click the Body tube component in the Rocket design view window, then, in the Add a new component window, left-click Trapezoidal fin set.
  2. Change the Number of fins to 3 (three tube fins).
  3. Change the Root chord to 2 cm.
  4. Change the Tip chord to 2 cm.
  5. Change the Height to 2.5 cm, the inside diameter of the tube.
  6. Change the Sweep length to 0 cm (the Sweep angle will automatically calculate).
  7. Change the Thickness to .1 cm.
  8. Now, create substitute flat fin sets for each tube fin, left-click the Split fins button.

At this point, three individual trapezoidal fin sets have been created, each fin set containing one fin.

  1. Left-click the Back view button so that you can see the fin angles for the remaining steps.
  2. Double left-click Trapezoidal fin set #1, and change the Number of fins to 3. Change the Fin rotation to 30 degrees (360 degrees divided by 12 fins [9 tube fins plus 3 flat fins]), then left-click the Close button.
  3. Double left-click Trapezoidal fin set #2, and change the Number of fins to 3. Change the Fin rotation to the previous rotation plus 30 degrees (30 degrees + 30 degrees = 60 degrees), then left-click the Close button.
  4. Double left-click Trapezoidal fin set #2, and change the Number of fins to 3. Change the Fin rotation to the previous rotation plus 30 degrees (60 degrees + 30 degrees = 90 degrees), then left-click the Close button.
    5. At this point, the rocket has three trapezoidal fin sets containing 3 fins each, plus the 3 flat fins, 12 fins in all.
  5. To finish, rotate the launch lug so that it is between fins. Double left-click the Launch lug component and change the Radial position to 15 degrees, then left-click the Close button.

The conversion is complete. Left-click the Side view button to see the completed rocket in profile. Although the tube notches can be seen, separate from the substitute tube fins, this design is the aerodynamic equivalent for simulation purposes.

Ring Tails

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Through-the-Wall Fin Mounting

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.

    Electronic and Dual Deployment

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    Clustering and Multi-staging

    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.

    Motor Clustering

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    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.

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    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."

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    Roll Stabilization

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    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

    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 or NAR official website 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.

      Tripoli Rocketry Association

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      User Guide Contributions

      Use the clustered, staged and dual-deploy example designs to cover the more advanced motor and recovery configuration options. Talk about using mass and CG overrides to improve accuracy. Special designs that might have their own sub-chapters:
      - clusters + staged (including rack rocket (https://sites.google.com/site/theskydartteam/projects/model-rocketry/rack-rocket-design))
      - dual-deploy and other more advanced recovery options (if we can think of any)
      - roll stabilization

      It would be good to include a description on how the different components affect the simulations and what the limits are (e.g. an inner tube won’t affect the aerodynamics even if you move it outside of the body tube).

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