A Scientific Guide to Hobby Rocketry Fundamentals of HPR Authors: Joseph, Edited by Matthieu & GJ 1/42 October 10th, 2018
Aerodynamics • 2/42
Aerodynamic Flight Regimes Low speed Compressible Transonic Supersonic Hypersonic Mach 0.3 0.7 1.2 5 3/42
Nose Cone Aerodynamics • Various geometries have different drag coefficients • Minimum drag bodies like the von Karman ogive have best across-the-board performance • Some shapes perform best in certain Mach regimes • Model rocketry nose cones are generally ogives 4/42
Effects of Rocket Length • Longer rockets lead to increases in skin friction drag • Increased length-to-diameter ratio (fineness ratio) leads to a decrease in pressure drag per rocket volume • Longer rockets are subject to extreme bending moments 5/42
Fin Aerodynamics Rectangular cross section •Simple to manufacture •Relatively high drag coefficient for airfoils with similar thickness-to-chord ratios Rounded cross section •Not too difficult to manufacture •Decent aerodynamic performance, but not the best Airfoil cross section •Optimal fin cross section for subsonic rockets, but prone to high drag and shocks at supersonic speeds •Should have a symmetric cross section Wedge cross section •Good aerodynamic performance at supersonic speeds •Decent aerodynamic performance at subsonic speeds 6/42
Stability • 7/42
Stable Rockets Center of mass Center of pressure Net aerodynamic force Net rotation of rocket 8/42
Unstable Rockets Center of mass Center of pressure Net aerodynamic force Net rotation of rocket 9/42
Why Stability Matters • Unstable rockets – BAD o Can spiral out of control under slight disturbances o Neutral stability is also BAD – rocket will not tend to “right” itself • Marginally Stable rockets – LESS BAD o Will be slow to “right” itself. Increasing velocity and nonzero α values generally cause static stability to drop, and it may become unstable o Base Drag is not considered in static stability margin; low aspect ratio rockets are often fine being marginally stable • Stable rockets – GOOD o Trajectory minimally perturbed by wind • Over-stable rockets – OKAY o Tend to weathercock, or fly into the wind o Not terrible, but can lead to horizontal flight on windy days 10/42
Effects of Geometry on Stability • 11/42
Effect of Weight on Stability • 12/42
Effect of Speed on Stability • 13/42
Structures • Cardboard tubes with plywood interior structure generally suitable for low-thrust, low-speed flight • Thicker structural materials needed for heavier, higher-thrust flights • Fiberglass and other composites become necessary for high-speed flight • Ductile metals as structural materials only permitted when deemed absolutely critical for structural integrity 14/42
Weight • Heavier rockets require more robust structures • Landing can cause poorly constructed components to be crushed from impact force or moments when tipping over • Heavy-weight rockets require much larger parachutes to land at safe speeds o Also need high-thrust motors to leave the launch pad at safe speeds 15/42
Fin Shapes • Stress tends to be higher in sharp corners and regions of abrupt area change. • Avoid highly swept fins with sharp corners o If sweep is necessary, use right or obtuse angles with reasonably large side lengths • Tapered fins that are not swept aft of the rocket tend to work really well • Same rules apply to forward sweep. When possible, avoid sweeping the leading edges of your fins forward. 16/42
Fin Dimensions • Fins with a long span can break easily due to excessive bending moments from aerodynamics and ground impacts • Thicker fins can carry much more load and bend less (generally good from a flutter perspective) • Try to minimize aspect ratio (span/chord) to minimize chance of breaking a fin o Too low of an aspect ratio leads to bad stability characteristics 17/42
Adhesives • 5-minute epoxy • Super glue o Short set time, but the bond is not o Forms bond almost instantly as high in strength o Weak, brittle bond o Good for quick repairs o Suitable for placing a component • 1-hour epoxy o Not suitable as only bond o Ideal for most structural • Wood glue components o Can use additives to enhance o Works well on porous materials various properties o Forms moderate strength bond • JB Weld (sufficient for some high power) o High strength, but more brittle o Great for fillets 18/42
Recovery • Good recovery is key for ensuring rocket safety • Landing speed should be slow, but not too slow o Too fast: things break o Too slow: things float forever and get lost • Ideal landing speeds are 15-20 ft/s o Some rocketeers recommend 17-22 ft/s o Lighter and/or sturdier rockets can generally survive the upper end of this range and somewhat beyond. • Typically achieved by one or two parachutes 19/42
Sizing a Parachute • 20/42
Sizing a Parachute • 21/42
Shock Cord • 22/42
Shock Cord • Rocket structure (materials and adhesives) must be capable of supporting loads, too • To reduce F during full shock cord extension, reduce Δ v and/or increase Δ t o Use drag force of rocket body to your advantage o Drag takes away some separation velocity so Δ v is smaller o The shock cord is slightly “stretchy”. This, and deliberate packing schemes, sacrificial stitching, addition of “spring” mechanisms, and other factors act to increase Δ t • To maximize effect of aerodynamics for reducing Δ v, make shock cord “infinitely” long o Not very practical, so use a minimum of 20 ft as a rule-of-thumb 23/42
Recovery Materials Parachutes • • Plastic o Melts easily o Does not support large loads, mainly for low power applications • Ripstop nylon o Traditional parachute material o Easy to manufacture, buy • Mylar o Expensive • Traditional fabrics o Heavy 24/42
Recovery Projection • Most recovery devices can be burned and damaged by hot gases from ejection charge • Fireproof cellulose insulation (aka “dog barf”) can be stuffed between ejection charge and recovery device o Wadding functions similarly for low power rockets • Kevlar or Nomex sheets often used to wrap parachutes o Much more expensive, but reusable and high quality • Strategically placed baffles can reduce exposure to hot gas 25/42
Launching a Rocket • Rockets launched using a rod or rail • As rocket accelerates, the rod or rail points the rocket in the correct direction o Rocket cannot achieve reasonable stability at low speeds • Rule of thumb: velocity of any rocket off the rod or rail should be at least 50 ft/s o Much easier to accelerate light rocket than heavy rocket o “Odd” designs or stability extremes will require more detailed analysis o NAR requires a thrust-to-weight ratio off the pad of no less than 5 26/42
Launch Lugs • Generally only used for low power rockets • Interface with launch rod (circular metal rod) • Common sizes are 1/8”, 3/16”, 1/4”, 3/8”, and 1/2” • Not used much for high power since the rod tends to “whip” • Single or multiple lug(s) (cardboard tube) aligned axially with rocket to keep motion near vertical • Rods vary in length depending on compatible motor sizes 27/42
Rail Buttons • Used for High Power Rocketry • “Rail buttons” screw into rocket and slide down the launch rail • Common sizes are 1010 (1” rail) and 1515 (1.5”rail) • Use two rail buttons aligned axially with the rocket • Bottom rail button should be ~2 inches from aft of rocket • Second rail button should be 12-18 inches forward 28/42
Rail Buttons • If the front rail button is too far forward, the rocket can pivot about the aft rail button once the first button has cleared the rail but velocity is not sufficient • Anchor rail buttons into rocket using expanding rubber well nut or a tee nut o Aft button usually requires well nut o Forward rail button can be placed (with planning) using tee nut • Rail length usually 8-10 ft for 1010 and 12+ ft for 1515 29/42
Propulsion • Commercial, off the shelf solution for hobby rockets • Generally uses an ammonium perchlorate (AP) composite propellant for high power, black powder (BP) for low power o Space Shuttle SRB used an AP-based propellant • Fine-grained AP and aluminum in HTPB rubber binder with other chemicals for effects • Solid propellant with annular grain geometry (generally) 30/42
Thrust & Impulse • 31/42
Identifying Motors Impulse class Propellant type Average thrust (N) Ejection charge delay 32/42
How High Will it go? • 33/42
National Rocketry Clubs • Must be a registered member of National Association of Rocketry (NAR) or Tripoli Rocket Association (Tripoli) to launch and attempt high power certifications • We are a NAR club, so NAR memberships help us maintain NAR national benefits o NAR members get a nice bi-monthly magazine • Tripoli Level 2 members may use experimental propellant o But not the Georgia Tech Fire Marshal … 34/42
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