A Scientific Guide to Hobby RocketryFundamentals of HPR

Authors: Joseph, Edited by Matthieu & GJ

October 10th, 2018 1/42

Aerodynamics

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Aerodynamic Flight Regimes

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Low speed Compressible Transonic Supersonic Hypersonic

0.3 0.7 1.2 5Mach

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

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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

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Fin Aerodynamics

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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

Stability

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Stable Rockets

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Center of mass

Center of pressure

Net aerodynamic force

Net rotation of rocket

Unstable Rockets

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Center of mass

Center of pressure

Net aerodynamic force

Net rotation of rocket

Why Stability Matters

• Unstable rockets – BADo Can spiral out of control under slight disturbanceso Neutral stability is also BAD – rocket will not tend to “right” itself

• Marginally Stable rockets – LESS BADo Will be slow to “right” itself. Increasing velocity and nonzero α values

generally cause static stability to drop, and it may become unstableo Base Drag is not considered in static stability margin; low aspect ratio

rockets are often fine being marginally stable

• Stable rockets – GOODo Trajectory minimally perturbed by wind

• Over-stable rockets – OKAYo Tend to weathercock, or fly into the windo Not terrible, but can lead to horizontal flight on windy days

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Effects of Geometry on Stability

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Effect of Weight on Stability

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Effect of Speed on Stability

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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

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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 speedso Also need high-thrust motors to leave the launch pad at safe speeds

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Fin Shapes

• Stress tends to be higher in sharp corners and regions of abrupt area change.

• Avoid highly swept fins with sharp cornerso 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.

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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 fino Too low of an aspect ratio leads to bad stability characteristics

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Adhesives

• Super glueo Forms bond almost instantlyo Weak, brittle bondo Suitable for placing a componento Not suitable as only bond

• Wood glueo Works well on porous materialso Forms moderate strength bond

(sufficient for some high power)o Great for fillets

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• 5-minute epoxyo Short set time, but the bond is not

as high in strengtho Good for quick repairs

• 1-hour epoxyo Ideal for most structural

componentso Can use additives to enhance

various properties• JB Weld

o High strength, but more brittle

Recovery

• Good recovery is key for ensuring rocket safety• Landing speed should be slow, but not too slow

o Too fast: things breako Too slow: things float forever and get lost

• Ideal landing speeds are 15-20 ft/so Some rocketeers recommend 17-22 ft/so Lighter and/or sturdier rockets can generally survive the upper end of

this range and somewhat beyond.

• Typically achieved by one or two parachutes

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Sizing a Parachute

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Sizing a Parachute

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Shock Cord

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Shock Cord

• Rocket structure (materials and adhesives) must be capable of supporting loads, too

• To reduce F during full shock cord extension, reduce Δv and/or increase Δto Use drag force of rocket body to your advantageo Drag takes away some separation velocity so Δv is smallero The shock cord is slightly “stretchy”. This, and deliberate packing schemes,

sacrificial stitching, addition of “spring” mechanisms, and other factors act to increase Δt

• To maximize effect of aerodynamics for reducing Δv, make shock cord “infinitely” longo Not very practical, so use a minimum of 20 ft as a rule-of-thumb

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Recovery Materials

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Parachutes• Plastic

o Melts easilyo Does not support large loads, mainly for

low power applications

• Ripstop nylono Traditional parachute materialo Easy to manufacture, buy

• Mylaro Expensive

• Traditional fabricso Heavy

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 deviceo Wadding functions similarly for low power rockets

• Kevlar or Nomex sheets often used to wrap parachuteso Much more expensive, but reusable and high quality

• Strategically placed baffles can reduce exposure to hot gas

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Launching a Rocket

• Rockets launched using a rod or rail

• As rocket accelerates, the rod or rail points the rocket in the correct directiono 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/so 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

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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

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Rail Buttons

• Used for High Power Rocketry

• “Rail buttons” screw into rocket and slidedown 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

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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 nuto 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

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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 powero 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)

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Thrust & Impulse

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Identifying Motors

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Impulse class

Average thrust (N)

Propellant type

Ejection chargedelay

How High Will it go?

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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 benefitso NAR members get a nice bi-monthly magazine

• Tripoli Level 2 members may use experimental propellanto But not the Georgia Tech Fire Marshal…

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High Power Rocketry

• Refers to any launch where any of the following are true:o Total impulse exceeds 160 Ns (H motor and above)

o Average thrust > 80 N

o Propellant mass > 125 g

o Rocket weight > 1500 g

o Airframe includes ductile metal

o Rocket uses a hybrid motor (Tripoli Research Launches only, N2O only)

• You must be a registered member of NAR or Tripoli before you can attempt a high power launch

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High Power Rocketry

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Certifications

• Level 1 certification procedures nearly the same for both NAR and Tripoli

• Must construct and fly a rocket on a single Level 1 high power motor and safely recover the rocketo Must not lose any components in flighto Must not break any components (zippering is at the discretion of the

certifier)o Generally, given a new motor, you should be able to immediately fly

the rocket again without modification

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Certifications

• Things that disqualify youo Landing in a tree or lakeo Motor CATOo Landing without successfully deploying a parachute

• For NAR memberso RRC certified members can sign off on your certification and should be

happy to do so• For Tripoli members

o Local Prefect must sign off on your certification paperwork• Must have certification form ready at the launch

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The End.