A Scientific Guide to Hobby Rocketry Fundamentals of HPR Authors: - - PowerPoint PPT Presentation

A Scientific Guide to Hobby Rocketry

Fundamentals 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 5 Mach

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 – BAD
• Can spiral out of control under slight disturbances
• Neutral stability is also BAD – rocket will not tend to “right” itself
• Marginally Stable rockets – LESS BAD
• Will be slow to “right” itself. Increasing velocity and nonzero α values

generally cause static stability to drop, and it may become unstable

• Base Drag is not considered in static stability margin; low aspect ratio

rockets are often fine being marginally stable

• Stable rockets – GOOD
• Trajectory minimally perturbed by wind
• Over-stable rockets – OKAY
• Tend to weathercock, or fly into the wind
• 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 speeds

• 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 corners
• 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
• f breaking a fin
• Too low of an aspect ratio leads to bad stability characteristics

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Adhesives

• Super glue
• Forms bond almost instantly
• Weak, brittle bond
• Suitable for placing a component
• Not suitable as only bond
• Wood glue
• Works well on porous materials
• Forms moderate strength bond

(sufficient for some high power)

• Great for fillets

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

as high in strength

• Good for quick repairs
• 1-hour epoxy
• Ideal for most structural

components

• Can use additives to enhance

various properties

• JB Weld
• High strength, but more brittle

Recovery

• Good recovery is key for ensuring rocket safety
• Landing speed should be slow, but not too slow
• Too fast: things break
• Too slow: things float forever and get lost
• Ideal landing speeds are 15-20 ft/s
• Some rocketeers recommend 17-22 ft/s
• 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

Δt

• Use drag force of rocket body to your advantage
• Drag takes away some separation velocity so Δv is smaller
• 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” long

• 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
• Melts easily
• Does not support large loads, mainly for

low power applications

• Ripstop nylon
• Traditional parachute material
• Easy to manufacture, buy
• Mylar
• Expensive
• Traditional fabrics
• 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 device

• Wadding functions similarly for low power rockets
• Kevlar or Nomex sheets often used to wrap parachutes
• 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

direction

• 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

• Much easier to accelerate light rocket than heavy rocket
• “Odd” designs or stability extremes will require more detailed analysis
• 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 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

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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 nut

• Aft button usually requires well nut
• 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 power

• Space Shuttle SRB used an AP-based propellant
• Fine-grained AP and aluminum in HTPB rubber binder with
• ther 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 charge delay

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 benefits

• NAR members get a nice bi-monthly magazine
• Tripoli Level 2 members may use experimental propellant
• 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:
• Total impulse exceeds 160 Ns (H motor and above)
• Average thrust > 80 N
• Propellant mass > 125 g
• Rocket weight > 1500 g
• Airframe includes ductile metal
• 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 rocket

• Must not lose any components in flight
• Must not break any components (zippering is at the discretion of the

certifier)

• 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 you
• Landing in a tree or lake
• Motor CATO
• Landing without successfully deploying a parachute
• For NAR members
• RRC certified members can sign off on your certification and should be

happy to do so

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

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The End.