A Scientific Guide to Hobby Rocketry A Guide to Everything You Need - - PowerPoint PPT Presentation

A Scientific Guide to Hobby Rocketry

A Guide to Everything You Need to Know Before Launching Your First High Power Rocket

Aerodynamics

• One of the three primary forces in hobby rocket flight
• Can greatly affect performance (altitude, etc.)
• Drag force can rip fins apart or cause structural buckling
• D=​1/2 ρ​v↑2 ​C↓D ​A↓ref
• Open Rocket can give you the drag coefficient

– Other variables easy to calculate

Aerodynamic Flight Regimes

Low ¡speed ¡ Compressible ¡ Transonic ¡ Supersonic ¡ Hypersonic ¡

0.3 0.3 0.7 0.7 1.2 1.2 5 5 Mach

Drag in Compressible Flows

• In subsonic flow

​C↓D ≈​C↓D,0 /√⁠1−​M↓∞↑2

• In supersonic flow

​C↓D ≈​C↓D,0 /√⁠​M↓∞↑2 −1

• Drag actually still increases in

supersonic flow because of the dependence on v∞

2!

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

Effect 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

Fin Aerodynamics

Rectangular ¡cross ¡sec:on ¡

• Simple ¡to ¡manufacture ¡
• Rela:vely ¡high ¡drag ¡coefficient ¡for ¡airfoils ¡with ¡similar ¡thickness-­‑to-­‑chord ¡ra:os ¡

Rounded ¡cross ¡sec:on ¡

• Not ¡too ¡difficult ¡to ¡manufacture ¡
• Decent ¡aerodynamic ¡performance, ¡but ¡not ¡the ¡best ¡

Airfoil ¡cross ¡sec:on ¡

• Op:mal ¡fin ¡cross ¡sec:on ¡for ¡subsonic ¡rockets, ¡but ¡prone ¡to ¡high ¡drag ¡and ¡shocks ¡at ¡supersonic ¡speeds ¡
• Should ¡have ¡a ¡symmetric ¡cross ¡sec:on ¡

Wedge ¡cross ¡sec:on ¡

• Good ¡aerodynamic ¡performance ¡at ¡supersonic ¡speeds ¡
• Decent ¡aerodynamic ¡performance ¡at ¡subsonic ¡speeds ¡

Stability

• Stability margin defined as:

m=​x↓CG −​x↓CP /Max ¡body ¡diameter

– Unstable: m<1 – Marginally stable: m=1 – Stable: 1<m ≤ 2 – Overstable: m>2

• Always mark the CP on your rocket

– Will not change with added weight/internal features like CG will

Stable Rockets

Center of mass Center of presssure Net aerodynamic force Net rotation of rocket

Unstable Rockets

Center of mass Center of presssure Net aerodynamic force Net rotation of rocket

Why Stability Matters

• Unstable rockets – BAD

– Can spiral out of control under slight disturbances

• Stable rockets – GOOD

– Trajectory not 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

Effect of Geometry on Stability

• Based on weighted average of normal force coefficient ​

C↓N,α

• Control surfaces such as fins have high values of ​C↓N,α

– Larger surfaces have greater effects

• To move CP aft, place large control surfaces further behind

the old CP location

– Note, larger surfaces also contribute more mass

• ​x↓CP =​∑↑▒​C↓N,α ↓i ​x↓i /∑↑▒​C↓N,α ↓i

Effect of Weight on Stability

• Center of gravity should be above center of pressure
• CG shifts upwards when mass is added above the old CG,

and downwards when mass is added below the old CG

• CG moves more quickly when mass is added further from
• ld CG (from the concept of a moment arm)
• Common solution to add dead weight (or payload) to the

nosecone

• ​x↓CG =​∑↑▒​x↓CG,i ​m↓i /∑↑▒​m↓i

Effect of Speed on Stability

• Like drag, normal force coefficient varies with Mach number
• In subsonic flow

​C↓N,α ≈​C↓N,α,0 /√⁠1−​M↓∞↑2

• In supersonic flow

​C↓N,α ≈​C↓N,α,0 /√⁠​M↓∞↑2 −1

• In general, stability margin drops approaching Mach 1

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

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

Fin Shapes

• Stress tends to accumulate in sharp (acute) corners
• 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

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
• Try to minimize aspect ratio (span/chord) to minimize

chance of breaking a fin

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

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

• 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

• Typically achieved by one or two parachutes

Sizing a Parachute

• Goal of parachute is to decelerate rocket

– Ideally, the rocket will reach terminal velocity (​dv/dt =0) – Statics problem (F = ma = 0), or weight equals aero forces

• W=​1/2 ρ​v↓term↑2 ​C↓D A
• Area=​2W/ρ​v↓term↑2 ​C↓D and Diameter=2√⁠​Area/π

Sizing a Parachute

• Diameter=​2/​v↓term √⁠​2W/ρπ​C↓D
• What values to use?

– W: weight of your rocket (after propellant burns out) – vterm: usually 15-20 ft/s (use higher end for light rockets) – ρ: approximately 1.12-1.2 kg/m3 at our launch site – CD: parachute drag coefficient, about 0.7-0.9 for Level 1 TFR

• Always check your units! You will have to do conversions!

Shock Cord

• Ejection charges usually apply 8-15 psi in a rocket

– Up to 106 lbf on a 3” rocket, 189 lbf on a 4” rocket – Leads to high separation velocity – Quick deceleration at full shock cord extension and parachute inflation

• Recall F≈​m∆v/∆t , where Δt is usually pretty small
• Shock cord must be able to load at full extension and also

entire rocket weight (much smaller) during descent

Shock Chord

• Rocket structure (materials and adhesives) must be

capable of supporting loads, too

• To reduce F during full shock cord extension, reduce Δv

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

• To maximize effect of aerodynamics, make shock cord

infinitely long

– Not very practical, so use a minimum of 20 ft

Recovery Materials

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

Shock cord

• Elastic

– Absorb ejection energy via stretching – Burn easily, so not suitable for HPR

• Tubular nylon (climbing webbing)

– High strength, but moderately heavy – Low cost, easily available – Preferred sizes 9/16” or 1”

• Kevlar

– Very high strength, flame resistance, and cost – Low in weight (typically use ¼” or ½”)

Recovery Protection

• 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 reduce exposure to hot gas

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

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 (cardboard tube) aligned axially with

rocket to keep motion near vertical

• Rods vary in length depending on compatible motor sizes

Rail Buttons

• Used for high power rocketry
• “Rail buttons” screw into rocket and slide

down the launch rail

• Common sizes are 1010 (1”) and 1515

(1.5”)

• 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

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

Propulsion

• Commercial, off the shelf solution for hobby rockets
• 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 in HTPB rubber binder with other

chemicals for effects

• Solid propellant with annular grain geometry (generally)

Thrust & Impulse

• Thrust is a function of time
• Impulse=∫↑▒Tdt

– Also approximately average thrust times burn time

• Average thrust should be at

least five times the rocket weight

• Very high thrust motors can

cause rocket to go supersonic

What’s in a Name?

Impulse class Average thrust (N) Propellant type Ejection charge delay

How High Will It Go?

• Depends on a number of factors, but you can use some
• rder of magnitude estimation:
• h=​1/2 ​t↓burn↑2 (​T/m −10)(​T/10m )

– h: apogee (m) tburn: motor burn time (s) T: average thrust (N) m: initial rocket mass (kg)

• Does not account for all forces and effects

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…

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

• You must be a registered member of NAR or Tripoli

before you can attempt a high power launch

High Power Rocketry

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

• f the certifier)

– Generally, given a new motor, you should be able to immediately fly the rocket again without modification

Certifications

• Things that disqualify you

– Landing in a tree or lake – Motor CATO – Landing without successfully deploying a parachute

• For NAR members

– I can sign off on your certification and would 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