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C,

J1

0k7

.

k9,110 1/1A.e.

(A-) V ( -1

equatio

n: (1

) water is in

com

pressib

le, (2) flo

w th

rough th

e nozzle is u

nifo

rm, (3

) velo

cities are rectilinear, (4

)

den

sity o

f water is m

uch

greater th

an d

ensity

of air, (5

) no v

iscosity

effects, (6) stead

y flo

w, (7

) velo

city o

f the

free surface o

f water is v

ery sm

all com

pared

to th

e velo

city o

f the n

ozzle, (8

) air pressu

re remain

s constan

t until

water ru

ns o

ut, (9

) nozzle v

elocity

remain

s constan

t until w

ater runs o

ut, an

d (1

0) th

ere are no v

iscous-frictio

n

effects from

the n

ozzle (see M

oody ch

art).

An in

dep

enden

t variab

le that in

fluen

ces peak

heig

ht is w

eight/m

ass. Dep

endin

g o

n th

e thru

st of th

e rock

et

pro

pulsio

n sy

stem, a ro

cket req

uires a m

inim

um

mass to

overco

me th

e deleterio

us effects o

f drag

. For ex

ample,

the g

reater the th

rust/th

e less the o

rigin

al weig

ht o

f the ro

cket, th

e more w

eight o

r mass m

ust b

e added

to th

e

rock

et to in

sure m

axim

um

apogee. T

he m

ass is gen

erally referred

to as b

allast. This p

rincip

le is dem

onstrated

by h

avin

g a stu

den

t thro

w a straw

with

and w

ithout a p

iece of clay

attached

to th

e 'nose' o

f the straw

. The straw

with

the g

reater mass w

ill travel fu

rther, p

rovid

ed th

at there is su

fficient th

rust to

overco

me th

e ballast o

r extra

mass.

Sources o

f gasied

itj

Sev

eral meth

ods fo

r pressu

rizing a ro

cket are u

sed in

cludin

g:

• k

likalle

illi*T

en

iettirn

resab

led

frad

Ate

Weeet6

k4

52

Diffa

t'

• W

ater pressu

re forcin

g all th

e air in an

emp

ty w

ater ho

se into

the ro

cket. P

ressure is th

e same as th

e water

main.

• A

n air co

mpresso

r, like th

ose u

sed in w

ork

shops to

pow

er pneu

matic eq

uip

men

t and to

ols. M

odify

ing a h

igh

pressu

re (greater th

an 1

5 b

ar / 15

00

kP

a / 20

0 p

si) com

presso

r to w

ork

as a water ro

cket p

ow

er sou

rce can

be d

ang

erou

s, as can u

sing

hig

h-p

ressure g

ases from

cylin

ders.

• C

om

pressed

gases in

bo

ttles, like carb

on

dio

xid

e (CN

, air, and

nitro

gen

gas (N

a). Exam

ples in

clude

in p

aintb

allcylin

ders an

d air in

industrial an

d S

CU

BA

cylin

ders. C

are must b

e taken

with

bottled

gases:

as the co

mp

ressed g

as exp

and

s, it coo

ls (see gas law

s) and

rock

et com

po

nen

ts coo

l as well. S

om

e

materials, su

ch as P

VC

and

AB

S, can

beco

me b

rittle and

weak

wh

en sev

erely co

oled

. Lo

ng

air ho

ses are u

sed to

main

tain a safe d

istance, an

d p

ressure g

aug

es (kn

ow

n as m

ano

meters) an

d safety

valv

es are

typically

utilized

on lau

nch

er installatio

ns to

avoid

over-p

ressurizin

g ro

ckets an

d h

avin

g th

em ex

plo

de

befo

re they

can b

e laun

ched

. Hig

hly

pressu

rized g

ases such

as tho

se in d

ivin

g cy

lind

ers or v

essels from

ind

ustrial g

as sup

pliers sh

ou

ld o

nly

be u

sed b

y train

ed o

perato

rs, and

the g

as sho

uld

be d

elivered

to th

e

rock

et via a reg

ulato

r dev

ice (e.g. a S

CU

BA

first-stage). A

ll com

pressed

gas co

ntain

ers are subject to

local,

state and n

ational law

s in m

ost co

untries an

d m

ust b

e safety tested

perio

dically

by a certified

test centre.

• Ig

nitio

n o

f a mix

ture o

f explo

sive g

ases above th

e water in

the b

ottle; th

e explo

sion creates th

e pressu

re to

launch

the ro

cket in

to th

e air.ot

Nozzlestedill

Water ro

cket n

ozzles d

iffer from

conven

tional co

mbustio

n ro

cket n

ozzles in

that th

ey d

o n

ot h

ave a d

iverg

ent

section

such

as in a D

e Lav

al no

zzle. Becau

se water is essen

tially in

com

pressib

le the d

iverg

ent sectio

n d

oes n

ot

contrib

ute to

efficiency

and actu

ally can

mak

e perfo

rman

ce worse.

There are tw

o m

ain classes o

f water ro

cket n

ozzles:

• O

pen

also so

metim

es referred to

as "standard

" or "fu

ll-bore" h

avin

g an

insid

e diam

eter of -2

2m

m w

hich

is

the stan

dard

sod

a bo

ttle neck

op

enin

g.

• ,120,S

trittecl'vvhich is anything smaller than the "standard". 6

popular restricted

nozzle h

as an in

side d

iameter

of 9

mm

riniffs k

now

n as a "O

eLden

ja.nozzle" n

amed

after a com

mon g

arden

hose q

uick

connecto

r used

to

mak

e th

en

ij

The 'iiiiii.cifth

e-h

ozzleT

.affeCtO

lia:thnistp

roduced

: ily,Jh

e rock

et: Larg

er diam

eter nozzles p

rovid

e faster

acceleration w

ith a sh

orter th

rust-p

heil, W

hile sm

aller nozzles p

rovid

e,lo

wer acceleratio

n w

ith alo

ng

eOh

rust

:-phage'A

It can b

e show

n th

at the eq

uatio

n fo

r the in

stantan

eous th

rust o

f a nozzle is sim

ply

:to

F =

2P.-1

1

where F

is the th

rust, P

is the p

ressure an

d A

t is area of th

e nozzle.

Fins[edit]

As th

e pro

pellan

t level in

the ro

cket g

oes d

ow

n, it can

be sh

ow

n th

at the cen

tre of m

ass initially

moves

dow

nw

ards b

efOre fin

ally m

ovin

g u

pw

ards ag

ain as th

e pro

pellan

t is dep

leted. T

his in

itial movem

ent

redu

ces stability

and

can cau

se water ro

ckets to

start tum

blin

g en

d o

ver en

d, g

reatly d

ecreasing

the

max

imu

m.sp

eed.an

djh

usth

epn

gth

.of glide (time that the rocket is flying under its ow

n mom

entum). T

_O-4

low

er the cen

tre of O

resSu

r6g

etitadd

itah, fin

Sfliiik

be ad

ded

wh

ich b

ring

the cen

tre of d

rag-fU

rthert?a0k, w

ell beh

ind th

e centre o

f mass at all tim

es, ensu

ring stab

ility.

Thus, sa

reT

exiriert

grl

aT

cra

Mita

'Wete

trocket,

,By ensuring stability, they are very likely to

,incre

ase

,,,

..ftsiaulT

dpfeig

hk F

ins in

crease drag

, but th

e stability

achiev

ed m

akes a m

uch

larger d

ifference to

the h

eight

the ro

cket w

ill fly. A

second th

ing th

at is verp

importan

tissths•p

ositio

n4fith

e4ln

s. It is best if th

ey are p

laced

nekih'eSiCk'ikflfie bottle,.w

ho

reittez:selt=

mittu

zr., A

wate

rpro

of, sta

ble

, light m

ate

rial to

make

the fin

s would

be 'lC

ditig

laSti. T

his is a ,'IW

Citioiid lik

e Material th

at is dafib

leiO

ttie. Th

e on

ly n

egativ

e it has is th

at it is hard

er to g

lue, b

ut w

ith th

e right g

lue it is p

ossib

le.

In th

e case of cu

stom

-mad

e rock

ets, where th

e rock

et nozzle is n

ot p

erfectly p

ositio

ned

, the b

ent n

ozzle

can cau

se the ro

cket to

veer o

ff the v

ertical axis. T

he ro

cket can

be m

ade to

spin

by an

glin

g th

e fins, w

hich

reduces o

ff course v

eering.

Anoth

er simple an

d effectiv

e stabilizer is a straig

ht cy

lindrical sectio

n fro

m an

oth

er plastic b

ottle. T

his

section is p

laced b

ehin

d th

e rock

et nozzle w

ith so

me w

ooden

dow

els or p

lastic tubin

g. T

he w

ater exitin

g th

e nozzle w

ill still be ab

le to p

ass thro

ugh th

e section, b

ut th

e rock

et will b

e stabilized

.

Aero

dynam

ic drag

acts on th

e fins as w

ell as on th

e rock

et body. F

ins ad

d to

the fro

ntal su

rface area on

wh

ich th

e drag

force acts (an

d th

erefore sh

ou

ld b

e desig

ned

no

t to ad

d to

o m

uch

drag

). Th

e drag

forces o

n

all frontal su

rfaces of th

e rock

et can b

e resolv

ed in

to o

ne fo

rce acting at th

e center o

f pressu

re Cen

ter of

pressu

re (fluid

mech

anics). T

his acts to

oppose th

e forw

ard m

otio

n, b

ut if th

e rock

et nose is n

ot p

oin

ted in

the d

irection o

f its motio

n at a g

iven

time (p

erhap

s due to

wobblin

g o

r instab

ility), th

en th

ere will b

e a torq

ue,

due to

the reso

lved

drag

force, actin

g aro

und th

e center o

f grav

ity. T

his to

rque w

ill stabilize th

e rock

et by

return

ing its n

ose to

the d

irection o

f travel.

Sin

ce the to

rque is th

e cross-p

roduct o

f the d

rag fo

rce mag

nitu

de an

d th

e mom

ent arm

, torq

ue can

be

max

imized

with

out in

creasing d

rag fo

rce by in

creasing th

e mom

ent arm

. The larg

er the d

istance b

etween

the cen

ter of g

ravity

and th

e center o

f pressu

re, the g

reater the m

om

ent arm

on th

e restorin

g to

rque.

Th

erefore, it is d

esirable to

hav

e the cen

ter of p

ressure, an

d th

erefore th

e fins, as far b

ack as p

ossib

le on

the ro

cket b

ody.

The lift fo

rce acts to p

ush

the b

ack en

d o

f the ro

cket so

that th

e nose w

ill face the flig

ht d

irection, an

d th

e

drag

force d

oes th

e same, ev

en th

ough it is p

oin

ting o

rthogonally

to th

e lift force. ra

Lan

din

g sy

stems[ed

it]

Stab

ilizing fin

s cause th

e rock

et to fly

nose-first w

hich

will g

ive sig

nifican

tly h

igher sp

eed, b

ut th

ey w

ill also

cause it to

to[witb,asignifidbiitt*tierV

ilocity than

it Would

if it tum

bled

to th

e gro

und,:an

d th

is may

O

arnag

eltheirO

Cket o

r whom

ever o

r whatev

er it strikes u

pon lan

din

g.

So

me w

ater rock

ets hav

eTarath

tid o

r oth

er recov

ery sy

stem to

help

prev

ent p

rob

lems. H

ow

ever th

ese

system

s can su

ffer from

malfu

nctio

ns. T

his is o

ften tak

en in

to acco

unt w

hen

desig

nin

g ro

ckets.

A steam

rock

et (or h

ot w

ater rock

et) is a rock

et which

uses steam

as its pro

pellan

t. Befo

re launch

, water

in th

e sealed ro

cket is h

eated. A

s the ro

cket rem

ains sealed

, the p

ressure in

creases. This p

ressure is

sufficien

t to k

eep th

e water assu

perh

eated w

ater rather th

an b

oilin

g in

to steam

, as the b

oilin

g

temp

erature o

f water in

creases with

pressu

re. On

laun

ch, th

e pressu

re vessel is v

ented

thro

ug

h a n

ozzle.

The released

water d

rops in

pressu

re as it passes th

rough th

e nozzle, allo

win

g it to

boil o

r 'flash' in

stantly

into

steam. T

he h

igh v

elocity

of th

e steam, an

d its ex

pan

sion th

rough th

e nozzle, g

ives rise to

the u

sual

reaction

force fo

r a rock

et

Th

e idea o

f such

rock

ets was co

nceiv

ed b

y G

erman

y b

efore th

e Seco

nd

Wo

rld W

ar with

the su

gg

ested u

se

of an

alternativ

e rock

et eng

ine fo

r laun

chin

g fig

hter lets. S

om

e of th

e few p

ractical steam ro

ckets

constru

cted h

ave b

een u

sed fo

r drag

racing an

d fo

r Evel K

niev

el's Skycy

cle X-2

canyon ju

mp.

How

Bottle R

ockets Work

Th

e intricate d

etails of th

e math

and scien

ce of w

ater rock

ets is reserved

for a ch

apter in

volu

me 2

of

The

Com

plete Water R

ocket Manual b

ut a b

rief explan

ation is in

ord

er befo

re we g

et into

build

ing y

our first w

ater

rock

et and lau

nch

er.

A w

ater rock

et uses th

e same p

hysics as a m

odel ro

cket o

r the S

huttle lau

nch

veh

icle. Very

simply

, asock

et,engin

e

uses h

igh

n

Iforce,a flu

id th

rough4 restricted

open

ing at a h

igh

velo

city an

d th

is creates a force th

at

pro

pels.th

e:laUndh %

/glid

e in,th

eepp

osited

irection

fron

t the ex

hau

sting

fluid

:

Tech

nically

, a fluid

is either a g

as or a liq

uid

. In ch

emical ro

ckets, th

e fluid

is a sup

erheated

gas g

enerated

from

a

bu

rnin

g fu

el. In a w

ater rock

et, the tem

peratu

res are norm

ally n

ear outsid

e temperatu

res and th

e fluid

is a

com

bin

ation o

f gas an

d liq

uid

, the g

as bein

g n

orm

ally air (alth

ough carb

on d

ioxid

e, or n

itrogen

are also so

metim

es

used

and

a hydro

gen

-oxygen

mix

is used

fore h

ydro

gen

rock

et) and th

e liquid

bein

g w

ater (though it co

uld

be

mix

ed w

ith salt o

r bubble b

ath o

r oth

er ingred

ients).

It is New

ton

's laws at w

ork

and

especially

his th

ird law

of m

otio

n sim

ply

stated as "E

very

action

has an

equ

al and

op

po

site reaction

.”

Five P

hases of Flight

A b

ottle ro

cket h

as four o

r five p

hases o

f flight as o

pposed

to th

ree for ch

emical ro

ckets.

1. A

cceleration d

ue to

pressu

re acting ag

ainst th

e launch

tube. (T

his is n

ot p

resent if a lau

nch

tube is n

ot

used

)

2. A

cceleratibri ,:d

ite to th

e reaction fo

rce of th

e water b

eing ejected

:

3. A

cceleration

olu

e.to th

e reaction

:force o

fthe air b

eing

ejected'.

4. 1Z

ifiettiphase::(eitding-atep

ogee, th

etighest attitu

de reach

ed), w

itiOh is th

e longest p

art of th

e Upw

ard:

fligh

t. Th

e acceleration

ph

ases usin

g ju

st the o

pen

neck

of th

e bo

ttle as the n

ozzle are in

milliseco

nd

s.

5. T

he reco

very

ph

ase wh

ere the ro

cketretu

rnS

to earth

after reachin

g ap

og

ei.

Ru

bb

er bu

mp

ers, Cru

mp

le zon

es and

safe laun

ch p

ractices can b

e utilized

to m

inim

ize dam

age o

r inju

ry

caused

by

a falling

rock

et.

An

oth

er po

ssible reco

very

system

invo

lves sim

ply

usin

g th

e rock

et's fins to

slow

its descen

t and

is

som

etimes called

backward sliding. B

yin

cregin

gfim

eiik'itio

re:dra4

1k

gen

eratedi,If th

e ceritte-of m

asaiS‘'

placed

forw

ard o

ftheT

hrie;-th

e'rcidk

etWill'n

osed

ive. In

the case o

f super ro

c or b

ack g

lidin

g ro

ckets, th

e

rock

et is desig

ned

such

that th

e relation

ship

betw

een cen

tre of g

ravity

and th

e centre o

f pressu

re of th

e

empty

rock

et causes th

e fin-in

duced

tenden

cy o

f the ro

cket to

tip n

ose d

ow

n to

be co

unteracted

by

the air

resistance o

f the lo

ng

bo

dy

wh

ich w

ou

ld cau

se it to fall tail d

ow

n, an

d resu

lting

in th

e rock

et falling

sidew

ays, slo

wly

.

Lalltle

httlb

e.S

ied

it]

Som

e water ro

cket lau

nch

ers use lau

nch

tubes. A

laun

ch tu

be fits in

side th

e nozzle o

f the ro

cket an

d

exten

ds u

pw

ard to

ward

the n

ose. T

he lau

nch

tube is an

cho

red to

the g

rou

nd

. As th

e rock

et beg

ins

accelerating u

pw

ard, th

e laun

ch tu

be b

lock

s the n

ozzle, an

d v

ery little w

ater is ejected u

ntil th

e rock

et

leaves th

e laun

ch tu

be. T

his allo

ws alm

ost p

erfectly efficien

t con

versio

n o

f the p

oten

tial energ

y in

the

com

pressed

air to k

inetic en

ergy

and

grav

itation

al po

tential en

ergy

of th

e rock

et and

water. T

he h

igh

efficiency

du

ring

the in

itial phase o

f the lau

nch

is imp

ortan

t, becau

se rock

et eng

ines are least efficien

t at

low

speed

s. A lau

nch

tub

e therefo

re significan

tly in

creases the sp

eed an

d h

eigh

t attained

by

the ro

cket

Lau

nch

tubes are m

ost effectiv

e when

used

with

long ro

ckets, w

hich

can acco

mm

odate lo

ng lau

nch

tubes.

Safe

tred

ill

Water ro

ckets em

plo

y co

nsid

erable am

ou

nts o

f energ

y an

d can-P

esdango%up4t4apdleprpproperly

cssesTo

tfaliltyaco

nstru

_c_

tton

:orm

ateriahfailu

re. Certain

safety p

roced

ures are o

bserv

ed b

y ex

perien

ced

water ro

cket en

thusiasts:

• W

hen

's rock

et is built, it is p

ressure tested

. This is d

on

e by

tilling th

e rock

et com

pletely

with

water, an

d

then

pressu

rizing

it to at least 5

0%

greater th

an an

ticipated

pressu

res. If the b

ottle ru

ptu

res, the

amount o

f com

pressed

air insid

e it (and

thus th

e poten

tial energ

y) w

ill be v

ery sm

all, and th

e bo

ttle will

not ex

plo

de.

• U

sing m

etal parts o

n th

e pressu

rized p

ortio

n o

f the ro

cket is stro

ng

ly d

iscou

raged

becau

se in th

e even

t

of a ru

ptu

re, they

can b

ecom

e harm

ful p

rojectiles. M

etal parts can

also sh

ort o

ut p

ow

er lines.

• W

hile p

ressurizin

g an

d lau

nch

ing

the ro

cket, b

ystan

ders are k

ept at a safe d

istance. T

ypically

,

mech

anism

s for releasin

g th

e rock

et at a distan

ce (with

a piece o

f string

, for ex

amp

le) are used

. This

ensu

res that if th

e rock

et veers o

ff in an

un

expected

directio

n, it is less lik

ely to

hit th

e op

erator o

r

bystan

ders.

• W

ater rock

ets sho

uld

only

be lau

nch

ed in

large o

pen

areas, away

from

structu

res or o

ther p

eop

le, in

ord

er to p

reven

t dam

age to

pro

perty

and p

eople.

• A

i•water:roZ

kirts- are.C

apab

Wg

f break

ingtO

nes.u

pon

imp

act they

sho

uld

never be fired at peoplee:

•„s

imffiv

eggi6

9,9

1,m.

• A

typ

ical two

-litre sod

a bo

ttle can g

enerally

'reach th

e pressu

re of 1

00 p

si (69

0 k

Pa) safely

, bu

t

prep

arations m

ust b

e mad

e for th

e even

tuality

that th

e bottle u

nex

pected

ly ru

ptu

res.

• G

lifeItraedlo

:pu

t tog

ether p

arts of w

sterrock

ets,mn

et he

;euitab

leto..u

se,on

r pleed

os, o

r else the g

lue

will ch

emicallY

.IiraWay

the b

citile, wh

ich in

ay th

en failtataS

trciph

iCallian

dcan

tarm b

ystan

ders

when

the ro

cket is lau

nch

ed.

Steam

rocke

tsiediti

-

- -

Main article: S

team

rocke

t

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Optim

um A

mount of W

ater to Use.

"How

much w

ater should I put in mhy w

ater rocket?" That is a frequently asked question. T

he:amount of w

ater

e6 in

the rocket m

akes a huge difference in th'e,altitude reached. Actually,..usingolom

atenat all but simply using

the compressed air for thrust can propel an aerodynam

ically efficient`iiiter rikket to surprising altitudes,:

If you put too much w

ater in. then the thrug'aVailable m

ust loft more w

eightowhich reduces the m

aximum

altitud

e., An air only rocket can fly higher than one w

ith too much w

ater.

1E

6'6

1M

:water is u

sed, th

en th

ere linf : tg

itotig

h_reactio

n•m

at to p

ropel th

eTpacet ro

g-efficientlyjT

he exact

most efficient am

ount of water varies depending on the overall em

pty weight of the rocket and w

hether or not a

launch tube is used. The graph below

shows a 2 liter bottle rocket w

eighing 150 grams and 250 gram

s, using a

launch tube and not, and with a fairly large drag coefficient for an exam

ple. For this rocket, the optim

al amount of

water w

ith a launch tube is 22% for 150-gram

rocket weight, 30%

for a 250-gram, and w

ithout a launch tube is 29%

for 150-gram one, and 37%

for a 250-gram rocket.

So y

ou can

see that th

e more th

e rock

et Weig

hifth

a more w

ater it need

stO:feach

thatn

axiiriin

t altitiidajn

d th

e

loffgar the launch rod, the less water it needs: A

lso notice that with a launch tube, actually the heavier rocket

reaches a higher altitude. That is because by using the pressure w

orking against the launch tube, the launch tube

can impart a greater m

omentum

so the inertia will carry it farther. It com

bines the affect of a bullet in a gun and a

rocket motor. Y

ou can find more details on this in the S

cience and Math chapter.

ocia-

J ?4

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Pu

mp

ing

up

the ro

cket to

50

psi

Th

is is the ro

cket after it h

as

landed

. It is a bit sq

uash

ed b

ut I

found th

at when

you p

ut th

e

pressu

red air in

it goes b

ack to

its

orig

inal sh

ape.

gA

k- M

c

S:e_covx.C

/c5

The ro

cket all set u

p. T

his is th

e

fatman

rock

et with

a foam

nose

and larg

e fins.

Th

is is a screen sh

ot fro

m o

ne o

f the v

ideo

s of th

e

rock

et of it b

eing released

.

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