# Jet engine for beginer.

I am a fourteen year old freshmen who just started my school year. I
was fortunate enough to get into my dream elective, intro to industrial
technology. This is the first time I have had acces to a full metal
shop and I am considering building a small jet engine. I only want
enough to get up to 30 mph on a bicycle. How many pounds of thrust
should it have and what kind of design would be best for a begginer?
Right now I'm thinking a presure jet would be the best. Pulsejets have
moving parts that wear down to fast, ram and scram jets have to be
moving really fast to even start, and turbo jets have complicated oil
and cooling systems. Pressure jets sound like the best of each design
and simple for a begginer to build.
The thrust should be the same as the force delivered to the road from your tires when you are pedaling 30 mph.
You can get an idea of that several ways.
Get your max speed pedaling up a steep short hill. Measure the distance on the road and measure the vertical drop.
Divide the vertical drop by the distance you pedaled then multiply that by the weight of your bike + your weight.
This is the thrust you delivered at your maximum output.
Now check your maximum output on level ground. If you can go 30 mph for the same length of time as on the hill climb, you already have your answer.
If it is less than 30 mph you can still calculate the thrust necessary to go 30 with the propeller rule: power increases with velocity cubed.
For example, if you can only pedal 15 mph then the power required to go 30 mph is 8 times more than what you can generate.
Since power = thrust X velocity you can calculate your maximum horsepower by multipling you max thrust from the hill climb by your max speed on flat land.
Then correct for units by multiplying 5280 feet/mile and dividing 3600 seconds/hour.
Take this number and multiply it by the cube of (30/(your max speed)).
This is the power necessary to go 30.
Divide the power by speed (30mph X 1.47ft/sec/mph).
This is the thrust necessary to go 30mph.
For comparison Armstrong goes just about 28 at 0.8 hp or 440 ft'lb/sec.
Divide power by velocity to get thrust.
440/(28 X 1.47) = 10 lbs.
The reason you see very little jet engine propulsion of vehicles and boats is because air and gases are very light. The only way to get enough thrust is to:
1. move a lot of air which requires a propeller which requires a shaft drive. If you have a drive you might as well run the wheels or a water prop.
or
2. move the air fast which uses a lot of energy. Kinetic energy is 1/2 mv^2 and yet vehicle propulsion only increases with velocity. The vehicle moves forward slowly while the jet exhaust mostly gets wasted as heat in turbulent friction, stirring up air for no reason at all.
Newton once made a steam car using a boiler with a nozzle. Newton ignored his own equations and never calculated the thrust that would be necessary or what thrust would be possible with a little vapor.
It didn't work.
Bret Cahill
then multiply them by height in feet and divide by the time in seconds.
To get that you keen to know what force you are putting out
I see Brett estimated 10 lbs of thrust for 30 mph on a bike.
Here's another way to estimate - the ever popular parachute jumper.....
If a 180 lb jumper drops at 120 mph and his cross section is 12 sq feet and you learn that air resistance is proportional to airspeed squared, then you know it takes 180 lbs of thrust on 12 sq feet to go at 120 mph
Lets guess your cross section to the wind is 10 sq ft thats 10/12 of 180 lb to go at 120 mph = 150 lb 150 lb X (30/120) squared = 9.4 lb to go at 30 mph.
That's two similar estimates!
How much power is that? force times speed is power that 560 watts or 3/4 HP
Brian Whatcott Altus OK
Ooops. Big error.
You power output can be assumed to be the same on a hill as on flat land but you cannot do this for thrust.
< The thrust should be the same as the
< force delivered to the road from
< your tires when you are pedaling 30
< mph.
So far so good.
< You can get an idea of that several
< ways.
< Get your max speed pedaling up a
< steep short hill. Measure the
< distance on the road and measure the
< vertical drop.
< Divide the vertical drop by the distance
< you pedaled then multiply that
< by the weight of your bike + your
< weight.
< This is the thrust you delivered at your
< maximum output.
Which only holds for that speed.
We need to first get max power.
Get the time it takes to go up the hill.
Take the height in feet and multiply it by the weight of you and your bike and then divide that by the time in seconds.
This is your max. power and will be the same on level ground as on a hill climb.
< Now check your maximum output on
< level ground. If you can go 30 mph
< for the same length of time as on the
< hill climb, you already have your
This is wrong.
Take your max power and divide it by your max speed in feet/sec. on level ground.
This is your thrust on level ground.
Bret Cahill
A rule of thumb is 1 hp will develop 2 lbs thrust.
Power and force are different quantities of course so this is only true at some particular exhaust speed -- probably that of a well designed propeller.
This means if a cyclist powered a prop instead of his rear tire he'ld have to put out 5 hp to go thirty -- 20% the efficiency of using the rear wheel.
Moreover a prop moves a larger amount of air more slowly than the hot exhaust gases leaving a jet. A high speed jet exhaust results in low propulsion efficiency at low vehicle speeds because most of the kinetic energy gets wasted as wind so the efficiency would be even worse.
All it takes is something like 6 psi gas flowing through a nozzle to get Mach 1 gas speeds. If you go to lower compression the thermal efficiency drops off even more. If you go to higher compression the propulsion efficiency drops off even more.
That's why you see turbo props and turbo fans in commercial aviation but no straight turbo jets. Even airliners go too slow for the super sonic exhaust gases to be efficient. You really need that power turbine driving a fan or a prop. Jet only or low bypass propulsion is only efficient in supersonic aircraft. Maybe some model airplanes use jet only propulsion.
A jet powered bike would be a fun way to wake up the neighbors but would get pretty lousy gas mileage, probably worse than a SUV.
Bret Cahill
Hmmm.... better not to mention this 1 HP = 2 lb thrust rule of thumb, which as you rightly say applies only at one speed.
It is a paradox of marine engines that the slower the vessel, the more the thrust that can be supplied for a given HP, given the appropriate prop and transmission.
We already showed that a cyclist having 3/4 HP at his disposal might get 10 lb of thrust at 30 mph
That implies the 3/4 HP could offer 20 lb of thrust at 15 MPH, 40 lb of thrust at 7.5 MPH, 80 lbs at 4 MPH and so on......
NOW, we are set up to estimate at what speed, 1 HP = 2 lb thrust. GIVEN: (approx) 3/4 HP = 10 lb thrust at 30 MPH THEN 1 HP = 2 lb thrust at [( 1 / 0.75 ) X 10 ] / 2 X 30 MPH = 200 MPH
Faster than your average bike, certainly!
Anyway, if you would like an efficient airprop at some particular airspeed, then having the prop propel sufficient airmass at THIS speed backwards leaves minimum turbulence
Brian Whatcott Altus OK
Cycling drag calculators are almost as fun as a strong tail wind. This one is simple and easy but not too accurate -- the drag is OK but the hp looks a bit low. Moreover there should be a transition when the Reynolds number goes over 100K -- about 17 - 23 mph for a cyclist -- but I couldn't find it.
If you want to get fancy these are better:

Bret Cahill
Do you have instructions on how to build a jet engine? They are complicated, and have a lot of parts.
You may want to consider a rocket motor.
He's thinking about a Gluhareff style pressure jet. That's welding several pipes and tubes up. Not a lot of parts
Brian W
Turbojet built from scrap automobile turbocharger:
Yes, but as I said in my original posting, Turbo jets need oil pumps and hydraulic cooling to run properly.
Unless you get the right air/fuel mixture, the thing is going to melt no matter what.
Bret Cahill
You can get an idea of the inefficiency of propulsion at low speeds just by watching birds that prefer to run, i. e., roadrunners and quail. They simply refuse to fly until the coyote's teeth are within inches of the tail feathers.
Bret Cahill

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