Specific Impulse Help

[Cheif Operations Director]

5 minutes ago, GoSlash27 said:

Negatory. Assuming you provide 7,500 pounds of propellant.

 

7,500 divided by 100 (fuel flow) = 75 

[GoSlash27]

Correct! 75 seconds burn time.

 

Now for a trickier one: What's the exhaust velocity?

[sevenperforce]

One thing to help you become more used to working with units and unit conversions: try messing around a little with the supercalculator at Wolfram Alpha (google it). It can do all the unit conversions automatically, but it will show its work so you can see what you end up with.

An even trickier one: let's switch from Imperial to metric. Suppose your engine produces 9800 kN of thrust and burns 2 kg of propellant every second. What is the specific impulse?

[Cheif Operations Director]

On 3/21/2018 at 11:04 PM, GoSlash27 said:

Negatory. Assuming you provide 7,500 pounds of propellant.

 

Don't mind this ^ 

@sevenperforce

@GoSlash27

 

can you verify just my answers for these assorted ones? 

Fuel Flow 5 Lbs

Thrust 15 Lbs

Isp: 3 Seconds

 

Fuel Flow 10 Lbs 

Thrust 10000 Lbs 

Isp: 1000 Seconds

 

Fuel Flow 10 Lbs 

Thrust 1000 Lbs

Isp: 100 Seconds

 

Fuel Flow 5.5

Thrust 1500

Isp: 272.7272727

 

Fuel Flow 100000 Lbs

Thrust 1000

Isp: 100 Seconds

 

Fuel Flow 53 

Thrust 1852

Isp: 34 Seconds (doesn't seem right)

 

Fuel Flow 1.5 

Thrust 100 

Isp: 66 (ish)

 

Fuel Flow 10

Thrust 1500

 

Isp: 150 

 

 

[sevenperforce]

23 minutes ago, Cheif Operations Director said:

can you verify just my answers for these assorted ones? 

Sure thing.

23 minutes ago, Cheif Operations Director said:

Fuel Flow 5 Lbs

Thrust 15 Lbs

Isp: 3 Seconds

Fuel flow should be 5 lbs per second, but yes, then that is correct.

23 minutes ago, Cheif Operations Director said:

Fuel Flow 10 Lbs 

Thrust 10000 Lbs 

Isp: 1000 Seconds

Again, this works if your fuel flow is 10 lbs per second. You've gotta keep track of your units. People can report fuel flow in a lot of ways: pounds per second, kilograms per minute, tonnes per hour, gallons per mile, liters per hour...it all varies. 

23 minutes ago, Cheif Operations Director said:

Fuel Flow 10 Lbs 

Thrust 1000 Lbs

Isp: 100 Seconds

If your fuel flow is 10 lbs per second and your thrust is 1000 lbs, then yeah, 100 seconds.

23 minutes ago, Cheif Operations Director said:

Fuel Flow 5.5

Thrust 1500

Isp: 272.7272727

Now we have no units at all.

23 minutes ago, Cheif Operations Director said:

Fuel Flow 100000 Lbs

Thrust 1000

Isp: 100 Seconds

Here's a good example of where the units are tripping you up. If your fuel flow is 100,000 lbs per second and your thrust is 1000 lbs, then your Isp is actually 0.01 seconds. Oops!

23 minutes ago, Cheif Operations Director said:

Fuel Flow 53 

Thrust 1852

Isp: 34 Seconds (doesn't seem right)

Well, if you are using lbs/sec for fuel flow and lbs for thrust, then yes, you get an Isp of 34.9 seconds. This is quite right; if you're dumping more than 50 pounds of fuel every second and getting less than 2,000 pounds of thrust, that's a really poor specific impulse indeed. Your propellant isn't even supersonic.

23 minutes ago, Cheif Operations Director said:

Fuel Flow 1.5 

Thrust 100 

Isp: 66 (ish)

Again, no units here. What am I looking at? You could be reporting fuel flow in mpg and reporting thrust in kN for all I know.  

23 minutes ago, Cheif Operations Director said:

Fuel Flow 10

Thrust 1500

Isp: 150 

Gotta get those units.

Suppose you want to know the specific impulse of a Cessna propeller engine during cruise. Chances are that its thrust and fuel flow are not going to be given in pounds and pounds/sec, respectively. Only by using units properly will you be able to solve it.

For that matter, most rocket engine thrust is given in kN and most fuel flow is given in kg/sec, so that's a whole separate conversion right there.

[Cheif Operations Director]

55 minutes ago, sevenperforce said:

Sure thing.

Fuel flow should be 5 lbs per second, but yes, then that is correct.

Again, this works if your fuel flow is 10 lbs per second. You've gotta keep track of your units. People can report fuel flow in a lot of ways: pounds per second, kilograms per minute, tonnes per hour, gallons per mile, liters per hour...it all varies. 

If your fuel flow is 10 lbs per second and your thrust is 1000 lbs, then yeah, 100 seconds.

Now we have no units at all.

Here's a good example of where the units are tripping you up. If your fuel flow is 100,000 lbs per second and your thrust is 1000 lbs, then your Isp is actually 0.01 seconds. Oops!

Well, if you are using lbs/sec for fuel flow and lbs for thrust, then yes, you get an Isp of 34.9 seconds. This is quite right; if you're dumping more than 50 pounds of fuel every second and getting less than 2,000 pounds of thrust, that's a really poor specific impulse indeed. Your propellant isn't even supersonic.

Again, no units here. What am I looking at? You could be reporting fuel flow in mpg and reporting thrust in kN for all I know.  

Gotta get those units.

Suppose you want to know the specific impulse of a Cessna propeller engine during cruise. Chances are that its thrust and fuel flow are not going to be given in pounds and pounds/sec, respectively. Only by using units properly will you be able to solve it.

For that matter, most rocket engine thrust is given in kN and most fuel flow is given in kg/sec, so that's a whole separate conversion right there.

I understand why you need units now! 

And that 10000 one I I flopped the thrust and fuel flow sorrry 0.01 is a wonderful Speciifc Impulse . 

They are all in Lbs and Seconds

57 minutes ago, sevenperforce said:

Sure thing.

Fuel flow should be 5 lbs per second, but yes, then that is correct.

Again, this works if your fuel flow is 10 lbs per second. You've gotta keep track of your units. People can report fuel flow in a lot of ways: pounds per second, kilograms per minute, tonnes per hour, gallons per mile, liters per hour...it all varies. 

If your fuel flow is 10 lbs per second and your thrust is 1000 lbs, then yeah, 100 seconds.

Now we have no units at all.

Here's a good example of where the units are tripping you up. If your fuel flow is 100,000 lbs per second and your thrust is 1000 lbs, then your Isp is actually 0.01 seconds. Oops!

Well, if you are using lbs/sec for fuel flow and lbs for thrust, then yes, you get an Isp of 34.9 seconds. This is quite right; if you're dumping more than 50 pounds of fuel every second and getting less than 2,000 pounds of thrust, that's a really poor specific impulse indeed. Your propellant isn't even supersonic.

Again, no units here. What am I looking at? You could be reporting fuel flow in mpg and reporting thrust in kN for all I know.  

Gotta get those units.

Suppose you want to know the specific impulse of a Cessna propeller engine during cruise. Chances are that its thrust and fuel flow are not going to be given in pounds and pounds/sec, respectively. Only by using units properly will you be able to solve it.

For that matter, most rocket engine thrust is given in kN and most fuel flow is given in kg/sec, so that's a whole separate conversion right there.

I'm used to American Units so that's why, also I'm not using this for KSP but for model rocketry so all my tools are going to be American units 

[PB666]

18 minutes ago, Cheif Operations Director said:

I understand why you need units now! 

And that 10000 one I I flopped the thrust and fuel flow sorrry 0.01 is a wonderful Speciifc Impulse . 

They are all in Lbs and Seconds

I'm used to American Units so that's why, also I'm not using this for KSP but for model rocketry so all my tools are going to be American units 

Thats where your problem starts right there.

[Cheif Operations Director]

3 hours ago, PB666 said:

Thats where your problem starts right there.

How so

[FleshJeb]

1 hour ago, Cheif Operations Director said:

How so

He's just joking. Metric and the Systeme International (SI) units were designed to be much easier to work with. They're all in powers of ten, and many of them are indexed to the physical properties of water (since water is really common and we interact with it a lot).

The most basic example of this is:

A volume of 1.0 liter of water has a mass of 1.0 kilograms. 1000 liters of water has a volume of 1.0 m^3 (cubic meter), and it weighs 1000 kilograms  = 1.0 metric tonne.

A volume of one gallon weighs 8.345404 pounds. There are 7.48052 gallons in 1.0 ft^3 (cubic foot) and it weighs some pain in the butt amount that is much harder to do in your head. I happen to know it's in the neighborhood of 62.4 pounds, but the real answer is 62.42718356 pounds per cubic foot.

5 hours ago, Cheif Operations Director said:

I understand why you need units now! 

Congratulations! I hope your project turns out well.

[Bill Phil]

The liter is not an SI unit. It's interchangable though, and is accepted for use with SI units.

Also, in America, model rocket motors are classed by total impulse. Usually in Newton-seconds. At least in my experience.