Specific Impulse Help

[Cheif Operations Director]

1 hour ago, sevenperforce said:

Not quite.

"Axis of engine exhaust divided by fuel flow" doesn't tell anyone anything. Forget that.

Now, if you happen to know the average velocity of the exhaust molecules, you can use that to calculate specific impulse. But it's not exactly an easy thing to measure, so let's forget it, too.

"thrust (lbs) divided amount of propellant (lbs) x burn time" isn't quite there, either.

To begin with, you need to know the propellant flow rate. The propellant flow rate tells you how fast your rocket engine (or any other engine) uses fuel. A big engine probably uses fuel quickly; a small engine probably uses fuel more slowly. An efficient engine uses fuel more slowly than an inefficient engine of the same size. 

The propellant flow rate is very easy to calculate. Run your engine for a few minutes, then shut it off. How much fuel did you use? If your engine used 10 gallons of fuel in 5 minutes, then the flow rate is 2 gallons per minute. If it used 20 gallons of fuel in 5 minutes, then the flow rate is 4 gallons per minute. Assuming you know the density of your fuel, it should be trivial to convert gallons into pounds and minutes into seconds. If your fuel has approximately the same density as water, then 4 gallons per minute is the same as 0.53 pounds per second.

Once you know your propellant flow rate, then dividing your total thrust by your flow rate will tell you your specific impulse.

Ok that's good for liquid rocket how bout solid rockets. Find the weight of the proplellant and burn out the engine. Correct?

[sevenperforce]

10 minutes ago, Cheif Operations Director said:

Ok that's good for liquid rocket how bout solid rockets. Find the weight of the proplellant and burn out the engine. Correct?

Unlike in KSP, real-world solid rockets do not have constant thrust. Their thrust, flow rate, and even specific impulse can all change over the course of the burn.

[Cheif Operations Director]

26 minutes ago, sevenperforce said:

Unlike in KSP, real-world solid rockets do not have constant thrust. Their thrust, flow rate, and even specific impulse can all change over the course of the burn.

Yes if there core is cut it that way I mean if it is cut so it's equal.

[sevenperforce]

6 minutes ago, Cheif Operations Director said:

Yes if there core is cut it that way I mean if it is cut so it's equal.

Then you do the same thing as with a liquid rocket. Let the rocket burn for a few minutes, see how much fuel was burned, and divide total fuel by burn time to get the mass flow rate. Then divide thrust by mass flow rate to get isp.

[GoSlash27]

3 hours ago, Cheif Operations Director said:

Ok that's good for liquid rocket how bout solid rockets. Find the weight of the proplellant and burn out the engine. Correct?

Again, it doesn't matter if it's liquid propellants, solid propellants, or Totino's pizza rolls. Establish your thrust and then divide that by the rate at which you're feeding the beast (supplying propellant) and you will find your efficiency (specific impulse). Conversely, find the velocity at which you are expelling the propellant and divide that by g0 and you will also find your efficiency (specific impulse).

A reaction engine is nothing more than a magic pipe from the mathematical point of view. You shove matter into it and it creates thrust. If it creates more thrust from the same matter, it's more efficient. If it creates the same thrust from less matter, it's more efficient. Looking at it the other way, it simply takes the matter and throws it away *really fast*. The faster it makes the exhaust move, the more efficient it is.

 Either way of looking at it yields the same measure of efficiency.

Best,
-Slashy

Edited March 21, 2018 by GoSlash27

[mikegarrison]

On 3/19/2018 at 7:11 AM, sevenperforce said:

Suppose you have a jet airliner powered by turbofan engines with a specific impulse of 3500 seconds. If this jet reaches cruising altitude with 2,000,000 pounds of fuel

If that plane reaches cruising altitude with 2,000,000 lb of fuel, it's a MONSTER. That's about 3.5x as much fuel as an A380 has with full tanks.

[Cheif Operations Director]

43 minutes ago, GoSlash27 said:

Again, it doesn't matter if it's liquid propellants, solid propellants, or Totino's pizza rolls. Establish your thrust and then divide that by the rate at which you're feeding the beast (supplying propellant) and you will find your efficiency (specific impulse). Conversely, find the velocity at which you are expelling the propellant and divide that by g0 and you will also find your efficiency (specific impulse).

A reaction engine is nothing more than a magic pipe from the mathematical point of view. You shove matter into it and it creates thrust. If it creates more thrust from the same matter, it's more efficient. If it creates the same thrust from less matter, it's more efficient. Looking at it the other way, it simply takes the matter and throws it away *really fast*. The faster it makes the exhaust move, the more efficient it is.

 Either way of looking at it yields the same measure of efficiency.

Best,
-Slashy

All I mean is how do you calculate your fuel flow with solid fuels?

[GoSlash27]

1 hour ago, Cheif Operations Director said:

All I mean is how do you calculate your fuel flow with solid fuels?

Same as with liquids. Note how much fuel you have at the beginning, then record how long it burns until it runs out of fuel. propellant mass (tonnes) divided by time (seconds)= fuel flow rate (tonnes per second).

Best,
-Slashy

Edited March 21, 2018 by GoSlash27

[Cheif Operations Director]

2 hours ago, GoSlash27 said:

Same as with liquids. Note how much fuel you have at the beginning, then record how long it burns until it runs out of fuel. propellant mass (tonnes) divided by time (seconds)= fuel flow rate (tonnes per second).

Best,
-Slashy

That's what I figured that's for your help do you mind if (tommorow) I give a few examples and you check my work?

[PB666]

6 hours ago, sevenperforce said:

Unlike in KSP, real-world solid rockets do not have constant thrust. Their thrust, flow rate, and even specific impulse can all change over the course of the burn.

Nor is the ISP constant. 

[GoSlash27]

6 hours ago, Cheif Operations Director said:

That's what I figured that's for your help do you mind if (tommorow) I give a few examples and you check my work?

Sure thing.

Best,
-Slashy

[wumpus]

14 hours ago, sevenperforce said:

Unlike in KSP, real-world solid rockets do not have constant thrust. Their thrust, flow rate, and even specific impulse can all change over the course of the burn.

This can often be used by rocket scientists to adjust the thrust needed.  Typically the internal channel is molded in various shapes to allow changes in thrust over time.

Dry mass of a solid rocket can be significant.  Not only do you have the payload, but a solid rocket can be described as a "pipe bomb with one end open": you need a strong container to keep the pressure of the escaping gasses inside the rocket and leaving the correct spot. (and shape).

[sevenperforce]

8 hours ago, PB666 said:

Quote

Their thrust, flow rate, and even specific impulse can all change...

Nor is the ISP constant. 

Right.

 

[PB666]

1 minute ago, sevenperforce said:

Right.

 

But the reasoning here is that as solid fuel burns, the dynamics of flow in the reaction chamber change, the absorption by the walls of the chamber increase, the intergral of friction with respect to distance increases. The momentum of exhaust passing the nozzle increases.

Collectively this means that to get a static fire appreciation of the SFRB is that you need to plug a dummy mass (say a soft limestone filler, an inert plug) and stop the rocket at points in time. If it burns for 30 sec. You would stop at 10 second (measuring force at 5) stop at 20 seconds (measuring force at 15) and 30 seconds (measuring force to 25) then determine the mass change by subtraction and F_section*t/δM_section*t = V_{e, section}.

The ISP would be the average of the three divided by 9.806 a. This is not perfect either, because you have estimate how fuel is burnt with respect to time to determine where to plug and measure the F_1/2 of the thrust at end burn as T₁.

 

[sevenperforce]

16 minutes ago, PB666 said:

But the reasoning here is that as solid fuel burns, the dynamics of flow in the reaction chamber change, the absorption by the walls of the chamber increase, the intergral of friction with respect to distance increases. The momentum of exhaust passing the nozzle increases.

Collectively this means that to get a static fire appreciation of the SFRB is that you need to plug a dummy mass (say a soft limestone filler, an inert plug) and stop the rocket at points in time. If it burns for 30 sec. You would stop at 10 second (measuring force at 5) stop at 20 seconds (measuring force at 15) and 30 seconds (measuring force to 25) then determine the mass change by subtraction and F_section*t/δM_section*t = V_{e, section}.

The ISP would be the average of the three divided by 9.806 a. This is not perfect either, because you have estimate how fuel is burnt with respect to time to determine where to plug and measure the F_1/2 of the thrust at end burn as T₁.

Of course the proper way to do it is simply measure thrust and propellant mass with respect to time. You end up with two different curves, f_T and f_M. The slope of the propellant mass curve is your mass flow; your Isp is given by a function f_S(t), where f_S(t) = f_T(t)/f_M'(t).

EDIT: If you have a small enough solid rocket motor, it's theoretically possible to measure both thrust and propellant mass as a function of time on a big enough test stand. You can have the rocket laying down on its side on a sled, with one forcemeter measuring the horizontal force on the sled and the whole apparatus on a scale to measure mass change.

[Cheif Operations Director]

9 hours ago, GoSlash27 said:

Sure thing.

Best,
-Slashy

Fuel flow rate: 100 pounds a second

Thrust: 10000 Lbs

Isp= 100

[PB666]

4 hours ago, sevenperforce said:

Of course the proper way to do it is simply measure thrust and propellant mass with respect to time. You end up with two different curves, f_T and f_M. The slope of the propellant mass curve is your mass flow; your Isp is given by a function f_S(t), where f_S(t) = f_T(t)/f_M'(t).

EDIT: If you have a small enough solid rocket motor, it's theoretically possible to measure both thrust and propellant mass as a function of time on a big enough test stand. You can have the rocket laying down on its side on a sled, with one forcemeter measuring the horizontal force on the sled and the whole apparatus on a scale to measure mass change.

Weighing a rocket while firing is not a trivial task even for a small rocket.  You cannot put it on a scale and fire exactly sideways. You could, in theory have an evacuated chamber and measure the increase in pressure, but the problem with this is temperature is not uniform, and as pressure rose ISP would fall. You could actively pump the room and measure the flow of the pumped exhaust using a flow meter, less lag but still lag. Vacuum pumps in general the best have the lowest flow, so you probably need concentrators for each pump. (10 or so Impeller based, coolant cooled). Again you are taking hot gas and compressing it, which means you are passing really hot gas through and hydrocarbon lubricated vacuum pump. So somewhere along the cooling inlet you need to cool the gas down. The other problem is that once you pass the gas through the pump, some of the gas will dissolve into the oil, since for SRB the exhaust is a combination of theoretical solids, liquids and gases. You could have a gigangtic flow meter with very low resistence behind the rocket. You could, if you cooled the gas without creating drag, use a combination of pitot tube and pressure sensors at the bell and at the outlet to compensate for changes in dynamic pressure.  As you know from AF447 fiasco, improperly designed pitots can foul up when particulates are present, disabling them. Again pitots are generally not rated for flows in km/sec at atmospheric pressure, so. .there might need to be a thrust diverter that slows down and laminarly spreads the gas. Again pressure drop but at a km/sec the gas should quickly arrive and escape.  . . .

Another way is to place thermocouples, small along the inside wall of the thruster and a rod spanning the center length of the rocket with thermocouples along its length. Each records the passing of fuel burnt with respect to time. If you know the mass per unit length of fuel, you know the rate. Another way is to use photocells places in the wall itself, measure the point in which the explosion is detected (need to be calibrated but in steady state should measure flow).

 

[sevenperforce]

47 minutes ago, Cheif Operations Director said:

Fuel flow rate: 100 pounds a second

Thrust: 10000 Lbs

Isp= 100

10,000 lbs / 100 lbs / sec means that the Isp is 100 seconds. You absolutely cannot let those units get away from you. 

25 minutes ago, PB666 said:

Weighing a rocket while firing is not a trivial task even for a small rocket.  You cannot put it on a scale and fire exactly sideways. 

Another way is to place thermocouples, small along the inside wall of the thruster and a rod spanning the center length of the rocket with thermocouples along its length. Each records the passing of fuel burnt with respect to time. If you know the mass per unit length of fuel, you know the rate. Another way is to use photocells places in the wall itself, measure the point in which the explosion is detected (need to be calibrated but in steady state should measure flow).

In practice, what you'd most likely do is create a surface area regression model, record burn time, and then adjust parameters (using a series of different geometries) until you have a rock-solid burn rate model. Flow rate is a function of burn rate, which is a function of surface area. But the burn rate also depends on chamber pressure, which is why you have to tweak your parameters until they fit the experimentally-derived burn times. Once you have a good mass flow model, then you can take the thrust curve and determine the specific impulse curve.

[GoSlash27]

COD,

^ What @sevenperforce said.

If you provided your engine with 7,500 pounds of propellant, how long would it run?

Best,
-Slashy

 

[Cheif Operations Director]

5 hours ago, GoSlash27 said:

COD,

^ What @sevenperforce said.

If you provided your engine with 7,500 pounds of propellant, how long would it run?

Best,
-Slashy

 

With my example? 

6 hours ago, sevenperforce said:

10,000 lbs / 100 lbs / sec means that the Isp is 100 seconds. You absolutely cannot let those units get away from you. 

In practice, what you'd most likely do is create a surface area regression model, record burn time, and then adjust parameters (using a series of different geometries) until you have a rock-solid burn rate model. Flow rate is a function of burn rate, which is a function of surface area. But the burn rate also depends on chamber pressure, which is why you have to tweak your parameters until they fit the experimentally-derived burn times. Once you have a good mass flow model, then you can take the thrust curve and determine the specific impulse curve.

I know I just didn't bother to write it.

5 hours ago, GoSlash27 said:

COD,

^ What @sevenperforce said.

If you provided your engine with 7,500 pounds of propellant, how long would it run?

Best,
-Slashy

 

I need the fuel flow rate and thrust 

[GoSlash27]

1 hour ago, Cheif Operations Director said:

I need the fuel flow rate and thrust 

But you provided those

[Cheif Operations Director]

1 minute ago, GoSlash27 said:

But you provided those

So doing thrust 10,000 and 100 pounds a second on fuel flow

I already answered it 

[GoSlash27]

1 hour ago, Cheif Operations Director said:

So doing thrust 10,000 and 100 pounds a second on fuel flow

Thrust 10,000 *pounds* and 100 pounds per second fuel flow. Yes sir...

[Cheif Operations Director]

3 minutes ago, GoSlash27 said:

Thrust 10,000 *pounds* and 100 pounds per second fuel flow. Yes sir...

100 seconds Isp

Oh wait

75 seconds of burn time correct?

Edited March 22, 2018 by Cheif Operations Director

[GoSlash27]

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