Solved Engine specs question

[Lt. Snakestrike]

I'm just wondering if the fuel consumption spec is supposed to be tons/sec, or if it's something else.

 

[TtTOtW]

I'm just wondering if the fuel consumption spec is supposed to be tons/sec, or if it's something else.

Yes that is correct.

 

[Lt. Snakestrike]

Was calculating Isp of my shuttle, and apperently the frontier's Isp is only 280 instead of the 285 it says... Based on the thrust and fuel consumption (mass expelled)

 

[Altaïr]

Was calculating Isp of my shuttle, and apperently the frontier's Isp is only 280 instead of the 285 it says... Based on the thrust and fuel consumption (mass expelled)

Are you sure about that? I built a simple rocket with a Frontier engine and measured its delta-V (by burning all fuel with drag and gravity disabled):

I end up at 3825 m/s, which corresponds to a specific impulse of 285.7s
I can't explain the exceeding 0.7s (I should have ended at 3815 m/s instead), but besides that the given value seems correct...

 

[Lt. Snakestrike]

Are you sure about that? I built a simple rocket with a Frontier engine and measured its delta-V (by burning all fuel with drag and gravity disabled):
View attachment 8924
I end up at 3825 m/s, which corresponds to a specific impulse of 285.7s
I can't explain the exceeding 0.7s (I should have ended at 3815 m/s instead), but besides that the given value seems correct...

It might be that one of the other values is wrong too, I was just using Isp=F/mdot e. My guess is its more likely the fuel consumption rate is rounded or something similar...

 

[Altaïr]

It might be that one of the other values is wrong too, I was just using Isp=F/mdot e. My guess is its more likely the fuel consumption rate is rounded or something similar...

Yes, that's probably the reason. That's why I prefer a delta-V measurement, using the formula:
Delta-V = g×Isp×Ln(1 + fuel_mass/dry_mass)

By the way, maybe I found the reason why the Isp I measured was slightly greater: I realized that with this value, the ejection speed (g×Isp) was practically exactly 2800 m/s. So it's possible that Stef actually specifies the ejection speed, and calculates the Isp from it (rounding it down). This should be verified though...

 

[Lt. Snakestrike]

Yes, that's probably the reason. That's why I prefer a delta-V measurement, using the formula:
Delta-V = g×Isp×Ln(1 + fuel_mass/dry_mass)

By the way, maybe I found the reason why the Isp I measured was slightly greater: I realized that with this value, the ejection speed (g×Isp) was practically exactly 2800 m/s. So it's possible that Stef actually specifies the ejection speed, and calculates the Isp from it (rounding it down). This should be verified though...

Yeah, that makes sense. My original question was because I'm working right now on calculating my shuttle's delta-V using that equation... it's hard though because it's a fairly complex launch...

 

[Lt. Snakestrike]

Yes, that's probably the reason. That's why I prefer a delta-V measurement, using the formula:
Delta-V = g×Isp×Ln(1 + fuel_mass/dry_mass)

By the way, maybe I found the reason why the Isp I measured was slightly greater: I realized that with this value, the ejection speed (g×Isp) was practically exactly 2800 m/s. So it's possible that Stef actually specifies the ejection speed, and calculates the Isp from it (rounding it down). This should be verified though...

Wouldn't it also make sense if he just specified that thrust and Isp of each engine as well though, as they're the values that people pay the most attention to... I mean, he'd have to have an actual fuel consumption value though too, but it wouldn't be a huge deal if it was just slightly different than stated.

 

[Altaïr]

Yeah, that makes sense. My original question was because I'm working right now on calculating my shuttle's delta-V using that equation... it's hard though because it's a fairly complex launch...

If that can help you I've made a calculator:
https://docs.google.com/spreadsheet...zOEbB5pSJMOjeiR-ES4y0HOzxy8/edit?usp=drivesdk
It's a sheet under google drive. I've left it in read only mode, so you have to make your own copy to work with it.

I wanted to improve it before making it official, but it can still help as is

 

[Lt. Snakestrike]

If that can help you I've made a calculator:
https://docs.google.com/spreadsheet...zOEbB5pSJMOjeiR-ES4y0HOzxy8/edit?usp=drivesdk
It's a sheet under google drive. I've left it in read only mode, so you have to make your own copy to work with it.

I wanted to improve it before making it official, but it can still help as is

Thanks, I've already got pretty much everything figured out, I just need to figure out the different mass cutoffs, which is proving hard right now because I keep messing up the launch due to attempting to screenshot mid-launch... Not to mention the use of ions after reaching orbit...

 

[Earlmilly1]

A delta V calculator I made, if this helps.

 

[Lt. Snakestrike]

A delta V calculator I made, if this helps.

The only thing stopping me right now is that I need to get the dry and wet masses for each different Isp the shuttle operates at during the launch.

 

[Altaïr]

The only thing stopping me right now is that I need to get the dry and wet masses for each different Isp the shuttle operates at during the launch.

For each different Isp??? What do you mean?
What prevents you from getting dry mass and wet mass?

 

[8BitCosmonaut]

A delta V calculator I made, if this helps.

Yours is very limited, there is a better one on strout net that can allow you to punch in any number you want.

 

[Lt. Snakestrike]

For each different Isp??? What do you mean?
What prevents you from getting dry mass and wet mass?

I turn different engines off at different times through the launch to balance thrust, so it gets more efficient through the launch. I need to get the fuel percents of the two main tanks to calculate the mass at each change in engine, which is hard to do while launching a shittle which is just trying to fall back to earth...

 

[Earlmilly1]

Yours is very limited, there is a better one on strout net that can allow you to punch in any number you want.

Same with this. Just click the value near the slider.

 

[8BitCosmonaut]

Same with this. Just click the value near the slider.

Nah, troublesome, just use pen and paper. And staring at the computer all day ruins my fucking eyes.

 

[TtTOtW]

Nah, troublesome, just use pen and paper. And staring at the computer all day ruins my fucking eyes.

Tell me about it... I'm a graphic designer. Nice job, but an eye killer.

 

[8BitCosmonaut]

Nah, troublesome, just use pen and paper. And staring at the computer all day ruins my fucking eyes.

Better dV calculator:
http://www.strout.net/info/science/delta-v/

Best dV calculator:
Get a scientific calculator, some 4B pencils and a lifetime supply of paper.

 

[Altaïr]

I turn different engines off at different times through the launch to balance thrust, so it gets more efficient through the launch. I need to get the fuel percents of the two main tanks to calculate the mass at each change in engine, which is hard to do while launching a shittle which is just trying to fall back to earth...

Oh, OK, I get your problem. Getting the fuel percentage is tough indeed, but the calculator I made can help you for the rest: you can specify how many engines of each kind you use, and it automatically calculates thrust, average Isp... There is also a field to specify that you don't use an engine (for that kind of situations): the calculator will still takes its mass into account, but will ignore it to calculate thrust, Isp and the rest.