[Video] Calculating delta V of solid boosters?

[Hooligan Labs]

If you take the delta V equation from here....

http://kspwiki.nexisonline.net/wiki/A_Guide_to_Basic_Kerbal_Rocket_Design_Through_Rocket_Science

... You would think that the trash can o' boom was the most efficient thing. However, if you try to actually fly through space with them it doesn't work so well.

I'm not talking about the inability to throttle them, I am just curious if solid fuel needs to be calculated differently when it comes to determining its performance.

Update: It looks like the issue was with too much drag from high velocity at low altitudes:

<iframe width="420" height="315" src="http://www.youtube.com/embed/dQjrrHcIFsg" frameborder="0" allowfullscreen></iframe>

Edited September 16, 2012 by Hooligan Labs

[Hillstache]

That's a really great reference page, thanks for that!

Regarding the SRB's, what did you calculate their delta V to be? It's seemed to me since the rebalancing of .16 that they're possibly overpowered.

[maltesh]

As engines, the RT-10 solid fuel booster is pretty efficient. As fuel tanks, they aren't so efficient.

The best full mass/dry mass ratio you can get using the RT-10 booster is just under 5:1. The best full mass/dry mass ratio you can get using stock 0.16 fuel tanks is 9:1.

As a result, even though the sfrs are more efficient at burning, the absolute best that a single-stage of RT-10's can do is a delta-V of about 6,984 m/s.

The absolute best that you can get out of a single stage of stock fuel tanks attatched to an LV-T30, the least efficient stock liquid boosters (assuming vacuum performance) is about 7,113 m/s.

[togfox]

So what is the Isp of a SRB - or alternatively, the density of the solid fuel?

[maltesh]

As per the Wiki's article on Parts, the specific impulse on the RT-10 is 443 seconds.

[Hillstache]

I calculate the delta v then of the RT-10 as 6994 m/s. Very efficient!

[Hooligan Labs]

Thanks to everyone for checking my calculations! This is a very useful tutorial, I use it to check my ships now before I fly them. It helps check for glaring problems with efficiency.

My problem is when I try to actually use SRBs to get somewhere. The spaceplane I have been working on uses 5 aerospikes (same ISP in and out of atmosphere) and can reliably get to the moon, land and return. It calculates to ~6000 m/s, which is significantly less than the 7000 m/s recommended by the tutorial but hey I'll just say I'm an awesome pilot.

So I re-calculated with a rocket assisted takeoff (still single stage). Suddenly it seems I should easily get over 7000 m/s! However, if I build the same plane but with SRBs, fire all rockets and aerospikes, and get out of atmosphere.... I actually have less fuel than if I did not bring the SRBs! Same thing happens if I take off, go vertical and then fire the SRBs.

As a final check, try taking off nothing but a few SRBs. You might calculate as having enough delta V to go to the moon and back, but you actually will have just enough to leave the atmosphere.

Anyone want to double check my findings in-game?

[Kosmo-not]

You need about 4500 m/s to get into Kerbin orbit (without overspeeding).

It sounds like you are forgetting some mass in your calculations.

[Hooligan Labs]

You need about 4500 m/s to get into Kerbin orbit (without overspeeding).

It sounds like you are forgetting some mass in your calculations.

Yes, and the delta v was calculated here as over 6000 m/s?

You would know, weren't you one of the original authors of the tutorial?

Edit: As an example, this morning I built a ship with 6 trashcans, a '16 fuel tank and an aerospike. I took all the parts into account and still had a delta v of over 9000 m/s. However, when I went to fly it I couldn't even fully achieve orbit with it.

I would link the excel file, but I appear to be not yet allowed attachments.

Edited September 13, 2012 by Hooligan Labs

[Hillstache]

My thought is that I assume you're firing all the 'trashcans' at takeoff, so much of their thrust is going to counteract air resistance, making our ideal delta v calculations not so accurate. That's just my guess.

[Kosmo-not]

Do you have a picture of your rocket?

[Ziff]

It sounds like you aren't calculating them correctly. How do you incorporate them into your space-plane design? I don't fly space-planes, only rockets, but my thoughts are this: Do you fire them at takeoff or later in the flight? If you are not firing the SRB's immediately after takeoff then that is part of your problem. You are carrying around dead weight, so to speak. If you are actually firing them immediately, you should be throttling down your other engines until the SRB's burn out, you lose a lot of delta-v to atmospheric drag when you are below 9km or so.

[Hooligan Labs]

Here is my test craft! Need to learn how to wrap this stuff in spoiler tags on this forum...

[ATTACH=CONFIG]32872[/ATTACH][ATTACH=CONFIG]32873[/ATTACH][ATTACH=CONFIG]32874[/ATTACH]

It can make it to about 97km at 1000 m/s. No where near capable of even going to the moon.

And thanks for the tip Ziff. Maybe part of the problem is that I'm going 500 m/s in atmosphere.

Darn, it won't let me upload the excel sheet I've been using for calculation.

Edited September 14, 2012 by Hooligan Labs

[EndlessWaves]

You should be allowed attachments after 27 posts.

spoilers are [spoiler="Click me"]content here[/spoiler]

[sal_vager]

Hooligan Labs, you should be able to attach stuff after 5 posts, are you using the Advanced Editor when making your post? Click on Go Advanced.

If you still have trouble, use Mediafire and post the link.

[Hooligan Labs]

Thanks for the spoiler explanation! Yes, I was able to attach the pictures no problem. However, it says that .xlsx files are not permitted.

[Nadrek]

If you get them into orbit, I've got a Mechjeb Autom8 module that has a function to experimentally calculate Isp in post #53 of the Autom8 megathread at http://kerbalspaceprogram.com/forum/showthread.php/16503-MechJeb-Autom8-Scripts-Megathread?p=238962&viewfull=1#post238962

One function does the calculation, and another actually will circularize your orbit and execute a set of burns (1 to N) to calculate Isp at different throttle settings, so you'd want just 1 for the burn.

To use it, get into orbit, make sure you've got only SRB's of the same type* in the stage you're on, and then in the Autom8 window:

require "SimpleKSPOrbitalOperations"
cotestSkooIspFromTmbiTmbtwnNsMpcs(2, 15, 1, 0.02)

The function definition and notes, if anyone cares, are:

cotestSkooIspFromTmbiTmbtwnNsMpcs(TimeBeforeInitial, TimeBetweenEach, NumberSlices, MaxPointingChangeStable)

-- MaxPointingChangeWeCallStable is the maximum degrees of change in heading + pitch

-- we'll call "stable" for purposes of detecting when mechjeb.attitudeTo() is done

-- i.e. 0.2 means 2/10ths of a degree change during TimeBetweenEach seconds

-- This is a wrapper that circularizes our orbit, verifies

-- a stable orbit, orients prograde, and then runs the Isp test

-- NOTE: This is designed to be run asynchronously to vessel control.

-- NOTE: This will only work anywhere close to properly with a small TimeBetweenEach,

-- in a circular orbit, with the vessel held in prograde or retrograde directions.

-- TimeBeforeInitial is the "spool up" delay in seconds we're going to use

-- this is how much time we think it takes our engines to come to stable thrust

-- i.e. 0.5 means set thrust to the desired value, then wait half a second, then start the measurement for this "slice"

-- For the bell rocket engines in 0.16, 0.25 is a reasonable minimum for TimeBeforeInitial

-- TimeBetweenEach is the time in seconds between initial and final measurements for each "slice"

-- For the bell rocket engines in 0.16, 0.25 is a reasonable minimum for TimeBetweenEach

-- NumberSlices is how many slices we're going to divide our thrust into, i.e.

-- 1 means 100 (100% max thrust) / 1 = one test at 100%.

-- 2 means 100/2 = 50% for the first test, and 100% for the second.

-- 4 means 100/4 = 25% for the first test, 50% for the second, 75% for the third, and 100% for the fourth.

-- Nothing is returned per se, but the Specific Impulse, kg mass lost, and Delta-V per mass is printed at each slice for human consumption.

-- While we could use the Skom reporting functions, because we need initial and final mass

-- instead of just differences, this is almost easier and is certainly simpler to understand.

*I've never tried this, but it should work - if they don't automatically fire, set a large initial delay (the first number) and fire them "by hand" when it throttles up

[Kosmo-not]

You can easily calculate the I_sp by looking at the part files.

For small SRB: I_sp=442s

For large SRB: I_sp=204s

[EndlessWaves]

Thanks for the spoiler explanation! Yes, I was able to attach the pictures no problem. However, it says that .xlsx files are not permitted.

It looks like the forum has a very restrictive allow list for file formats, stick it in a zip file and you should be able to upload it.

[Nadrek]

It looks like the forum has a very restrictive allow list for file formats, stick it in a zip file and you should be able to upload it.

For pictures, please don't upload then in a Microsoft Excel format (much less an Excel 2007+ format) - .jpg or another picture format would be better. Remember, not everyone has Excel or something good to read them, and Excel is expensive.

For spreadsheets, Libreoffice (http://www.libreoffice.org/) is at least free and available for Windows, Mac, and Linux.

[Hooligan Labs]

You can easily calculate the I_sp by looking at the part files.

For small SRB: I_sp=442s

For large SRB: I_sp=204s

Yes, but Nadrek is looking to experimentally prove it, which I applaud!

I'm playing without mods at the moment, so I did another test... Looks like the air resistance theory was right. Bad news is, we've lost Jeb.

<iframe width="420" height="315" src="http://www.youtube.com/embed/dQjrrHcIFsg" frameborder="0" allowfullscreen></iframe>

[Nadrek]

Yes, but Nadrek is looking to experimentally prove it, which I applaud!

I'm playing without mods at the moment, so I did another test... Looks like the air resistance theory was right. Bad news is, we've lost Jeb.

<iframe width="420" height="315" src="http://www.youtube.com/embed/dQjrrHcIFsg" frameborder="0" allowfullscreen></iframe>

My condolences on Jeb. Once the family is done grieving, talk to his brother Jeb, also an excellent pilot.

And thank you for the applause; the experimental measurements have three major advantages over the parts file. First, they allow those who look at the code to see the actual math involved. Second, they don't care how many engines of what specific impulses you have on rocket. Third, they are _actually_ accurate within a reasonable margin of error... unlike the Parts file in 0.16, which lies!

For example, an LV-T45 on a rocket starting the test at approximately 13.3 tons:

At thrust: 10% Isp is: 3490.6740910247
   Used 0.407934188842773 kg fuel, for 1.05126962080158 DeltaV, or 2.57705691151756 DeltaV per kg fuel and 1.40169282773544 DeltaV per second
At thrust: 20% Isp is: 1797.90199416966
   Used 1.45530700683594 kg fuel, for 1.9319236255069 DeltaV, or 1.32750245579261 DeltaV per kg fuel and 2.57589816734253 DeltaV per second
At thrust: 30% Isp is: 1215.11984362804
   Used 3.66973876953125 kg fuel, for 3.29362980189717 DeltaV, or 0.897510697285376 DeltaV per kg fuel and 4.39150640252956 DeltaV per second
At thrust: 40% Isp is: 903.884828955461
   Used 6.70003890991211 kg fuel, for 4.47590106928692 DeltaV, or 0.668041056099723 DeltaV per kg fuel and 5.96786809238256 DeltaV per second
At thrust: 50% Isp is: 725.654721031727
   Used 10.1935863494873 kg fuel, for 5.47232665973979 DeltaV, or 0.536840173038317 DeltaV per kg fuel and 7.29643554631972 DeltaV per second
At thrust: 60% Isp is: 608.021967457222
   Used 14.2822265625 kg fuel, for 6.43367939469454 DeltaV, or 0.450467535054168 DeltaV per kg fuel and 8.57823919292605 DeltaV per second
At thrust: 70% Isp is: 521.458302751379
   Used 20.519495010376 kg fuel, for 7.94516284354404 DeltaV, or 0.387200700578959 DeltaV per kg fuel and 10.5935504580587 DeltaV per second
At thrust: 80% Isp is: 460.325639631327
   Used 26.8013477325439 kg fuel, for 9.1855244198191 DeltaV, or 0.342726213304021 DeltaV per kg fuel and 12.2473658930921 DeltaV per second
At thrust: 90% Isp is: 410.633801563746
   Used 33.0276489257813 kg fuel, for 10.1319807593941 DeltaV, or 0.306772691636706 DeltaV per kg fuel and 13.5093076791921 DeltaV per second
At thrust: 100% Isp is: 370.224071690755
   Used 41.8767929077148 kg fuel, for 11.6319489877646 DeltaV, or 0.277765993527688 DeltaV per kg fuel and 15.5092653170195 DeltaV per second

[maltesh]

That looks a lot like the v0.16 fuel bug. Are you running the fuel bug fix?

[Hooligan Labs]

My condolences on Jeb. Once the family is done grieving, talk to his brother Jeb, also an excellent pilot.

And thank you for the applause; the experimental measurements have three major advantages over the parts file. First, they allow those who look at the code to see the actual math involved. Second, they don't care how many engines of what specific impulses you have on rocket. Third, they are _actually_ accurate within a reasonable margin of error... unlike the Parts file in 0.16, which lies!

For example, an LV-T45 on a rocket starting the test at approximately 13.3 tons:

At thrust: 10% Isp is: 3490.6740910247
Used 0.407934188842773 kg fuel, for 1.05126962080158 DeltaV, or 2.57705691151756 DeltaV per kg fuel and 1.40169282773544 DeltaV per second
At thrust: 20% Isp is: 1797.90199416966
Used 1.45530700683594 kg fuel, for 1.9319236255069 DeltaV, or 1.32750245579261 DeltaV per kg fuel and 2.57589816734253 DeltaV per second
At thrust: 30% Isp is: 1215.11984362804
Used 3.66973876953125 kg fuel, for 3.29362980189717 DeltaV, or 0.897510697285376 DeltaV per kg fuel and 4.39150640252956 DeltaV per second
At thrust: 40% Isp is: 903.884828955461
Used 6.70003890991211 kg fuel, for 4.47590106928692 DeltaV, or 0.668041056099723 DeltaV per kg fuel and 5.96786809238256 DeltaV per second
At thrust: 50% Isp is: 725.654721031727
Used 10.1935863494873 kg fuel, for 5.47232665973979 DeltaV, or 0.536840173038317 DeltaV per kg fuel and 7.29643554631972 DeltaV per second
At thrust: 60% Isp is: 608.021967457222
Used 14.2822265625 kg fuel, for 6.43367939469454 DeltaV, or 0.450467535054168 DeltaV per kg fuel and 8.57823919292605 DeltaV per second
At thrust: 70% Isp is: 521.458302751379
Used 20.519495010376 kg fuel, for 7.94516284354404 DeltaV, or 0.387200700578959 DeltaV per kg fuel and 10.5935504580587 DeltaV per second
At thrust: 80% Isp is: 460.325639631327
Used 26.8013477325439 kg fuel, for 9.1855244198191 DeltaV, or 0.342726213304021 DeltaV per kg fuel and 12.2473658930921 DeltaV per second
At thrust: 90% Isp is: 410.633801563746
Used 33.0276489257813 kg fuel, for 10.1319807593941 DeltaV, or 0.306772691636706 DeltaV per kg fuel and 13.5093076791921 DeltaV per second
At thrust: 100% Isp is: 370.224071690755
Used 41.8767929077148 kg fuel, for 11.6319489877646 DeltaV, or 0.277765993527688 DeltaV per kg fuel and 15.5092653170195 DeltaV per second

Can you break that down for us?

That looks a lot like the v0.16 fuel bug. Are you running the fuel bug fix?

... What?

[maltesh]

KSP v.0.16 has a bug that afflicts all engines that have a Specific impulse rating in the VAB.

What should happen is that, for a specific engine, fuel consumption should be directly proportional to the throttle fraction used: At 1/2 throttle, your fuel consumption should be 1/2 the rate of it would be at full throttle. 1/3 throttle -> 1/3 fuel consumption rate and so on. This results in a constant specific impulse regardless of throttle setting, which is the way it worked in all previous versions of KSP, and in general, in reality.

In v.0.16, the fuel bug results in fuel consumption is proportional to the square of the throttle fraction used: At 1/2 throttle, you wind up using 1/4 the fuel consumption rate at full throttle. 1/3 throttle -> 1/9 fuel consumption rate. The bug results in an unrealistic higher efficiency any throttle amount below full throttle, and effectively infinite specific impulse and delta-V if you're capable of setting the throttle low enough.

The bug is fixed in v0.17, and there's a makeshift fix in the addons section.