Flight Question How do you calculate fuel usage?

[Jeorbit]

Hi guys,

Last night I achieved my first Moon orbit (using Altea's XR-2). But I did not have enough fuel left to perform my deorbit burn.

I'm coming from KSP where I actually never had to take fuel consumption in account directly. There, my only reference was Dv. I would simply build my ships as to have the proper amount of Dv required at each stage (I now realize just how real-time feedback during construction made life easier!).

Of course, this meant in turn trying different fuel quantities / engines performance combos, but I never had to actually care about computing the fuel quantity needed. It was just a by-product of getting the right Dv and TWR for each stage / mass.

Obviously things are different within Orbiter and I wonder how advanced orbinauts proceed regarding fuel consumption?

My lacking fuel once in orbit around the moon can probably be attributed in part to:

- A bad ascent profile
- A distinct plane alignment burn done once I reached LEO (instead of making it during the ejection burn)
- A sub-optimal ejection burn (I used the default TransX, although I know there is a specific moon transfert MFD I have yet to try)

But if indeed even a topped-up XR2 in capable hands does not have enough fuel to land at Brighton Beach, I still know I can get extra fuel as payload in the service bay. I just don't know how much and I'l like to understand how I can estimate it for all my subsequent flights & crafts.

Thanks for your help!

 

[Topper]

I use BurnTimeCalcMFD, it shows you the "estimated remaining dv" of your ship and the required fuel and dv for a manouver (If you enter the dv).
But of course you need to know how much dv you need, that depents on your exact flight plan. I hope that helps...

 

[Jeorbit]

Thanks!

Indeed I could always look up the needed DVs from DV maps (btw are there any popular DV maps you guys like to use?) and find those XR-2's engines specs (in the dock I guess?).

But then I'd still need the math to use those values to derive a fuel quantity payload!

 

[Thorsten]

Once you're orbital, you can basically map fuel into Delta v via the rocket equation - so this is the part you ought to be used to.

During ascent, it's more difficult because you always need some percentage of thrust to keep you up against gravity, so fuel usage depends a lot on your acceleration capabilities (aka payload, target inclination,...) and ascent profile. It's not a pen and paper computation, you probably need a numerical tool to do it. Or you simply try a few cases and make a table...

In the atmosphere it's yet more complicated since there's drag but also lift, so you need to expend fuel to overcome drag but if you have a lifting body you need less fuel to overcome gravity.

The rule of thumb is that you want to get out of the atmosphere fast and then accelerate to orbital speed fast without wasting too much time and fuel climbing to high altitude. On Earth you'd climb to ~120 to 150 km to be clear of the atmosphere and then level off to blast to Mach 26.

 

[Marijn]

Based on David Courtney's spreadsheet which is visible in some of his video's, I made my own version in Google Spreadsheets in order to plan all kinds of flights. I am able to plan the fuel very precisely and predict available delta-v at every part of the mission with very high accuracy.

Have a look at Tsiolkovsky rocket equation on Wikipedia. This formula is rather easy to implemend. The results will match perfectly with BurnTimeMFD.

 

[Jeorbit]

Ahh, indeed... I just need to find XR2's engines exhaust velocity in metres/second to derive my Delta V budget once in orbit.
The panel already gives total and dry mass of the ship at any given time.

I guess the variables are now just the ascent and moon landing, but I probably can go for approximations here (found a Dv map here : http://i.imgur.com/SqdzxzF.png)

Edit: based on the map above map I'd need a bit over 15k Dv to land on the moon from a earth take-off.

Thanks for your help!

 

[Ripley]

Have you already looked in XR Flight Operations Manual?

 

[Jeorbit]

Have you already looked in XR Flight Operations Manual?

Yes, but mainly for the ascent phase, which is very well explained - actually I could never have taken it to orbit using SCRAM engines without it

But I might have missed the engine's specifications. I'll need to get back to it (and I guess I'll find that in the configuration file too).

 

[Marijn]

@Jeorbit, the first thing you need to decide is the Main Fuel ISP setting in the XR2 config file. I suppose it is set at the default (2). This determines the amount of thrust for every kilogram of fuel. So if you want to compare any delta-v value with someone, you need to know the ISP settings.

If you're interested, I could share the spreadsheet with you. But I think you will learn more if you create your own.

Remember, If you carry too much fuel, the trip will cost more fuel because all this left-over fuel can be considered as cargo (weight).

 

[Jeorbit]

@Jeorbit, the first thing you need to decide is the Main Fuel ISP setting in the XR2 config file. I suppose it is set at the default (2). This determines the amount of thrust for every kilogram of fuel. So if you want to compare any delta-v value with someone, you need to know the ISP settings.

If you're interested, I could share the spreadsheet with you. But I think you will learn more if you create your own.

Remember, If you carry too much fuel, the trip will cost more fuel because all this left-over fuel can be considered as cargo (weight).

Thanks, it's indeed more satisfying if I do it myself

The only thing I still can't get is how do you go from the ISP value in XR-2's config file (0,1 or 2) to a specific impulse in seconds or an exhaust velocity in metres/second that i can feed directly in the rocket equation.

Am I missing something obvious here ?

 

[Marijn]

The rocket equation lets you play with 4 variables: DV, M0, M1 and ISP. If you know at least 3 of these 4 variables, you can calculate the 4th.

The ISP is always given: It's the setting in your config file. Let's assume you're at the default ISP2, which is 25962. This value is only true in the vacuum of space. While landed on earth, the ISP is lower, like 20775. This is because of atmospheric drag I figured. Also on Mars, the ISP will be slightly lower, like 25950. On the moon however, the ISP will match 25962. You can check this for yourself. The ISP value is displayed by BurnTimeMFD.

M0 is the total weight. M1 is M0 minus the weight of the fuel (main and RCS). Be aware that BurnTimeMFD deals with the scram fuel in a way I do not fully understand. I never look at the weight numbers of BurnTimeMFD before I used all the scram fuel. I have a some sort of hack in my sheet which deals with that.

Delta-v values can only be obtained by practicing and taking notes. I do not believe you can grab them from a table. Even the smallest mistakes during a launch from earth have a very big impact on the remaining delta-v budget. So it really depends on your skills.

So now you know the ISP and the delta-v. M1 is also known, so you can calculate M0 which is your answer. Remember that when you use APU fuel and LOX, both M0 and M1 do change.

 

[Thorsten]

While landed on earth, the ISP is lower, like 20775. This is because of atmospheric drag I figured.

It's pressure - the engine has to shove the exhaust against the ambient pressure, and this is less effective on the ground than in space (also, the engine bell expanding the exhaust gas has to be designed for an ambient pressure, and unless you have a configurable engine, it's usually designed for space, which again reduces the effectivity inside the atmosphere).

 

[Marijn]

The only thing I still can't get is how do you go from the ISP value in XR-2's config file (0,1 or 2) to a specific impulse in seconds or an exhaust velocity in metres/second that i can feed directly in the rocket equation.

DV=ISP*LN(M0/M1) & M0=M1*EXP(DV/ISP). I think that's all the math you need.

---------- Post added at 07:06 PM ---------- Previous post was at 06:56 PM ----------

@Thorsten, thanks for that. I noticed the ISP on earth is not always the same. On the Edwards base it seems to be always higher than KSC. Any idea how Orbiter is calculating these numbers? Latitude perhaps?

 

[Thorsten]

Edwards is not at sea level, KSC is - so the pressure at Edwards is lower due to the altitude.

 

[Marijn]

Edwards is not at sea level, KSC is - so the pressure at Edwards is lower due to the altitude.

Ah, that makes a lot of sense.

In order to calculate the fuel needed until MECO, I did set the ISP at 21300, like some sort of average. This number seems to work for me. Any thoughts on whether this is the correct way of doing this?

 

[Topper]

Hi I found this cool image at wikipedia :thumbup:

 

[Jeorbit]

Thanks a lot guys! This has been very helpful!

As a bonus, I found this chart on Wikipedia dealing with specific impulse of air breathing engines (and rockets at the bottom). It shows where SCRAMs are relative to other types of engines, and how low rocket engines ISP really is.

And it reminds us that with a 2k+ near constant ISP, XR-2's engines are really, really nice

 

[Thorsten]

In order to calculate the fuel needed until MECO, I did set the ISP at 21300, like some sort of average. This number seems to work for me. Any thoughts on whether this is the correct way of doing this?

It's not 'correct' (i.e. it won't work for any ascent trajectory), but since it works for you it seems to be 'good enough'. I'm not familiar with the ascent procedure of the spacecraft in question (my experience is rather Shuttle-centered), so I can't comment further.

 

[Urwumpe]

Thanks a lot guys! This has been very helpful!

As a bonus, I found this chart on Wikipedia dealing with specific impulse of air breathing engines (and rockets at the bottom).

Funnily, the J-58 is actually operating as a Ramjet at Mach 3... The turbine section is only needed to suck in enough air to make sure the inlet shock waves have the right position to compress the air and not reverse the flow suddenly...