BurnTimeCalcMFD BurnTimeCalc MFD Estimate Total dV?

Wedge313

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Using BurnTimeCalc MFD, what exactly am I looking at with the Estimate Total dV number? While sitting on the ground with a full load of fuel, why is this number so low? (XR-2, full fuel load, the Estimate Total dV shows less than 7.000 km/s) Thanks.
 

asbjos

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BurnTimeCalculator calculates the total dV for one propellant tank alone, with the current engine specifications. With the XR-2, it will therefore not include any dV from the scram engines/propellant.
Also, the XR-2 changes the engine thrust with the air pressure, and you will therefore see that the effectiveness of the engine (the dV) will increase as you escape the atmosphere.
 

Wedge313

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Thanks. I've seen how the dV available goes up as the mass goes down, so the number I see is only the dV available at the moment I look at it., and I'm not clear how that helps me. I guess my real question is this: How much fuel do I need to get from A to B? Let's take an earth-moon flight. It seems that to get to orbit I'll need around 9.3-10 km/s, TLI 4.1 km/s, lunar orbit 0.7 km/s, and 1.6 km/s to deorbit. So that's around 16 km/s dV. Knowing that, how do I figure out how much fuel I need for an XR-2 with known crew/LOX/cargo etc. short of crunching numbers (I stink at calculus)?
 

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It seems that to get to orbit I'll need around 9.3-10 km/s..
That's a bit much. 4.5km/s is more typical.

What you need to do is put your budget in a spreadsheet and use the rocket equation to calculate each step. Then you will find the answer to how much fuel you need or what the maximum payload is for a certain mission.

Have a look at this screenshot. It's from an old spreadsheet and the numbers aren't checked, but it shows the idea. When you know 3 of the 4 parameters of the rocket equation (in black), you can calculate the missing one (in red) and use it on the next row. Note that the ISP is lower when landed to reflect the correct amount of delta-v as indicated by BurnTimeMFD.

 

Wedge313

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Thanks. Maybe I'm missing something? I'm using dV numbers I've seen on numerous websites, they all show something like 9.3-10 km/s to get into orbit. My orbital speed at 200k is about 7.700 km/s. Most sites I've read allow about 1.5-2.0 km/s due to atmospheric drag. The dV due to the earth's spin at Cape Canaveral is only around 0.4 km/s, so it doesn't make a huge difference. So that's how I came up with those numbers. But that spreadsheet looks helpful, do you know where I might find a link? I know David Courtney has one he uses, but I've been unable to find a link.
 

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Share all relevant settings first (MainFuelISP, SCRAMFuelISP, LOXLoadout, LOXConsumptionRate, LOXConsumptionMultiplier, APUFuelBurnRate). And mention the scenario you are using if it is one of the defaults, or share it as well.

It's an old spreadsheet I made by eyeballing the spreadsheets David and Tex used in their video's. Creating one yourself helps a lot to understand the flight plan.

Once I know your settings, I see if I can make a simple budget for a earth-moon trip and a video showing how to fly into orbit (inclusing a SCRAM ascend of course).
 

Wedge313

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Thank you for your offer, but you're right, doing this myself would be the correct way to go. FWIW, I've been using Orbiter 2016 and starting with one of the stock XR-2 scenarios (Ready for Takeoff to ISS). From that point I've deleted or added LOX or fuel as necessary for trips to the moon or Mars, but it's all been by trial and error (land at Brighton Beach with excess fuel? OK, next time start with less). It's just that I know there's a way to figure this out while on the ground, and I would enjoy doing that. I can get to the moon or Mars using TransX or IMFD without issue; I just want to learn how to do it efficiently. Texfilms has some great flight planning videos (good discussion on his Saturn planning), but the link he provides to the spreadsheets he uses doesn't work. As you said, it may be best for me to plow ahead and do this on my own.
 

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Be aware that there is a little bug in the calculation of the total crew mass in the XR series when used with Orbiter 2016. It is not the sum of the body masses you find in the scenario or xrcfg files. I think the Orbiter sim uses 903kg for the XR2. Not sure.

Once you have all the numbers and calculations right, BurnTimeMFD and a spreadsheet will match 100%.
 
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