Advanced Question Enough fuel??

DutchPlayer

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Hello,

I want to go to Mars, I have a trip of 321 days. By the ascent I burnt a lot of fuel. (this will not always happen) But my question is: How do I set deltaV into kilograms? I want this to know if I have enough fuel to do the ejection burn.

Has anyone a solution???


Kind regards,
Dutchplayer
 

RisingFury

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Hello,

I want to go to Mars, I have a trip of 321 days. By the ascent I burnt a lot of fuel. (this will not always happen) But my question is: How do I set deltaV into kilograms? I want this to know if I have enough fuel to do the ejection burn.

Has anyone a solution???


Kind regards,
Dutchplayer


Top left you have MAIN PROP with a number like 1.00k or something, which indicates how much delta-v you have left. You can check the fuel mass using the scenario editor.
 

Izack

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Top left you have MAIN PROP with a number like 1.00k or something, which indicates how much delta-v you have left. You can check the fuel mass using the scenario editor.

That number actually denotes remaining fuel mass. [math]\Delta_v[/math] must be calculated manually or by utility.
 

worir1

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to get into earth orbit you use around 7.5k delta-v.You cant really convert delta-v to kilograms. delta, Greek for change i thin, v which is velocity. delta v means change in velocity so. This cannot be measured accurately as fuel in kg. The actual amount of fuel needed depends on the ships engines. The question you need the answer to is how much delta v do i need to get to mars. For an accurate answer we need a departure date and information on your earth orbit. e.g ApA, PeA, Are you at the correct inclination?
 

worir1

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Im not so sure. im no expert but i hazard a guess at around 1.5k for ejection burn then maby 1k for orbit insert. What vessel are you using?
 

DutchPlayer

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Departure: 56662.5269
Encounter: 56983.9290

ApA: 223.2 Km
PeA: 205.4 Km

XR-2

BTW, I see no where MAIN PROP. Where can I see it?
 

worir1

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If you are using the stock delta glider you don't really have to worry about delta v as you will have loads to spare.
 

DutchPlayer

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Ok, I'll keep it in mind. You can change your fuel tank in the Delta Glider IV, can you also do that in the XR-2
 

worir1

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Ok, I'll keep it in mind. You can change your fuel tank in the Delta Glider IV, can you also do that in the XR-2

Yes. In the launch pad enable the senario editor then press ctrl f4 in game. choose senario editor then find your vessel and you can add fuel. works for all craft i think. You can also carry fuel cargos in both the DG-IV and the XR2
 

Cras

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Burn Time Calc MFD will provide plenty of information. It will give you the dV you got left, and the number of seconds you have fuel to burn the engines full throttle.
 

asbjos

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As Cras says, [ame="http://orbithangar.com/searchid.php?ID=4530"]Burn Time Calculator[/ame] show everything what you are searching after.

Also, on the lower panel of the stock Delta Glider, there is an indicator showing remaining delta-v.

Delta-v.jpg
 

sorindafabico

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Also, if you want to find the numbers by yourself...

[ame="http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation"]Tsiolkovsky rocket equation - Wikipedia, the free encyclopedia[/ame]
 

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He said he's using XR-2.
Keep in mind that the whole XR fleet is highly customizable through its per-vessel cfg files.

Look at page 17 of the file "XR Flight Operations Manual.pdf"
 

C3PO

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[ame="http://www.orbithangar.com/searchid.php?ID=1176"]Equation MFD[/ame] can show you the data for your craft. (Except in cases like DGIV with its turbopump) It can calculate the remaining DV.

It's not only a useful tool because the manual includes tutorials that are a very good way for a beginner to start calculating orbits and burns.
 

BruceJohnJennerLawso

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If you want to calculate your Delta V manually, its not too hard

DV (m/s) = engine isp * ln(m0/m1)

engine isp is typically given in seconds for some bizarre reason, so multiply an isp of say 400 sec by 9.81 to get the metric equivalent. Then calculate the mass fraction, ie mass before burn divided by mass afterwards. If you make sure to account for additional mass beyond the dry value (Oxygen, cargo, crew, RCS, etc), you should be able to accurately calculate your delta-vee. Good luck :thumbup:
 

Izack

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If you want to calculate your Delta V manually, its not too hard

DV (m/s) = engine isp * ln(m0/m1)

engine isp is typically given in seconds for some bizarre reason, so multiply an isp of say 400 sec by 9.81 to get the metric equivalent. Then calculate the mass fraction, ie mass before burn divided by mass afterwards. If you make sure to account for additional mass beyond the dry value (Oxygen, cargo, crew, RCS, etc), you should be able to accurately calculate your delta-vee. Good luck :thumbup:

The equation is:

[math]\Delta v=v_{e}\cdot\ln\frac{m_{0}}{m_{1}}[/math]Where [math]v_{e}[/math] is exhaust velocity in metres/second, not engine specific impulse [math]I_{sp}[/math] in seconds. It's confusing, because what Orbiter defines as specific impulse is actually exhaust velocity.
 

BruceJohnJennerLawso

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The equation is:

[math]\Delta v=v_{e}\cdot\ln\frac{m_{0}}{m_{1}}[/math]Where [math]v_{e}[/math] is exhaust velocity in metres/second, not engine specific impulse [math]I_{sp}[/math] in seconds. It's confusing, because what Orbiter defines as specific impulse is actually exhaust velocity.

Yes I was just explaining the conversion above. apparently you have to multiply by "standard gravity", 9.81 or so to go from seconds to m/s. Apparently there is some method to the madness of using seconds, since the number can accurately represent the number of seconds the rocket can hover in the air.
 

Urwumpe

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The equation is:

[math]\Delta v=v_{e}\cdot\ln\frac{m_{0}}{m_{1}}[/math]Where [math]v_{e}[/math] is exhaust velocity in metres/second, not engine specific impulse [math]I_{sp}[/math] in seconds. It's confusing, because what Orbiter defines as specific impulse is actually exhaust velocity.

Actually, specific impulse and exhaust velocity are the same. What is wrong is calling the value with the unit seconds an specific impulse. It is more a specific combustion endurance.

Specific impulse, by the words is "impulse per propellant mass". Impulse is force integrated by time, which means it is measured in SI units in "Ns" or "Newton seconds". so, specific impulse is measured in "N s /kg" which, by the definition of Newtons as " kg m/s²" means, that this unit for the specific impulse is also a velocity, since the units reduce themselves to "m/s".

Specific impulse and average exhaust velocity of one or more engines, are both equivalent. They just come together from different directions.

Thrust is mass flow multiplied by exhaust velocity. Impulse is Thrust force integrated by time or, at constant thrust, thrust force multiplied by time.

I = Ve * mdot * t

Mass flow at constant thrust is total propellant mass (consumed) divided by time.

I = Ve * mp / t * t = Ve * mp

Specific impulse is (again) Impulse divided by propellant mass (consumed):

Isp = I/mp = Ve * mp/mp = Ve

Now, if the time would be infinitely small, all mass mp would be consumed instantly. There would be just a single short impulse (like an explosion) and the velocity would be changed by:

dv [m/s]= (I[Ns]/m0 [kg]) [kg m s/(kg s²)]

But the reality is not like that. Instead, it is a sum of many infinitely small impulses, with the rocket getting lighter with every impulse. The lighter the rocket gets, the more velocity is gained by every (constant) impulse. That reality is described with the rocket equation.

---------- Post added at 05:25 PM ---------- Previous post was at 05:23 PM ----------

since the number can accurately represent the number of seconds the rocket can hover in the air.

One more fallacy that should be added to the list of reasons why specific impulse should never ever be measured in seconds again.

Can a rocket with a specific impulse of 350 seconds hover its own mass for 350 seconds? Sure not without quite many other conditions to be fullfilled, so that the claim works out.
 

BruceJohnJennerLawso

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One more fallacy that should be added to the list of reasons why specific impulse should never ever be measured in seconds again.

Can a rocket with a specific impulse of 350 seconds hover its own mass for 350 seconds? Sure not without quite many other conditions to be fullfilled, so that the claim works out.

The wiki article on the subject might need some editing then. Is the seconds of hover analogy even close?
 
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