Jump to content

Node Burn Time Formulae(RESOLVED)


MrWilliam932

Recommended Posts

Hi, I started doing calculations of Orbit translations.

I know how to calculate:

Data related to the rocket

  • TWR
  • Total DV
  • Total burn time
  • ISP
  • Fuel consumption (kg/s)

Data related to the orbit

  • Speed at certain point of a orbit (round or elliptical orbit)
  • Escape Velocity
  • DV needed for change orbit (plain or inclined)
  • DV needed for an inclination change

Now I want to know how many time I have to make a burn to get the DV i need, but I didn't find nothing conclusive. So, what I need to know ? 

 

Edited by MrWilliam932
Link to comment
Share on other sites

I not completely sure what you are asking for, but are you asking for how long your burn will need to be to complete a maneuver that you know the delta-v for? If so first you need to know your acceleration, you can take you TWR and multiply it by 9.8m/s2. You can then take the delta-v for your maneuver and divide it by your acceleration to get your burn time in seconds. This won't be entirely correct since you will lose mass and increase your TWR while you burn, but it should be close enough if you aren't doing massive maneuvers or losing a large portion of your mass during your burn.

TL/DR: Delta-v/(TWR*9.8m/s2)

Edited by Redjoker
Link to comment
Share on other sites

4 minutes ago, Redjoker said:

I not completely sure what you are asking for, but are you asking for how long your burn will need to be to complete a maneuver that you know the delta-v for? If so first you need to know your acceleration, you can take you TWR and multiply it by 9.8m/s2. You can then take the delta-v for your maneuver and divide it by your acceleration to get your burn time in seconds. This won't be entirely correct since you will lose mass and increase your TWR while you burn, but it should be close enough if you aren't doing massive maneuvers or losing a large portion of your mass during your burn.

It's a good aproximation, but i need the formula for exact burn time, including the lose of the burned fuel, but thank you anyway :)

Link to comment
Share on other sites

If you need the exact burn, it gets more difficult because calculus is the only way I see of finding it. You would need a function or set of functions that models the acceleration of your rocket as it loses mass. You would then need to solve for the end time with an integral that equals the delta-v of the maneuver and starts at 0.

Link to comment
Share on other sites

Actually you could use the rocket equation and solve for the final mass and subtract that from the initial mass to find out how much fuel you used. You could then divide the difference by your fuel usage.

 

 \Delta v = v_\text{e} \ln \frac {m_0} {m_1}     and    v_\text{e} = g_0 I_\text{sp},

or

Delta-V = Isp g ln( Minitial / Mend )

or

Mend = Minitial / e Delta-V / (Isp · g)

 

Link to comment
Share on other sites

4 hours ago, Redjoker said:

Actually you could use the rocket equation and solve for the final mass and subtract that from the initial mass to find out how much fuel you used. You could then divide the difference by your fuel usage.

That is the correct way of doing it. You can also integrate fuel usage using similar technique to deriving rocket formula in the first place, and you will get the same answer.

Link to comment
Share on other sites

Yes i have:

  • ISP= 270s
  • Fuel consumption= 63,73kg/s
  • TWR=2,49(5,93)
  • Total Burn Time=62,75s
  • Max deltaV=2295,5m/s
  • Thrust = 200kN
  • Mf = 6900kg
  • Me= 2900kg
  • so Fuel mass is = 4000kg

If I need to get a deltaV of 117m/s how much time I have to burn the engine

I only need the equation.

Link to comment
Share on other sites

This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...