Difference between revisions of "Tutorial:Advanced Rocket Design/fr"

Revision as of 08:04, 23 April 2014

Page originale Par Vincent McConnell et Kosmo-not, traduite par Maneth. Traduction non finie.

Introduction:

Apprendre à construire une fusée basique pour un jeu de simulation spatiale comme Kerbal Space Program peut être extrêmement important pour le succès de la construction de fusées souhaitées dans le cadre d'une mission précise. Dans ce guide, nous allons couvrir des sujets tels que calculer du Delta-V de notre vaisseau, expliquer comment réussir un transfert de manoeuvre, obtenir le rapport de poussée sur poids, calculer les poussées de force gravitation (G-force) durant une séquence d'accélération particulière, ou encore calculer le Delta-V nécessaire pour un transfert Hohmann complet et beaucoup plus encore.

Delta-V

${\displaystyle \Delta v}$ (change in velocity) is the bread and butter of rocket science. It is probably the most important thing to know about your rocket because it determines what your rocket is capable of achieving. Among the several things we will explain in this basic tutorial, ${\displaystyle \Delta v}$ is most likely the most useful thing you will apply to Kerbal Space Program while building a rocket. To find the ${\displaystyle \Delta v}$ of your rocket for each stage at a time we have to sum up the part masses of every single part of the stage.

• Total mass: ${\displaystyle m_{\text{total}}}$
• Fuel mass: ${\displaystyle m_{\text{fuel}}}$
• Dry Mass: ${\displaystyle m_{\text{dry}}=m_{\text{total}}-m_{\text{fuel}}}$

The equation only needs the total and dry mass, but as it is easier to get the fuel mass we calculate the dry mass by subtracting the fuel mass from the total mass. Of course other combinations like calculating the total mass and measuring the fuel and dry mass are also possible.

The next important part of this set of calculations is to find your engine's specific impulse. Specific impulse is a measure of how fuel efficient an engine is (the greater the specific impulse, the more fuel efficient it is). For example, the non-vectoring stock engine LV-T30 has a vacuum specific impulse of 370 s. So here, we must apply the Tsiolkovsky rocket equation. More informally known as "The Rocket Equation".

It states:

${\displaystyle \Delta v=I_{sp}\cdot \ln \left({\frac {m_{\text{total}}}{m_{\text{dry}}}}\right)}$

If the specific impulse is given in seconds it is necessary to multiply this value by ${\displaystyle 9.82{\frac {m}{s^{2}}}}$ (see also Terminology about I_sp).

So go ahead and sum up your stage's total mass with fuel. Then, go ahead and sum up the mass minus the fuel (this can be done by just adding up the 'dry mass' where given). Input these into the equation in the place of ${\displaystyle m_{\text{total}}}$ and ${\displaystyle m_{\text{dry}}}$. Following is a quick example, where the surface gravity of Earth ${\displaystyle 9.81{\frac {m}{s^{2}}}}$ is used:

Example rocket

Stage 3 (TMI, Mun lander, Return)
Full mass:	${\displaystyle 3.72t}$
Dry mass:	${\displaystyle 1.72t}$
Isp:	${\displaystyle 400s}$
Δv:	${\displaystyle 3027.0{\frac {m}{s}}}$
Stage 2 (Kerbin orbit insertion)
Full mass:	${\displaystyle 7.27t}$
Dry mass:	${\displaystyle 5.27t}$
Isp:	${\displaystyle 370s}$
Δv:	${\displaystyle 1167.8{\frac {m}{s}}}$
Stage 1 (Ascent):
Full mass:	${\displaystyle 38.52t}$
Dry mass:	${\displaystyle 14.52t}$
Isp:	${\displaystyle 350s}$ (estimated due to atmospheric flight)
Δv:	${\displaystyle 3349.9{\frac {m}{s}}}$
Total
Δv:	${\displaystyle 7544.6{\frac {m}{s}}}$

Multiple engines

To calculate the I_sp for multiple engines with different I_sp values, you need to find total thrust and mass flow:

${\displaystyle I_{sp_{avg}}={\frac {\sum \limits _{i}^{n}(thrust_{i})}{\sum \limits _{i}^{n}({\dot {m}}_{i}\cdot g_{0})}}={\frac {\sum \limits _{i}^{n}(thrust_{i})}{\sum \limits _{i}^{n}\left({\frac {thrust_{i}}{I_{sp_{i}}}}\right)}}={\frac {thrust_{1}+thrust_{2}+\dots +thrust_{n}}{thrust_{1}\div I_{sp_{1}}+thrust_{2}\div I_{sp_{2}}+\dots +thrust_{n}\div I_{sp_{n}}}}}$

This will give you the correct I_sp to use for your Δv calculation. If all engines are the same, they act as one engine in this calculation so the sums aren't needed.

Calculating fuel flow

Next, we will explain how to calculate fuel flow in mass to see how much fuel a burn uses up in a specific amount of time.

If we know the ${\displaystyle \Delta v}$ needed for the burn and the total mass

of the rocket before the burn, we can calculate how much fuel is 

required to complete the burn.

First, we calculate the mass of the rocket after the burn is complete. To do this, we use the Tsiolkovsky Rocket Equation, inputting the initial mass and ${\displaystyle \Delta v}$ of the burn. We can then solve the equation for the final mass after the burn. The difference between these

two masses will be used to determine the length of time that is needed 

to complete the burn.

The equation for mass flow rate of fuel, given ${\displaystyle I_{sp}}$ and thrust, is:
${\displaystyle {\dot {m}}={\frac {\frac {thrust}{9.81{\frac {m}{s^{2}}}}}{I_{sp}}}}$
where ${\displaystyle {\dot {m}}}$ is the mass flow rate of fuel consumed (in seconds)

Dividing the difference between initial mass and final mass for the burn by the mass flow rate of fuel, we arrive at how many seconds are required.

Note: The

mass flow rate of fuel can be converted into the consumption rate of 

the fuel units used in KSP (Liters, I presume). The conversion ratio is 1

mass unit per 200l of fuel.

Orbital velocity

Rather easy is the formula to calculate the orbital velocity of an orbit. This assumes circular orbit or the velocity of a specific point in an orbit. For this, we simply do this calculation:

${\displaystyle {\sqrt {\frac {\mu }{r}}}}$

Where:
${\displaystyle \mu }$

= Gravitational Parameter of parent body. (km³/s²)

${\displaystyle r}$

= radius of orbit. (km)

If we input the radius of the orbit in Kilometers, our orbital velocity will come out in Kilometers per second. In a 100km orbit, our radius will be 700km. Meaning our velocity will be ~2.2458 kilometers per second (km/s), or 2245.8 m/s.

Delta-v map

A ${\displaystyle \Delta v}$ map consists of approximate amounts of ${\displaystyle \Delta v}$ needed to get from one place (whether it is on the ground or in space) to another. The ${\displaystyle \Delta v}$ values we have for our ${\displaystyle \Delta v}$ map are approximate and include a fudge factor (in case we slip up on our piloting). Our map is as follows:

from Mun and return to Kerbin: || 1000 m/s

If

we design our rockets to have 7600 total ${\displaystyle \Delta v}$, and the acceleration of the launch stages are adequate, 

we can have confidence that our rocket is able to land on the Mun and return to Kerbin. A rocket with a little less ${\displaystyle \Delta v}$ can accomplish this goal, but it is less forgiving of less efficient piloting.

==Thrust to Weight Ratio== Calculating Thrust to Weight Ratio is only three very simple steps.

It is important to know the thrust to weight ratio of your rocket to ensure your rocket will actually liftoff.

If your TWR is less than 1, you can bet that you won t make an inch in 

altitude when starting from the launch pad. The minimum optimal TWR to have for your rocket at launch is 2.2.

The formula for this is simply the thrust of all of your current stage engines divided by the weight (mass * 9.81 m/s²) of your ship, fully fueled. At the same

time, this will give you the minimum G-force you can expect on the 

current stage. Your peak G-force will occur instantly before fuel depletion. The way to calculate this is to simply divide thrust by the dry mass of your stage+the fully fueled stages above it.

In

conclusion: This guide will hopefully have helped with designing your 

rockets to allow you to get the job done -- whatever it may be -- with no test flights first. We hope this guide has been helpful to new and continuing KSP pilots alike.