whos's been stealing my delta v?

[tomthenose]

hi folks, i'm making a couple of addons just now and i'm trying to work out the delta v ranges of various craft but i never seem to have the same delta v in game as i'd projected. for example:

name: cargo longboat
drymass+payload: 44800kg
wetmass:242764kg
ISP:9993
thrust:810000N

so i thought that would give me:

mass ratio: wetmass/drymass=5.42
delta v range:165608

This should be heaps of delta v for a mars transfer, i thought i'd have fuel for the return Journey too, and yet i burn nearly all my fuel getting to mars at a delta v of about 5500

can anyone help me understand this delta v black hole i have? i'm not sure if i'm doing the maths wrong or failing to take something into account?

am i correct that thrust doesn't effect the total delta V just how quickly you can acheive it?

 

[Urwumpe]

I only get a specific impulse of 16889.13 m/s, when I calculate with your numbers. Remember that Orbiter uses the specific impulse in SI units: N * s / kg (m/s) not seconds, like some lonely backward country does.

Next, you can have gravity losses, control losses, plane changes, etc. Do you take off from Earth? that alone means about 9200 m/s go away for leaving the planet, 9200 + 5500 m/s is almost what you have.

 

[tomthenose]

Aah, so thats ISP* ln(mass ratio)?

The transfer was orbit to orbit, i was reading about gravity losses and the like but doesn't it take them into account in IMFD? I am leaving from quite a low orbit so should i be combining the delta v requirement from the 'orbit eject' program with the delta v from the target intercept program? To my untrained eye 16889.13 would be enough for mars and back?

I'm also not sure if its something i've missed in the config:

[CONFIG]
MESHNAME="tomspace/ts_CARGOlongboat"
SIZE=37
EMPTY_MASS=32800
FUEL_MASS=197964
MAIN_THRUST=810000
RETRO_THRUST=36000
ATTITUDE_THRUST=18000
ISP=9993
TRIM=0.05
PMI=(242,258,34)
CW_Z_POS=8
CW_Z_NEG=8
CW_X=3
CW_Y=3
COG=1
CROSS_SECTION=(230,240,84)
PITCH_MOMENT_SCALE=0.00001
BANK_MOMENT_SCALE=0.00001
ROT_DRAG=(0.1,0.1,0,1)

My understanding of exactly how alot of this effects performance is a little hazy, i'm doing alot of reading on this stuff but it takes a while to sink in!

 

[Loru]

How much your trajectory differs from perfect hohmann? How Are your planes perfectly aligned? "Out of a hat" trajectory can eat a lot of dV

---------- Post added at 03:00 PM ---------- Previous post was at 02:40 PM ----------

basic rocket equation gives me this (I've taken drymass+payload as empty mas in calculations):

so clearly your trajectory is far from optimal or your payload mass may be off.

 

[dgatsoulis]

Your problem is not the delta V, but the ship's poor acceleration.

You mentioned a burn of 5.5 km/s to get to Mars. (BTW, that's a bit high. You should be able to get to Mars wth less than 4 km/s from LEO).

If we know the ship's total mass and thrust, we can calculate the burn-time:

[math] T_{burn}=\frac{{m_0} \cdot\ (1-e^{-\frac{\Delta V}{u_e}})\cdot u_e}{F_{thrust}} [/math]
So we have m0 = 242764 kg (dry+payload+fuel), ΔV=5.5 km/s, Ve = 9993 m/s and F_thrust = 810000 N

That gives us a burn-time of 1267,7 seconds.

In LEO the orbit has a period T of about 90 minutes. If you need almost a quarter of an orbit to complete the burn, you will have huge losses.

There is nothing wrong with your ship, just the acceleration.
You can see that at the start of the burn you have an acceleration of a = F/m = 810000/242764 = 3.34 m/s² (0.34g)

Try to perform the burn, using multiple periapsis kicks.

EDIT: @Loru
What if the Mars transfer is not optimized for dV, but for Time Of Flight? Sometimes the transfer doesn't need to be a Hohmann one.

 

[tomthenose]

I attempted some more mars transfers yesterday but couldn't find a window for under 4000 m/s, I used the trajectory optimisation tool to get my window on the 4th of April 2033. With extra fiddling on imfd i got it down to about 6000-7000 m/s. I was starting from only 200km above earth so theres alot of work to do just getting away and i'm reading again about aligning myself to mars on the ecliptic plane as i think i'm getting that wrong.

thanks for your patience! I'm kinda hooked on the mechanics of all this but i'm ashamed to admit its taking a while to get used to all the maths i've forgotten since school. Orbiter is an awesome educational tool!

 

[dgatsoulis]

For the same year, I am getting much better results with a departure on April 29th 2033 and an arrival on January 28th 2034.

The "outbound" (oV, aka heliocentric transfer dV ) is 2775 m/s and the "inbound" (iV, aka heliocentric encounter velocity) is 4376 m/s.

Even if you don't open the Orbit-Eject program, you can easily calculate the TMI burn ΔV. We know the transfer ΔV (ΔVtr=2275 m/s) and the altitude of the parking orbit (200km).

First let's find the escape velocity of the parking orbit:

[math] V_{esc} = \sqrt{\frac{2\cdot G\cdot M}{R+alt}} = 11014.58 \; m/s[/math]
Now let's find the Injection Velocity:

[math] V_{inj} = \sqrt{\Delta V_{tr}^2+V_{esc}^2} = 11358.77 \;m/s [/math]
To find the Injection Delta V, we simply subtract the orbital velocity of the parking orbit:

[math] V_{orb} = \frac{V_{esc}}{\sqrt{2}} = 7788.48\; m/s \;\;\;\;\;\; \Delta V_{inj} = V_{inj}-V_{orb} = 11358.77 - 7788.48 = \textbf{\underline{3570.29\; m/s}} [/math]
If you want to string it all in one equation:

[math] \Delta V_{inj} = \sqrt{\Delta V_{tr}^2+\frac{2\cdot G\cdot M}{R+alt}}-\sqrt{\frac{G\cdot M}{R+alt}}[/math]
Where ΔVtr is the heliocentric transfer delta v (m/s), G is the gravitational constant (m³/kg*s²) , M is the mass of the Earth (kg), R is the radius of the Earth (m) and alt is the altitude of the parking orbit (m).

Of course, IMFD's Orbit-Eject program makes all those calculations for you:

(The small difference comes from the 0.03° R.Inc of my parking orbit, to the hypothetical injection trajectory).

Now that we know the TMI burn dV we can calculate the burn-time for your ship:

[math] T_{burn}=\frac{{242764} \cdot\ (1-e^{-\frac{3571}{9993}})\cdot 9993}{810000}=899.76 \; seconds [/math]
(If you don't want to do the math, you can use BurnTimeCalc MFD)

I'd start ~2.5 days earlier from the date of the Interplanetary Course program and brake the burn into at least 3 periapsis kicks. The first two would be 1500 m/s each, bringing the spacecraft back at the periapsis of the third pass 2.42 days later.

You could even go for 5 kicks of ~700 m/s each. It would add less than half a day.