# General QuestionWhat is inward dV and outward dV?

#### Cris

##### New member
Hello,

Was looking at IMFD's Target Intercept program. I see it gives out dV and iV. What is inward and outward velocity, exactly? I can't seem to find this info anywhere.

Thanks.

#### Mojave

##### Wilding the West
Moderator
Have you ever heard of the hokey pokey? You put your dV in, you put your dV out, you put your dV in and you slingshot all about

- - -

But in all seriousness, I believe those values relate to your dV required for the journey, and the dV you need at intercept to make it into a stable orbit at the target. I'm going off fuzzy memory, so I apologize in advance if I'm a little off.

#### Arvil

##### Member
The direction of travel in an orbit is prograde, the opposite direction is retrograde, orbit normal+ is perpendicular to the orbital plane toward the north, normal- is toward the south. If you point your ship directly toward the center of the earth and burn, that would be inward velocity, directly away would be outward velocity. So, when you set up a burn to leave, let's say earth orbit to go to mars, for example, you will likely do a burn that incorporates vectors in all three axes, pro- or retrograde, plane change (normal+, normal-) and inward/outward. Time is the final adjustment, to burn at the right time. The effect is more or less, prograde will certainly allow you to escape earth's gravity, but in regard to the solar portion of the transfer orbit will make the size of the orbit larger, the plane change vector will change the inclination, and the inward/outward will change the eccentricity. Hope that helps.

#### Cris

##### New member
Thanks both of you for the replies. It does help. Now my follow-up question (which i hoped i wouldn't have to ask) is why does the Target Intercept program in IMFD only burns dV during the eject burn, but not iV? That still counts towards the total. Inward velocity makes sense the way you explained it, Arvil, but looks like when it comes to IMFD the norm doesn't apply. I think Samuel's explanation works a bit better for IMFD, since the iV i was getting at eject time was very similar to the dV i needed to circularize orbit at destination.

#### Mojave

##### Wilding the West
Moderator
I would trust anyone else over me right now as I am super rusty with Orbiter and IMFD at the moment. I was just trying to remember what I've seen. I think @Arvil gave a more complete and correct answer,

#### Arvil

##### Member
I missed the dV part of the question. Outward velocity is oV and inward is iV. If your orbital velocity is 7.5k m/s in LEO, then you need to increase your orbital velocity so it goes to the moon, the difference between the two is delta velocity (dV), change in velocity. So, the calculations used for transfer from surface to orbit, and from one orbit to another is usually given in terms of delta velocity, how much faster you need to get to get there, and then dV, how much you need to slow down to get into orbit at the moon or other body, then how much dV you need to lose to descend to the surface. Then you could calculate from fuel capacities, and ship's mass how much fuel you need. Or just let IMFD or TransX do the math.

#### Cris

##### New member
Ok, i think i figured it out... The manuals i was reading for IMFD are wrong, i think. iV is not inward delta velocity, it's injection (or maybe insertion) delta velocity. I got the hint from this conversation: https://www.orbiter-forum.com/threads/orbiter-planning-vs-trajectory-browser.39648/

Of course Arvil's explanations are on point, the problem is i was asking the wrong question Thanks to both of you for the help.

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#### Arvil

##### Member
KSC to Brighton Beach might look something like this: KSC to LEO, 9 or 10K m/s, LEO to Lunar orbit 4.1k/s, lunar orbit to surface 1.6k m/s. Wikipedia has an article here: https://en.wikipedia.org/wiki/Delta-v_budget

#### Arvil

##### Member
@chris:
iV is not inward delta velocity, it's injection delta velocity.

I didn't realize that. Hadn't read IMFD for a while to remember that as a term.

If you're flying a DG, I think Dr. Martin included some math to calculate from ship's mass and fuel to delta velocity in the Orbiter Manual.

#### Mojave

##### Wilding the West
Moderator
I missed the dV part of the question. Outward velocity is oV and inward is iV. If your orbital velocity is 7.5k m/s in LEO, then you need to increase your orbital velocity so it goes to the moon, the difference between the two is delta velocity (dV), change in velocity. So, the calculations used for transfer from surface to orbit, and from one orbit to another is usually given in terms of delta velocity, how much faster you need to get to get there, and then dV, how much you need to slow down to get into orbit at the moon or other body, then how much dV you need to lose to descend to the surface. Then you could calculate from fuel capacities, and ship's mass how much fuel you need. Or just let IMFD or TransX do the math.

Oh wow, so I actually was kind of right. I'm glad to know my knowledge is not wholly degraded!

#### Arvil

##### Member
It’s nice when one discovers that one is almost smart. lol

#### IDNeon

##### Member
The direction of travel in an orbit is prograde, the opposite direction is retrograde, orbit normal+ is perpendicular to the orbital plane toward the north, normal- is toward the south. If you point your ship directly toward the center of the earth and burn, that would be inward velocity, directly away would be outward velocity. So, when you set up a burn to leave, let's say earth orbit to go to mars, for example, you will likely do a burn that incorporates vectors in all three axes, pro- or retrograde, plane change (normal+, normal-) and inward/outward. Time is the final adjustment, to burn at the right time. The effect is more or less, prograde will certainly allow you to escape earth's gravity, but in regard to the solar portion of the transfer orbit will make the size of the orbit larger, the plane change vector will change the inclination, and the inward/outward will change the eccentricity. Hope that helps.
You have no idea how much this just UNFUZZIED all the things I've been messing around with.

Now I think I an make a moonshot without a computer...but I have no way to manually burn the SIV-B?

#### Arvil

##### Member
You have an SIV-B? Cool. It’ll be kinda expensive to fill, won’t it? lol