Best way to launch from a pole to an equatorial orbit?

[uncle_jew]

Hello,

assuming my spacecraft is sitting on a (North) pole, and I need to launch it into an equatorial orbit. How to do it with the least delta-v needed?

I can obviously simply launch it on a suborbital trajectory towards the equator, then do a big burn to change the direction by 90 degrees and to reach the orbital speed, but this requires almost twice the delta-v needed to launch from the equator.

Are there any significantly cheaper ways of achieving the same goal?

[Lelitu]

Nothing really good.

About the best option is to launch into a *very* eccentric orbit, with the apoapsis almost on the soi, and at an/dn. Then a small burn will complete the plane change. But that leaves you in a very eccentric equatorial orbit, and it's usually quite expensive to get that extreme eccentricity, since it takes nearly enough deltav to escape.

Your best plan was scuppered before you landed. If you absolutely have to launch into an equatorial orbit, for cheap. Land on the equator, it's the only good option.

[Spricigo]

 

Basically you can't. To launch into a given orbit you need to be below it at launch time. Which means the target orbit need to be inclined enough to reach the launch site's latitude. Which means launching from the pole the orbit will be always have 90° inclination. If you are at the north pole the only direction you can go to is south and vice-versa.

The only way is a plane change, either during the launch (which means lots of cosine loss due change in direction of thrust) or after orbit acquired (which means a inefficient burn perpendicular to the velocity).

 

 

[CSE]

Instead of a single big dogleg turn at the equator, try making one long gradual turn: launch south, obviously, then after the first stage is away, hold a steady five to ten degrees east of prograde, straightening up for staging events if necessary. See what angle you meet the equator at.

[bewing]

If there is anything at all interesting on the equator, you really don't lose anything by doing a suborbital hop to any spot on the equator, and launching again.

[Pand5461]

Well, if you're launching from Kerbin/Duna, you may use gravity assist from Mun/Ike to change inclination. If you are VERY careful, you may even manage to get multiple encounters. This should save a few hundreds m/s.

Otherwise, the advice from @Lelitu is probably the best you can get.

[uncle_jew]

6 minutes ago, bewing said:

If there is anything at all interesting on the equator, you really don't lose anything by doing a suborbital hop to any spot on the equator, and launching again.

Wouldn't you lose even more delta v this way? Because when landing on the equator, I would have to do a retro burn.

But when changing the direction by 90 degrees at the equator without stopping, I would do a burn at like 45 degree angle, killing the latitudal velocity and building up the velocity along the new orbit at the same time.

[bewing]

Yes, but orbital velocity is a fair bit higher than what you need for a suborbital hop. You need approximately your orbital velocity (for liftoff) + your orbital velocity / root 2 (to do the plane change) to do it all in one shot. You need suborbital velocity (for liftoff) + suborbital velocity (for landing at the equator) + orbital velocity (for equatorial liftoff) to do it with a hop. So you get a low-cost landing at the equator into the deal.

 

Edited May 15, 2017 by bewing

[FancyMouse]

1 hour ago, bewing said:

suborbital velocity (for liftoff) + suborbital velocity (for landing at the equator)

From my experience, if the hop distance is comparable to the size of the body, each suborbital is going to be close to a full orbital velocity. Do you really have the numbers to show it's going to save fuel? I feel really skeptical that you can do each hop within 0.707*orbital velocity to cover the distance between pole and equator.

(and you said /root2 - you probably meant *sqrt2)

[bewing]

I didn't say it was going to save fuel. I was saying that you get an extra landing spot to take another set of readings at a discount to the full dV price that it would normally cost. It costs a little more than doing a plane change, but not much.

 

[Spricigo]

I think is worth asking:

Where You are?

What is the deltaV budget available?

Why the equatorial orbit is necessary?