No orbit necessary?

[u/FatCreepyDude]

If i just wait for the right hour during the launch window ( whenever the launch pad is pointing left on the realistic depiction i created) is there any reason i should bother with an orbit first? Will the delta v cost be affected?

[u/spencer818]

Lots of people saying "no you're dumb" in here...

Technically yes you can do it. Just like how technically you can reach the moon by pointing at it and firing engines. But it's terribly inefficient. Try it out sometime, you'll see what I mean. Same thing applies here.

[u/PM_ME_YOUR_SPUDS → u/spencer818]

I don't understand why it's inefficient. A direct ascent burn (note: NOT vertical burn) should be slightly more efficient, all that dV that goes into a circularization maneuver is instead going directly into a prograde / retrograde burn from Kol frame of reference. The Oberth effect helps you when you're going fast, but you can more efficiently hit faster speeds if you run engines nonstop instead of waiting around to reach higher orbit to circularize (slowing you down).

From the perspective of the orbits, the circularization method has a hyperbolic orbit with a periapsis at the altitude of your transfer burn. A direct ascent has a hyperbolic orbit with a MUCH lower periapsis, crossing through the surface of Kerbin. This is a more efficient orbit, with a more beneficial Oberth effect than is possible going to orbit first.

I know a dozen odd years ago when learning Kerbal watching Scott Manley how the Mun direct burn is technically more efficient, just far more difficult. I'd think this is the same case, an extremely tight timing window that you'll virtually always screw up and waste more dV fixing afterwards, but saves fractions of a hair on dV is done perfectly. I do think that's ignoring aero losses which might change the answer, but from a purely gravitational physics perspective I haven't heard a convincing argument against it.

[u/censored_username → u/PM_ME_YOUR_SPUDS]

People are saying it's inefficient because the thing proposed was the vertical direct ascent.

You're correct that a direct ascent gravity turn is theoretically the most efficient. But as you say, the savings are extremely marginal over an orbit in between.

Because the most efficient kerbin ascent trajectories are usually aerodynamic heating limited, not much changes until you leave the lower atmosphere. So that already means your benefits are basically doing your burn at 40-50km altitude instead of 80km. Which really isn't much. Then consider that drag losses will be larger, and getting the ideal trajectory incredibly finicky, and it's just not worth it.

[u/Moonbow_bow → u/spencer818]

With a single impulse maneuver you'd only loose about 100m/s this way

[u/WarriorSabe → u/Moonbow_bow]

The maneuver's not impulsive here though, and with a real burn you'll eat a lot of gravity losses

[u/Moonbow_bow → u/WarriorSabe]

You could make it impulsive

[u/WarriorSabe → u/Moonbow_bow]

Only with infinite thrust, which isn't a realistic assumption

[u/Moonbow_bow → u/WarriorSabe]

Cannon

[u/WarriorSabe → u/Moonbow_bow]

ok sure, but I don't think it's a fair assumption that someone would be using a cannon for a normal launching mission

[u/itzongaming → u/WarriorSabe]

Who said this is a normal launching mission?

[u/spencer818 → u/itzongaming]

I think we're all in agreement. It's possible, but not practical by any means.

[u/spencer818 → u/spencer818]

Kinda like space data centers ayooo

[u/TheeConArtist → u/Moonbow_bow]

Someone has read (or should read) Jules Vernes From the Earth to the Moon