the vertical launch approach

[darthgently]

No feathers ruffled here.  Just honestly don't understand your statement

[PakledHostage]

Me neither. They're two different things (Oberth effect and vis-viva). @kerbiloid makes a good point with reference to the vis-viva equation,  but it neglects the reality that we don't have infinite thrust to instantaneously make orbital adjustments and that there's an atmosphere.  Somewhere there's an optimal solution. I am very skeptical that going straight up is it.

[farmerben]

I now think that, if you can accelerate fast enough the vertical approach can save dV.  The key thing is to reach escape velocity quickly which is the same as saying below a certain altitude. 

[darthgently]

24 minutes ago, PakledHostage said:

Me neither. They're two different things (Oberth effect and vis-viva). @kerbiloid makes a good point with reference to the vis-viva equation,  but it neglects the reality that we don't have infinite thrust to instantaneously make orbital adjustments and that there's an atmosphere.  Somewhere there's an optimal solution. I am very skeptical that going straight up is it.

I have trouble comprehending the Oberth effect without seeing it through a vis viva lens.  Different strokes I suppose.  Well not so much Oberth per se but the fact that prograde burns at PE raise AP most efficiently 

Edited March 23 by darthgently
clarify

[jimmymcgoochie]

6 hours ago, darthgently said:

1. Still seems to only be relevant if your goal is local orbit as the DV req'd for that horizontal velocity isn't free either and keeps you in the atmo longer.  Not sure the lower velocities make it worth it.  Would depend a lot on design.

Travelling directly against gravity uses 9.81m/s of delta-V for each second you're doing it, just to have a net zero change in velocity. Travelling perpendicular to gravity uses 0m/s of delta-V per second to counter gravity. Atmospheric drag in the upper atmosphere is more of a suggestion than something that actually makes a difference to a rocket launch. On Earth, it's just not worth trying to launch straight up; on Kerbin, the penalties are much lower and escape velocity can be reached much faster so it's not nearly so clear-cut.

I saw someone saying something about direct ascent trajectories earlier- launching to the Moon without stopping in Earth orbit, or launching from the Moon back to Earth without orbiting the Moon first. Launching straight up from the Moon to return to Earth is possible and I've done it myself in RSS/RP-1, and it makes sense because the Moon has low gravity and is tidally locked so landing on the trailing side guarantees you can launch straight up and get a return trajectory (it also allows for unguided stages, saving mass on all prior stages).

Launching from the Earth to the Moon without "orbiting", however, isn't a shoot-straight-up thing: the Soviet Union didn't have a restartable upper stage rocket engine at that time and so had to send their Luna probes straight at the Moon without stopping in Earth orbit first. The US did have a restartable upper stage engine- Agena B- and so could stop in low Earth orbit before performing a lunar transfer burn.

[magnemoe]

On 3/22/2024 at 9:32 PM, kerbiloid said:

First Luna crafts were sent (in)to the Moon without intermediate LEO.
https://en.wikipedia.org/wiki/Luna_1

Luna-16 return rocket has targeted the Earth vertically.

LK-700 had two return options, and the main one was direct vertical trajectory, without LLO.

The intermediate orbit just help to soften the possible ballistic errors, or simplify the transfer, or land on the invisible hemisphere.

It did not enter earth orbit but it did do an gravity turn according to the stamp at least  
Few deep space missions enter orbit first, Apollo was an exception as it was manned. 

It makes sense to do an gravity turn, you are no longer fighting gravity just getting velocity. 

Now many rockets launch steeper than optimal, yes this is nice for return to launch site but atlas 5 also does this but here its more about giving the centaur upper stage time to circulate. Its why they need two engines for the starliner, astronauts would come down to steep if rocket failed around separation. 
Same is common for GTO launches, your Ap is above GEO so the satelite has time to circulate. 

[kerbiloid]

11 minutes ago, magnemoe said:

It did not enter earth orbit but it did do an gravity turn according to the stamp at least  

Everything but Luna-16 return rocket needs some kind of gravity turn, as the target is rarely right above head.

(For Luna-16 rocket it was. So, its control system was keeping it straight vertical).

Edited March 24 by kerbiloid

[Hotel26]

On 3/24/2024 at 3:32 AM, jimmymcgoochie said:

Launching straight up from the Moon Mun to return to Earth Kerbin is possible

Every now and again in this forum: paydirt!  Thanks for the above insight.  (SFC Rad Out)

137.4E on the Mun.  And I happen to already have an 8 drill-rig combine ("Snowflake") right at 139E on a pad with high-grade ore.

It's a small site now, but the next thing that happens is a swarm of bulldozers will be landed followed by everything else required to build a major spaceport.

Spoiler

For vehicles already in LMO, I already have a landmark/base called "Express" at 18W to mark the accleration point (+230 m/s dV) for return to Kerbin.

 

Edited March 25 by Hotel26

[magnemoe]

14 hours ago, kerbiloid said:

Everything but Luna-16 return rocket needs some kind of gravity turn, as the target is rarely right above head.

(For Luna-16 rocket it was. So, its control system was keeping it straight vertical).

Its the Moon, and yes an gravity turn would save some dV but this was simpler and could probably use more reliable software, I assume other option was lunar orbit and then an return burn. 

[kerbiloid]

6 hours ago, magnemoe said:

I assume other option was lunar orbit and then an return burn. 

Luna-16 didn't have other option. It was barely able to quit the lunar well vertically, moving towards the Earth in zenith.
And it was able to do that only from the center of the Moon visible hemisphere.

Thus, it didn't have a full-featured control system, only local vertical, fixed (zero) vertical angle automate, and acceleration integrator to cut off the engine before the Earth turns with a non-USSR part of surface on its coming re-entry.

Edited March 25 by kerbiloid

[farmerben]

Isn't it better to do vertical launches from the day-night terminator?

[sevenperforce]

On 3/22/2024 at 4:32 PM, kerbiloid said:

First Luna crafts were sent (in)to the Moon without intermediate LEO.
https://en.wikipedia.org/wiki/Luna_1

"Without intermediate LEO" doesn't mean that the rocket was launched vertically straight up all the way to the moon. The Luna 8K72 three-stage rocket (2.5 stages by US accounting) still executed a normal gravity turn, etc.; it just didn't stop off in a parking orbit. The Blok E upper stage burned continuously from stage separation all the way through to translunar injection. The RD-0109 engine most likely used a pyrotechnic igniter like the RD-0110 that came after it, so it was not designed to be restarted for that mission, requiring a single continuous burn.

You never burn straight up all the way.

On 3/22/2024 at 4:32 PM, farmerben said:

If you have TWR of 1.5 or so, then the vertical approach is way less efficient.  With TWR greater than 3 the amount wasted is very small.  

With T/W ratio of more than 3, you begin losing substantial amounts of Δv to aerodynamic drag. I don't believe there is any situation where thrust to weight balance makes a vertical launch more efficient. You're not really "losing" anything at all by entering a stable orbit first, except perhaps a very tiny amount of Oberth advantage.

On 3/23/2024 at 8:30 AM, darthgently said:

the DV req'd for that horizontal velocity isn't free either and keeps you in the atmo longer

This is the wrong way of thinking about it. While the Δv required for horizontal velocity isn't free, it's not wasted, either. The Δv you spend getting to orbit is preserved as your orbital velocity, which is then added to your eventual injection burn.

The only thing that is "wasted" is thrust spend pointing at the ground, resisting gravity.

Gravity turns are designed specifically to start the pitchover such that drag will be minimized. MaxQ happens very early in flight; drag after MaxQ is negligible. So "keeps you in the atmo longer" is not an issue.

On 3/23/2024 at 9:23 AM, farmerben said:

The inclination difference is no big deal.  It's easy to adjust inclination after leaving the atmosphere.

Not really, no. You want to change your inclination while moving at the lowest possible velocity. When you choose a particular launch direction, you are picking your inclination at a velocity of zero, which is optimal.

Burning out of the atmosphere and then changing directions is a huge waste of Δv.

On 3/23/2024 at 9:23 AM, farmerben said:

 Nobody claimed the vertical approach would save dV.  But if the difference is not significant, what advantages does it have?

The difference is significant.

On 3/23/2024 at 9:23 AM, farmerben said:

One of the big advantages in game is you can have oversized payloads without a fairing.  The aerodynamics hurt you less.  

If you are going fast enough to make up for the gravity drag losses, you will run into aerodynamics issues much sooner.

[kerbiloid]

5 minutes ago, sevenperforce said:

"Without intermediate LEO" doesn't mean that the rocket was launched vertically straight up all the way to the moon

Not "to", but "from". It's about the mini-rocket on top of the lunar lander (lunder?).

Got it. As I said before, unless you are aiming zenith, any rocket follows a curve, more or less vertical.

Edited March 25 by kerbiloid

[Superfluous J]

11 hours ago, farmerben said:

Isn't it better to do vertical launches from the day-night terminator?

Near it, yes.

I don't think you should do vertical launches unless you don't care about efficiency, but if you do it anyway, the most efficient way to do it to go interplanetary is to launch straight up just before sunrise to go to outer planets, or just before sunset to go to inner planets.

"just before" is to account for the rotation of the planet pushing you "up" or "down" (or "left" or "right" depending on how you look at it) and get you aimed as close to exactly pro- or retro-grade from Kerbin after you take off.

[PakledHostage]

5 hours ago, Superfluous J said:

Near it, yes.

You can do that from orbit, too. If you use a circular orbit, it's relatively easy to figure out how many seconds after sunset or sunrise you need to wait before doing your ejection burn, so that your escape trajectory out of Kerbin's SOI is parallel with Kerbin's orbital velocity vector.  I once used that technique to set up a flyby of Duna with a return to Kerbin on a single orbit. 

Edit: I also used it to achieve a gravity assist off the Mun, to reach Duna with minimal delta-V. I reached Duna from LKO using about 80 m/s less than the minimum predicted by the delta-v charts. I had to carefully time my burn to flyby the Mun and have it bend my trajectory just right to reach Duna. I recall that was complicated to set up, although they had at least added maneuver nodes to the game by then so I had that tool available to help make it work.

Edited March 26 by PakledHostage

[magnemoe]

4 hours ago, PakledHostage said:

You can do that from orbit, too. If you use a circular orbit, it's relatively easy to figure out how many seconds after sunset or sunrise you need to wait before doing your ejection burn, so that your escape trajectory out of Kerbin's SOI is parallel with Kerbin's orbital velocity vector.  I once used that technique to set up a flyby of Duna with a return to Kerbin on a single orbit. 

Edit: I also used it to achieve a gravity assist off the Mun, to reach Duna with minimal delta-V. I reached Duna from LKO using about 80 m/s less than the minimum predicted by the delta-v charts. I had to carefully time my burn to flyby the Mun and have it bend my trajectory just right to reach Duna. I recall that was complicated to set up, although they had at least added maneuver nodes to the game by then so I had that tool available to help make it work.

This should work, Mun is known to kick stuff out of Kerbin SOI, it can even capture asteroids in KSP 1  Happen in real life with the moon too. An Apollo 3rd stage was first kicked out by the moon, then captured 30 years later, spent some orbits around earth and got kicked out again. 

[mikegarrison]

Launching straight up from the Mun to a return at Kerbin is totally doable. If you are on the equator at the trailing terminator, and you launch perfectly straight up, you can simply make sure you are at 0 velocity relative to Kerbin as you leave the Mun's SOI. That will let you just drop straight down the well to Kerbin.

However, it's not the most efficient way to do it, because you fight the Mun's gravity the entire time. If you choose a path that spirals around the Mun until it reaches the SOI boundary point with the same 0 speed relative to Kerbin, it takes more time but uses less fuel.

Edited March 28 by mikegarrison

[darthgently]

1 hour ago, mikegarrison said:

Launching straight up from the Mun to a return at Kerbin is totally doable. If you are on the equator at the trailing terminator, and you launch perfectly straight up, you can simply make sure you are at 0 velocity relative to Kerbin as you leave the Mun's SOI. That will let you just drop straight down the well to Kerbin.

However, it's not the most efficient way to do it, because you fight the Mun's gravity the entire time. If you choose a path that spirals around the Mun until it reaches the SOI boundary point with the same 0 speed relative to Kerbin, it takes more time but uses less fuel.

The operative word is time though.  The spiralling makes sense at normal or low TWR where time fighting gravity would pile up.  But at some point as the design TWR increases the time spent fighting gravity will drop and reach a break even point.  What am I missing?

[farmerben]

I just tested with 50 TWR vessel on Minimus.  Going straight up uses 248 dV to escape the SOI.  Changing to a near horizontal trajectory and burning in a straight line uses almost the same amount of dV.

However, throttling down and taking a gradual spiral used 279 dV.

Edited March 28 by farmerben

[sevenperforce]

19 hours ago, farmerben said:

I just tested with 50 TWR vessel on Minimus.  Going straight up uses 248 dV to escape the SOI.  Changing to a near horizontal trajectory and burning in a straight line uses almost the same amount of dV.

However, throttling down and taking a gradual spiral used 279 dV.

Once you are past escape velocity, you no longer need to worry about gravity turns. Minmus has such low gravity that with substantial T/W you can achieve escape velocity within the first few seconds of launch, obviating the need for a gravity turn.

Note here that you are still achieving an orbit; it's just an orbit with a periapsis under the surface of Minmus.

[magnemoe]

20 hours ago, farmerben said:

I just tested with 50 TWR vessel on Minimus.  Going straight up uses 248 dV to escape the SOI.  Changing to a near horizontal trajectory and burning in a straight line uses almost the same amount of dV.

However, throttling down and taking a gradual spiral used 279 dV.

Yes back in KSP 1 I found that adding chemical engines on my tankers helped, with nuclear they used to long getting up to an speed there I could turn over, still with an TWR of less than 2 you can tip over very fast and mostly gain velocity.