Just how much better is a gravity turn than direct ascent?

[Eric S]

19 hours ago, Rodhern said:

Hi Eric, how do you arrive at 1.73g ?

Good question, it is wrong.... OK, I think I got it.  1.73 would be the square root of 3. 1.43 squared plus 1 would be 3, so I counted the 1, not the 0.43 for the vertical component.  Apologies, I haven't been sleeping well for the last two weeks and this is hardly my worst sleep-deprivation induced goof.  I should probably go back to actually drawing this stuff rather than just doing it in my head until I get back to my usual sleep cycle.  Much smaller benefit, but still quite a bit larger than the extra orbital energy required for the higher periapsis.  Unless I botched that math as well, that is.

I've done the math when fully awake in other threads, and the conclusion was the same, just off on the amount.

 

[Norcalplanner]

On 8/26/2016 at 8:06 AM, Stoney3K said:

That was not what I was really wondering about. Why is the orbiter ascent upside down instead of flying SRBs down, cockpit up orientation?

 

Found this on an old "cosmoquest" forum to answer this question:

Two main reasons are usually given for this: 

(1) It optimally orients the vehicle in case of an RTLS (Return To Launch Site) abort.

Oriented "heads down", a simple powered pitch around maneuver is needed to return. Oriented "heads up", a more complicated roll plus pitch maneuver would be required.

(2) It decreases structural loading on the wings vs a "heads up" orientation. Heads down, the vehicle pitch is oriented to a slight negative angle of attack (wind flow relative to wings). Wings generate lift -- the higher speed the more lift. A slight negative angle of attack nulls this out thus lowering wing stress.

There are also less important reasons: it helps with comm coverage (antennas are on top of the orbiter), it provides the crew with a better horizon view, etc.

The vehicle is physically capable of flying a "heads up" profile and in fact that helps payload slightly. However because of the RTLS abort and wing loading issues I doubt they'll ever do that.
 

[GoSlash27]

Did @bewing ever respond in this thread?

[OhioBob]

1 hour ago, GoSlash27 said:

Did @bewing ever respond in this thread?

No

[Plusck]

On 26/08/2016 at 10:05 PM, Stoney3K said:

For an interplanetary transfer or a transfer to the edge of Kerbin's SOI, it doesn't really matter as much anymore. Why bother with estabilishing a parking orbit if you're just going to leave it anyway to escape Kerbin's gravity well altogeher? When you're burning for another planet, you might as well burn straight up at the proper moment.

Absolutely... as long as you have the TWR...

On 27/08/2016 at 0:10 AM, foamyesque said:

 

That's incorrect, because you still save on deltaV and needed TWR by burning horizontally in a direct ascent mode than straight up. This is the exact argument @bewing made in his OP, you realize.

No, he's right. I gave some figures for it on the first page too. And you're also right about "saving TWR" because it's only advantageous with a high TWR.

However, having a low TWR will necessarily cost you in terms of dv - since you're either going to to be fighting gravity for most of your escape burn, or you're going to have to waste dv on circularising at a higher pre-escape-burn orbit. And though you can split the burn over several orbits, you can't split the last post-escape burn (which means that going to Eeloo or Moho is necessarily wasteful of dv if you have a very low TWR).

Still, bewing's assumptions are undoubtedly wrong. I did some tests back in 1.0.4 or 1.0.5, with a launchpad TWR of about 8 or so based on one of bewing's designs, and sure enough it was equally efficient to go straight up as long as you were heading out to Minmus or further. However, simply downsizing the engines gave far better results for an orbital approach, as the first answer in this thread shows.

 

On 27/08/2016 at 1:38 AM, Eric S said:

 

Pythagoras still matters, just not the way you think.  We can all see that the specific orbital energy between two orbits with a Mun-level apoapsis, with one a center-of-Kerbin periapsis and the other a low-Kerbin-orbit periapsis, favors the former but is minimal.  The point of the non-vertical ascent is that it's adding to its orbital energy in a more efficient manner.  It should also be pointed out that the extra energy of the latter orbit isn't entirely wasted as it means that you're closer to matching the Mun's orbital velocity when you get there.

2g worth of vertical thrust yields 1g worth of acceleration (two steps forward, one step back).  2g worth of prograde thrust 45 degrees off of the horizon yields 1.73g worth of acceleration (1.43 steps forward, one step back, and 1.43 steps to the side).  Now, as you increase the TWR, this effect diminishes, but it doesn't go away until you're talking an infinite TWR.  Even at a TWR of 10.0, the vertical ascent is still losing about 10% more acceleration to gravity. 

This doesn't mean that the entire first case launch will be 73% more efficient because even a gravity turn spends some time going close enough to vertical that it doesn't really benefit from this during that time.  In addition, as the craft burns fuel, it's TWR goes up, so even a low-TWR craft should have a higher TWR by the time it has pitched over enough for this to make a significant difference.  On the other hand, as the craft starts reaching orbital velocity, the craft will wind up losing even less acceleration.  But really, given that the specific orbital energy between the two target orbits only differs by about 3%, it doesn't take much to overcome that.

Yes, you can say that you'll just design a craft with a higher TWR, but then you're using a more expensive craft to launch the mission, which is a better method of determining efficiency than pure delta-v.  

But all of this is the theoretical math that got dismissed by the OP, so really, we need to fix the test case and rerun the tests.  As others have mentioned, we need a defined payload to be delivered but treated as inert or at least mostly inert.  If there's a probe core there I don't mind the idea of it controlling the craft, but if it has fuel and/or an engine, neither should be used or altered.

I see where you're coming from, but you're missing a couple of parts of the question. And at 45° its root 2 along each side = 1.414, and the final velocity after deducting gravity is root (2+0.414²) = 1.474g.

2g acceleration at 45° - starting on Kerbin - gives a horizontal velocity of 1.414g = 13.9 m/s². Since you need to attain orbital velocity, you need to burn for about 170 seconds, but in this time your vertical velocity will be 0.414*170*9.81 = 690 m/s. You will have reached an altitude of 58km...

However, burning upwards for those 170 seconds gives a vertical velocity of 1*170*9.81 = 1668 m/s. At an altitude of 283km...

So while there is surely going to be an advantage to burning at 45°, if you plug these altitudes and velocities into the orbital equation to get the semi-major axis, you'll find that SMA for 45° after 170s is 752km while burning straight up gives a SMA of 677km. The difference is not huge - and the top of that vertical "orbit" might actually be higher than the 45° burn.
On the other hand, the figures start getting shaky towards the end of those 170 seconds as Kerbin's curvature starts having an influence on the true angle of the burn. Clearly, if you continue burning for a significant amount of time, the orbital approach will start winning hands-down.

 

Which brings us back to what @Stoney3K was saying. If you are just aiming at getting to a given altitude, burning straight up is much more efficient than most people give it credit. As your example shows, a mere TWR of 2 is enough to gain a higher altitude than the "47% more efficient" 45° burn after 170 seconds. Increase the TWR and you necessarily decrease the burn time, and the vertical approach starts to look very reasonable.

What the vertical burn does not give, though, is any orbital velocity at Ap. However, that "sideways" velocity is increasingly small as the burnout velocity increases, becoming largely irrelevant. Going to Jool, Eeloo or Moho, you might as well consider the orbit to be purely vertical by the time you've finished, and therefore the majority of the escape burn will be - in effect - directed directly against Kerbin's gravity...

So bewing's numbers are not really that bad. What they discount is the fact that lowering the TWR allows more fuel and/or more payload, for less funds, and therefore much greater "efficiency" in general. But it's interesting that the pure dv numbers are actually a lot closer than one might think (intuitively, or, after KSP experience, intuitively after learning counter-intuitivity...).

[foamyesque]

A horizontal burn is a vertical burn, @Plusck. It just doesn't fight gravity while you're burning. It will therefore be more efficient on the same hardware, and it will allow use of more hardware layouts than a vertical manouver does, which can save weight, cost, or both. The only circumstance where an actual straight-up ascent is going to be better is if the cost of the control mechanisms to execute a pivot to the horizontal exceeds that of the additional fuel and engine needed.

That particular case would be very unusual.

 

EDIT: And if you or @Stoney3K think otherwise, feel free to provide a rocket, mission, and your numbers and screenshots for a vertical ascent. I am certain I will be able to beat your numbers.

Edited August 28, 2016 by foamyesque

[Superfluous J]

1 hour ago, Plusck said:

What they discount is the fact that lowering the TWR allows more fuel and/or more payload, for less funds, and therefore much greater "efficiency" in general. But it's interesting that the pure dv numbers are actually a lot closer than one might think

Funny that you put quotes around "efficiency" for funds, implying it's not "real" efficiency, while then implying that optimizing for pure dV *is* real efficiency in your next sentence. This is at best a difference of opinion and a far better case could be made that it's simply flat-out wrong than that it's purely correct.

mk1 pod on top of an orange tank on top of a mainsail has X dV. Radially attaching 6 more orange tanks each with 6 more Mainsails at the bottom adds a smidgen of dV and a wisp more TWR, but adds a MASSIVE cost. Speaking purely by dV numbers, it's better. But I can't think of any objective situation where anybody (but Whackjob) would prefer it.

[GoSlash27]

1 hour ago, 5thHorseman said:

Funny that you put quotes around "efficiency" for funds, implying it's not "real" efficiency, while then implying that optimizing for pure dV *is* real efficiency in your next sentence. This is at best a difference of opinion and a far better case could be made that it's simply flat-out wrong than that it's purely correct.

 Agreed (not harping on Plusck, but the concept). Minimal DV expended is an excellent way to track the relative efficiencies of maneuvers, but not spacecraft. Minimum DV does not mean minimum fuel expended, minimum stage mass, or minimum launch cost.
 It's an easy metric to track, but it's the wrong metric. Kinda like the drunk guy losing his keys in the alley and then searching for them around the lamppost across the street because the light is better over there.

Best,
-Slashy

[Eric S]

4 hours ago, Plusck said:

So while there is surely going to be an advantage to burning at 45°, if you plug these altitudes and velocities into the orbital equation to get the semi-major axis, you'll find that SMA for 45° after 170s is 752km while burning straight up gives a SMA of 677km. The difference is not huge - and the top of that vertical "orbit" might actually be higher than the 45° burn.

 

 

The difference in SMA between a 100km altitude periapsis (gravity turn) and a center of Kerbin periapsis (vertical ascent) is 5.8% in the case of a Mun-altitude apoapsis, so if the gravity turn yields an 11% greater SMA, it is more efficient. Also note that the farther the gravity turn pitches over, the more efficient it becomes, as long as you're not drastically increasing losses from aerodynamic drag.  Even ignoring centrifugal force, 45 degrees isn't the most efficient angle, I just gave those numbers because it was easier to visualize (and I got it wrong anyway). At 30 degrees above the horizon, you cancel out 1g of gravity and get 1.73g (2g * cos(30 degrees)) of horizontal acceleration.  Of course, if you try to launch that far over right away, you get into the case of drastically increasing your aerodynamic drag.

Also note that the higher the apoapsis, the smaller the difference in the SMA the LKO periapsis makes, so leaving the SoI doesn't change this.

There are probably people that overstate the difference.  I've seen people claim a 40% advantage overall, but I'd actually be surprised if the difference is that great for craft with a reasonable TWR.  But it is is there.

[maccollo]

36 minutes ago, Eric S said:

There are probably people that overstate the difference.  I've seen people claim a 40% advantage overall, but I'd actually be surprised if the difference is that great for craft with a reasonable TWR.  But it is is there.

Just tested one of my designs, which has a high thrust first stage followed by a low TWR upper stage, and it looses 60% of it's payload capacity to Kerbin escape velocity.

Edited August 29, 2016 by maccollo

[Plusck]

3 hours ago, foamyesque said:

A horizontal burn is a vertical burn, @Plusck. It just doesn't fight gravity while you're burning. It will therefore be more efficient on the same hardware, and it will allow use of more hardware layouts than a vertical manouver does, which can save weight, cost, or both. The only circumstance where an actual straight-up ascent is going to be better is if the cost of the control mechanisms to execute a pivot to the horizontal exceeds that of the additional fuel and engine needed.

That particular case would be very unusual.

 

EDIT: And if you or @Stoney3K think otherwise, feel free to provide a rocket, mission, and your numbers and screenshots for a vertical ascent. I am certain I will be able to beat your numbers.

I don't think you're reading what I'm writing, but rather taking the gist of it and objecting on principle.

I said "However, simply downsizing the engines gave far better results for an orbital approach, as the first answer in this thread shows."
The first answer in this thread is your answer.

I also insisted on the question of TWR. And I said "Clearly, if you continue burning for a significant amount of time, the orbital approach will start winning hands-down.".

And in my answer to you I said "you're also right about 'saving TWR' because it's only advantageous with a high TWR".

 

So, I played around in a freshish sandbox game. It's here: Two TWR>8 ships on Tylo
Turns out that from the surface of Tylo (3km altitude) you can escape the SOI with a dv of 3004m/s. The ship with a Mainsail can do it (action group 8 toggles gimbal on that ship, but not on the other one, if you feel like trying). Took a few tries though...

It takes 30s of burn to do so. 30s times Tylo's gravity is about 235 m/s the gravity losses suffered by vertical ascent). The second ship (Mammoth) has 3243 m/s. And it can escape Tylo going straight up. Ap is at about -12k. Of course, going horizontally gets you further (about -6k, maybe better). I think it's neat how the numbers add up almost perfectly.

So no, there is no doubt and never has been that a vertical ascent is inefficient. However: if you start increasing TWR, and you have an atmosphere to go through, the numbers start evening out. You can't go horizontally, you can't use all that thrust, and by the time you've lugged it up to orbit you could be halfway out of the SOI. I just built a stupid ship which nearly makes Eve with 3953 vacuum dv. Is that even possible going the orbital route? I don't know.

 

3 hours ago, 5thHorseman said:

Funny that you put quotes around "efficiency" for funds, implying it's not "real" efficiency, while then implying that optimizing for pure dV *is* real efficiency in your next sentence. This is at best a difference of opinion and a far better case could be made that it's simply flat-out wrong than that it's purely correct.

mk1 pod on top of an orange tank on top of a mainsail has X dV. Radially attaching 6 more orange tanks each with 6 more Mainsails at the bottom adds a smidgen of dV and a wisp more TWR, but adds a MASSIVE cost. Speaking purely by dV numbers, it's better. But I can't think of any objective situation where anybody (but Whackjob) would prefer it.

Only because there are so many ways of defining "efficiency". There was no other implication there. You can build an efficient ship, or you can fly a bad design efficiently.

Again, I've never suggested that a vertical ascent is a good idea. All I'm saying is that the dv numbers aren't as terrible as many people seem to assume, and the reason for that is orbital mechanics.

But of course it is needlessly wasteful - and I've been saying that all along too.

[Superfluous J]

Then we agree, that to make a vertical ascent similarly efficient to a horizontal ascent, we need to make the ship itself inefficient. That's fair.

[Stoney3K]

1 hour ago, 5thHorseman said:

Then we agree, that to make a vertical ascent similarly efficient to a horizontal ascent, we need to make the ship itself inefficient. That's fair.

Moar boosterz?

In other words, a high TWR, vertical ascent ship is perfectly fair if you have some ridiculous payload, like an entire mothership or a station, that you are unable to turn on ascent (ie. the amount of off-center drag would be insane causing your ship to flip if you move an inch off prograde), and you would lose more fuel fighting drag than you would lose fighting gravity losses. It's difficult to do any numbers on that, because there is no way to determine the (numeric) amount of drag on a ship.

Slamming a high drag ship straight up through the atmosphere so fast that it doesn't have any chance to lose energy due to drag may be more efficient in that case. But that's such an edge case that it may not be worth considering.

[Plusck]

7 hours ago, Eric S said:

At 30 degrees above the horizon, you cancel out 1g of gravity and get 1.73g (2g * cos(30 degrees)) of horizontal acceleration.  Of course, if you try to launch that far over right away, you get into the case of drastically increasing your aerodynamic drag.

Cool - it never even crossed my mind that a 30° angle with 2g acceleration would be perfectly flat until the ground started dropping away.

[foamyesque]

24 minutes ago, Stoney3K said:

Moar boosterz?

In other words, a high TWR, vertical ascent ship is perfectly fair if you have some ridiculous payload, like an entire mothership or a station, that you are unable to turn on ascent (ie. the amount of off-center drag would be insane causing your ship to flip if you move an inch off prograde), and you would lose more fuel fighting drag than you would lose fighting gravity losses. It's difficult to do any numbers on that, because there is no way to determine the (numeric) amount of drag on a ship.

Slamming a high drag ship straight up through the atmosphere so fast that it doesn't have any chance to lose energy due to drag may be more efficient in that case. But that's such an edge case that it may not be worth considering.

 

Funny thing is, the larger the payload (and hence rocket) the less you need to care about drag and the more you need to worry about gravity. Largest drag losses (in modern aero) I've ever seen on a streamlined launch happened when I was firing something that was all of 0.6t, fully fuelled on the pad, into orbit.

 

If I were launching something truly gargantuan -- which I have, a time or two -- I'd put my time and effort into making it controllable through a gravity turn with as high a TWR as I could get without blowing out the size of the rocket. Anything else is just making life hard on yourself.

[Corona688]

On 8/26/2016 at 2:03 PM, Corona688 said:

Not really.  Just because you're in Kerbin orbit, doesn't mean you've actually escaped gravity.  I'll have to launch a gravioli to check, but the force of gravity in near orbit is should be nearly as large as the force on the ground, as it is on Earth;  it's only the constant fall which creates "free fall" conditions.  Meaning, escape is still quite a long journey away.

OK, I checked, gravity in low kerbal orbit is still over 0.7 g's.

[Tyko]

On 8/26/2016 at 8:06 AM, Stoney3K said:

That was not what I was really wondering about. Why is the orbiter ascent upside down instead of flying SRBs down, cockpit up orientation?

(Also, time to scale up the Kickbacks and ModuleManager them into having proper gimbals, so we can make decent shuttles.)

@Corona688: Your chart shows why a gravity turn is more efficient for LKO. But the more the trajectory is 'vertical' as opposed to circling around Kerbin (ie. going to Mun, Minmus or interplanetary) the horizontal line becomes more and more tiny and therefore less relevant.

Here you go: https://en.wikipedia.org/wiki/Roll_program

During the launch of a space shuttle, the roll program was simultaneously accompanied by a pitch maneuver and yaw maneuver.^[2]

The roll program occurred during a shuttle launch for the following reasons:

• To place the shuttle in a heads down position
• Increasing the mass that can be carried into orbit
• Increasing the orbital altitude
• Simplifying the trajectory of a possible Return to Launch site abort maneuver
• Improving radio line-of-sight propagation
• Orienting the shuttle more parallel toward the ground with the nose to the east

I'd also read that the launch profile keeps the 3G of force in a downward orientation relative to the crew so they're not experiencing negative G forces during launch

[Stoney3K]

2 hours ago, tjt said:

Here you go: https://en.wikipedia.org/wiki/Roll_program

During the launch of a space shuttle, the roll program was simultaneously accompanied by a pitch maneuver and yaw maneuver.^[2]

The roll program occurred during a shuttle launch for the following reasons:

• To place the shuttle in a heads down position
• Increasing the mass that can be carried into orbit
• Increasing the orbital altitude
• Simplifying the trajectory of a possible Return to Launch site abort maneuver
• Improving radio line-of-sight propagation
• Orienting the shuttle more parallel toward the ground with the nose to the east

I'd also read that the launch profile keeps the 3G of force in a downward orientation relative to the crew so they're not experiencing negative G forces during launch

The reason I'm puzzled by the use of the roll program is because the shuttle is supposed to be launching due east (for an equatorial target orbit), so is it necessary because the lauch pad is oriented north-south? I suspect it would never have mattered with rockets when they first built it, but it was more of an issue with shuttles which aren't symmetrical across 2 axes.

With respect to the negative G part in the "heads down" orientation: That makes sense, because the thrust is not straight into the crew's back, but on an angle (the component that wants the shuttle to fly sideways and veer off prograde). If they were launching heads-up, that force would push the crew out of their seats, so a heads down orientation is more comfortable.

[Palaceviking]

Oh not a-bloody-gain...yes, gravity turn. No l, direct ascent.

Any more of this and I'm getting my calculator out!! 

[Superfluous J]

1 hour ago, Stoney3K said:

The reason I'm puzzled by the use of the roll program is because the shuttle is supposed to be launching due east (for an equatorial target orbit)

The shuttle does NOT want to launch due East. It wants to launch into an orbit that matches its target or its mission. Or, it used to want that back when it flew

When the ISS flies overhead, it could be going north-to-south or south-to-north (and east of course) but it's never going directly east. This is because its inclination is such as to be reachable by launch sites in Russia. Also, Florida is not on the Equator like KSC is, so even by launching East the Shuttle wouldn't get into an equatorial orbit - even if it wanted to which it never does.

The roll program is because it's easier to roll the shuttle in flight than it is to turn the entire launchpad to align the shuttle the way we want it to go on any given launch. They don't have the benefit of the rotation gizmo in their VAB.

Edited August 30, 2016 by 5thHorseman

[Palaceviking]

7 hours ago, 5thHorseman said:

The shuttle does NOT want to launch due East. It wants to launch into an orbit that matches its target or its mission. Or, it used to want that back when it flew

When the ISS flies overhead, it could be going north-to-south or south-to-north (and east of course) but it's never going directly east. This is because its inclination is such as to be reachable by launch sites in Russia. Also, Florida is not on the Equator like KSC is, so even by launching East the Shuttle wouldn't get into an equatorial orbit - even if it wanted to which it never does.

The roll program is because it's easier to roll the shuttle in flight than it is to turn the entire launchpad to align the shuttle the way we want it to go on any given launch. They don't have the benefit of the rotation gizmo in their VAB.

And aerodynamics 

[Stoney3K]

20 hours ago, 5thHorseman said:

The shuttle does NOT want to launch due East. It wants to launch into an orbit that matches its target or its mission. Or, it used to want that back when it flew

The roll program is because it's easier to roll the shuttle in flight than it is to turn the entire launchpad to align the shuttle the way we want it to go on any given launch. They don't have the benefit of the rotation gizmo in their VAB.

I was referring to launching due east when its target orbit is equatorial. Which, due to Earth's geography, may almost never be the case, and because the Kennedy Space Center is not on the equator, it needs to turn somewhat for every mission.

What makes me wonder is that the shuttle makes a full 90 degree turn and then some, instead of simply pitching over to orient heads-down and bank slightly to turn to the correct heading (returning to wings level orientation once the proper heading is reached). This leads me to believe the shuttle is positioned in the 'wrong direction' (due North-South) at launch because that is the only way the launch facility allows it.

[Jarin]

On 8/18/2016 at 2:08 PM, Red Iron Crown said:

Your gut is leading you astray. A not-stupidly-draggy craft will get to orbit most efficiently by going as fast as possible as low as possible without burning up. Gravity losses trump drag losses in nearly every case, so getting sideways as quickly as possible is key to high efficency. (All this from a delta-V-to-orbit perspective.) Early stages should almost never be throttled back.

Also: I think you may be conflating "normal" with "radial" in your post.

Does this mean I should be running my SSTO spaceplanes lower, hour and faster? Assuming they can take the heat, that is. Flying flat at 30km or so seems like drag would still be an issue.

[Red Iron Crown]

12 minutes ago, Jarin said:

Does this mean I should be running my SSTO spaceplanes lower, hour and faster? Assuming they can take the heat, that is. Flying flat at 30km or so seems like drag would still be an issue.

IME yes, as fast as you can as low as you can without burning up is generally best.

[Jarin]

2 minutes ago, Red Iron Crown said:

IME yes, as fast as you can as low as you can without burning up is generally best.

I'll give it a shot. Been figuring out some tricks to minimize shock heating, so I should be able to snag some decent gains if this works.