Direct assent to interplanetary transfer

[codepoet]

After lots of faffing around on the Mum, and learning to build big rockets, mine and set up a fuel depots, I have finally turned my attention to getting to other planets. It occured to me that it might be a good idea to attempt a launch directly into an interplanetary transfer orbit. I waited until the launch pad was pointing in the kerbin prograde direction, and the launch vertically and just keep going until I have reach escape velocity. Then I keep burning until the solar apoapsis / periapsis reaches the orbit of the planet I am aiming for. So there are a few critical factors:

1) Launch at the right time (for the interplanetary transfer window) Obviously this is always the case, but if you burn interplanetary from LKO you can time it down to the nearest orbin, but if you launch from the surface then it is the nearest kerbin day (6ish hours?) Does this matter much?

2) Launch at the right time in terms of getting your escape velocity directly along the kerbin prograde. Your resultant velocity when you leave the Kerbin SOI needs to be in the kerbin prograde. However when you launch you have about 200m/s perpendicular to the vertical launch velocity direction from the kerbin spin. Does this matter when you are trying to get up to 4000/5000m/s?

So I have a few questions:

1) Is anyone else doing this?

2) Is there any point in doing this? I am hoping that is saves some DeltaV, but I am not sure if it really does. I suspect hat it is helpful because:

a) I do not have to worry about raising my kerbin periapsis as I will never reach it (but you raise your periapsis by increasing your velocity and this is velocity can have its direction changed by gravity by simply orbiting to a certain point before the ejection burn, so perhaps the DeltaV is not wasted.

By not bothering with a gravity turn I an not trying to minimise drag lasses against gravity losses in the ascent - I simply go straight up which means I take the most direct route out of the atmosphere, which should minimise the potential for drag loses.

3) Are there any tools to help? Navigation is hard. Is it possible to get the direction of the kerbin prograde on the nav-ball when you are in kerbin orbirt (or even on the kerbin atmosphere or surface?)

[Superfluous J]

Your idea IS the best way to do an interplanetary transfer, for the reasons you expect. Also, you don't have to waste fuel flying sideways to get into Kerbin orbit when you're just going to leave it again anyway. That's a lot of delta V you really don't need to spend. Kerbin Escape is a certain velocity at each altitude and every ounce of fuel you spend NOT increasing the speed portion of that vector is wasted fuel. So if you want to escape Kirbin, you need to burn toward prograde at all times. Starting with your prograde aiming in the correct direction and just going is logical and smart.

Leaving within 3 hours of the window (either before or after) isn't a big deal. Likewise the 200 or so m/s from being on the surface isn't enough (for me, at least) to worry about. The further you are from perfect, the further you have to correct but I always end up with more fuel than need anyway so don't worry about it.

The only way in Stock to get the correct aim on the nav ball is to make a maneuver node. Note: You don't have to set up the correct maneuver node, just set one up with a maneuver aimed in the right direction. You can do this by setting the node, then zooming out enough to see the setup of the solar system. Then click the already-created maneuver node and you can edit it You can also drag it around the orbit path to get the direction nodes to line up right. With any luck, you can aim ONE of them in the direction you want to go. Then aim your ship in that direction and fire the rockets.

[codepoet]

... you don't have to waste fuel flying sideways to get into Kerbin orbit when you're just going to leave it again anyway. That's a lot of delta V you really don't need to spend. Kerbin Escape is a certain velocity at each altitude and every ounce of fuel you spend NOT increasing the speed portion of that vector is wasted fuel. So if you want to escape Kirbin, you need to burn toward prograde at all times. Starting with your prograde aiming in the correct direction and just going is logical and smart.

I am not sure about this. If I get into LKO at 100km then I will have an orbital velocity of 2250m/s (ish), but kerbin gravity is constantly accelerating me towards the centre of kerbin, and of course acceleration changes velocity, so even if I have gained velocity in the wrong direction I just have to wait until a different moment in the orbit when kerbin gravity will have changed the direction of that velocity into the direction I want it to be. (well actually a bit before, taking into account all the swanky maths to do with angles for escape trajectories and all that jazz. Tell me I am wrong - I would love to be!

I do agree with you agreeing with me about the most efficient route out of the atmosphere being straight up. What is the most efficient velocity - terminal velocity all the way?

[Eric S]

1) Probably, but I'm not one of them.

2) It might save delta-V over a regular launch to orbit then transfer, but it's still at least slightly less optimal than a gravity turn straight into an interplanetary transfer, and the lower your TWR, the more significant the difference. Scott Manley did a video on it. Basically, going straight up maximizes gravity losses, which are not insignificant.

Think of it this way. If you have a rocket with a WTR of 2.0 and thrust straight up, the first 1.0 of your TWR goes to cancelling out gravity. On the other hand, should you thrust at the correct angle, you'll split the thrust into 1.0 vertical thrust and 1.732 horizontal thrust. The 1.0 vertical cancels out gravity, leaving you with a TWR of 1.732 accelerating the craft, or 73% more than you'd have going straight up. As you build up horizontal velocity, a portion of that horizontal velocity starts cancelling out part of gravity, allowing you to use more of your TWR for acceleration.

Yes, you still have to take aerodynamic losses into effect, which is why traditionally KSP players have started their gravity turn at 10km height. At that point, they're past the thickest of the atmosphere, so it's time to start worrying more about gravity losses than aerodynamic losses.

Edited August 19, 2013 by Eric S

[SRV Ron]

To do so directly, you would need to launch when the target planet offers the best opportunity for efficient capture. That may mean waiting several years for the intercept window to present itself. Then, it is launch straight up at dawn for reaching the outer planets, or dusk for the inner ones. Otherwise, orbit as circular as possible, set up a Mun escape slingshot prograde to Kerban's orbit for the outer planets, retrograde for the inner ones, You will have to wait for Mun to present itself in the correct position when you enter its SOI, try to get the escape orbit around the sun as circular as possible without using extra fuel to do so, and set up the transfer maneuver to get a retrograde capture. You can also go direct when Kerbal is in the right position, or get into a circular solar orbit and find the best time to do the transfer burn from there, much easier.

[SRV Ron]

I am not sure about this. If I get into LKO at 100km then I will have an orbital velocity of 2250m/s (ish), but kerbin gravity is constantly accelerating me towards the centre of kerbin, and of course acceleration changes velocity, so even if I have gained velocity in the wrong direction I just have to wait until a different moment in the orbit when kerbin gravity will have changed the direction of that velocity into the direction I want it to be. (well actually a bit before, taking into account all the swanky maths to do with angles for escape trajectories and all that jazz. Tell me I am wrong - I would love to be!

I do agree with you agreeing with me about the most efficient route out of the atmosphere being straight up. What is the most efficient velocity - terminal velocity all the way?

A escape burn near the midnight side of Kerban while in orbit, assuming a normal 90* launch will be a burn prograde to Kerban's orbit around its sun. That will create an elliptical orbit that extends to the outer planets. An escape burn on the noon side of Kerban will be retrograde to Kerban's orbit around the sun. That will create an elliptical orbit that dips closer to the sun. Those can be most efficient providing you leave Kerban when an intercept window to th target planet presents itself. Otherwise, try to get as circular a solar orbit as possible to make the later intercept burn as efficient as possible.

An interesting experiment would be to use one interplanetary design and launch several probes to one location under different scenarios and keep a log on the amount of fuel needed to complete the maneuver to a given intercept;

1. Direct from Kerban to target launch at dawn.

2. Direct from Kerban to target launch at dusk.

3. Direct from Kerban to target launch at noon.

4. Direct from Kerban to target launch at midnight.

5. Launch to orbit, then launch direct to the target.

6. launch to orbit, then launch to Kerban escape and note the orbit location in relation to the time of day under the orbit, then launch to target when it's window of intercept is available.

7. Launch to orbit, Then use a Mun slingshot to escape when it is in the most favorable position, then complete the intercept.

The results will be interesting and will likely vary depending on your rocket design as to which method is most efficient. I actually found that one design, with its high acceleration rate, was actually most efficient launched straight up at midnight. That might not be the case with a lower acceleration model or one that you can keep from spinning so you can do a proper gravity turn into orbit.

Edited August 19, 2013 by SRV Ron

[SRV Ron]

Results of the experiment on this interplanetary design;

1. Exactly the same amount of fuel is needed to reach Kerban escape on vertical launch to escape no matter the time of day.

2. Burn to orbit, then burn to escape used 28 more units of fuel. That could have been from the difficulty to get this rocket into orbit.

3. A Mun encounter of 965K prograde saved 10 units of fuel. A closer one will save even more. However, follow through the time warp on that rocket or the speed boost will not be computed in the path. That probe crashed back into Kerban.

A midnight launch gave the best circular path for planning a maneuver burn for visiting the outer planets. A noon launch best for the inner planets. Morning and evening launch made the solar orbit more elliptical with a risk of a Kerban capture during a maneuver burn.

The launch vehicle reaches near Mun orbit on burnout. A short burn of the LV-N completes the escape maneuver. There is 2628 units of fuel, 3212 units of oxygen left for the LV-N for the rest of the mission. That can easily reach orbit even to Eeloo if its encounter window is available, conduct a voyager mission with plenty of fuel to spare, or do a suicide run into the sun.

I did not try for a window to attempt a direct escape to Jool encounter.

Edited August 19, 2013 by SRV Ron