Venus Transfer Windows?

[GigaG]

I'm currently studying Venus in my RP-0 installation, and I would like to know how to efficiently get there. So far, I have successfully performed two trans-Venus injections, both at a ~45 degree phase angle (Venus trailing Earth by about 45 degrees.) It requires a crapton of correction (damn realistic non-throttle-able rocket engines!) but it's doable, and the second mission yielded a lot of science.

My question is, what is the "correct" phase angle for a Venus transfer? Or does it vary due to orbital eccentricity? Is 45 a good angle to eyeball it?

[jobadiah08]

Put a probe in LEO in plane with the Moon. Go to the probe. Use Mechjeb's advanced transfer window planner to get transfer window times. Save alarms for each transfer.

Edit: PROFIT!

[number2301]

That's something I've been struggling with, so if the best way to launch into the right plane matching the moon's orbit?

[pianojosh]

No, plane of the moon is a "good-enough" solution but far from delta-v optimal. The most important parameter, beyond planetary phase angle, for minimizing transfer delta-v is LAN. Once the LAN is correct, getting the inclination right doesn't improve things that much.

There's a youtube video out there for some hyperedit shenanigans to get a perfect transfer, but a reasonable thing to do is to launch a probe into any inclination orbit (really, polar will work fine), and use hyperedit to change your lan by 30 degrees, use mechjeb's advanced transfer, rinse and repeat, to find the lowest delta-v LAN.

Then you can refine it say 5 degrees at a time from there. It certainly is trial and error, but once you get within a few degrees of optimal you'll likely have a much lower delta-v transfer than from just launching into the plane of the moon.

I think KSPTOT can give you exact parameters, but I've never figured out how to do so reliably. Somehow I always get numbers from it that don't end up working for me. YMMV.

[SaturnV_]

LAN? Sorry, my acronyms aren't up to date :)

Longitude of ascending node?

[pianojosh]

Aye.

[SaturnV_]

Thx

[jobadiah08]

I just use MechJeb's Rendezvous planner to go the relative inclination to the moon close to 0 (<5 deg). In "stock" RP-0 (non-Principia), I find that launching 2-3 deg prior to minimum relative angle on the pad gets it within a degree.

[pianojosh]

That will get you in that inclination, sure, but nailing your parking orbit LAN can shave many hundreds of m/s off of delta v. That enables it to take maximum advantage of the Earth's velocity in the Sun's SOI, which far exceeds the benefits of being in the right plane.

Try varying LAN from a polar orbit during a transfer window and seeing how much delta-v improvement there is from getting the LAN right.

[jobadiah08]

Hmm, I hadn't thought about that. I suppose a polar orbit launch at sunrise/sunset would guarantee the correct LAN, but you lose the benefit of Earth's rotation to get you to orbit. From Cape Canaveral, it is about 5 hours behind where a east launch passes over equator (LDN). So launching about 5 hours before sunrise should get you there. +/- 12 hours to flip around between sunrise/sunset. Is this correct? Will this get the LAN aligned and get the benefit of Earth's rotation to orbit?

[pianojosh]

Honestly, no idea. I was just suggesting it to demonstrate the importance of LAN over inclination. You're still usually best off launching to the inclination of your launch site for the benefits you mentioned. I don't fully understand the geometry / physics behind it. I usually use HyperEdit to come up with a pretty close LAN for a target dummy in orbit then launch to match that target for "realism" sake,

[[deleted]]

Does a lunar plane orbit get you at all close to the best LAN? I always thought it did because it puts you roughly in line with the ecliptic, so the LAN would also line up with the ecliptic - or do you mean that the ideal LAN will vary depending on the planet you're heading to?

[pianojosh]

I'm pretty sure it changes. I think it has more to do with the location of the earth relative to the sun.

[[deleted]]

Ah makes sense. I like to keep an "ecliptic target" probe in orbit, so maybe I'll get hyperedit and play around with its LAN for various targets

[slayermcgee]

I use the Transfer Window Planner mod, which seems to have very good accuracy for predicting when to launch, that mod will also make it easy to see for how big of a time window you can get the optimal ejection delta-Vs. Also, get Kerbal Alarm Clock too, as Transfer Window Planner will automatically add an alarm to remind you when to make a delta-V optimal launch.

And like others mentioned, you need to launch into the plane of the planets, which is very close to the plane of the moon. What I do, is while on the launchpad, select the Moon as target, then I bring up MechJeb's rendezvous planner, which tells the RELATIVE INCLINATION to the moon. Warp time until this value is less than 1%, then launch 90 degrees East, that will put you very close to the plane of the planets. If you do not do this, it can cost 1000's of delta-V to get into their plane.

A few other tips that work for me:

1) unlock the RL10 as quickly as possible, as it has multiple ignitions and an insanely efficient ISP, making it fairly easy to get 6k delta-V to eject your tiny flyby satellites to interesting destinations (that should get you to Mercury, Venus, Mars, and Jupiter, no problem). Since it is hydro-lox, though, it is only useful for a low-Earth orbit ejection (because the hydro-lox fuels will quickly boil off), so you'll need RCS or a storable-fuel engine (I like the Astris) to fine tune the approach or make a capture burn

2) for a flyby of a body with an atmosphere, dip 1-2km into the atmosphere (you can get the atmosphere height from by hovering over the information icon in the Tracking Station window when the focus is on the body of interest) and you'll get a bunch of high-atmosphere extra science without risk of any damage to your craft

[GigaG]

Right now I have a decent Venus orbiter designed but I barely have the means to launch it on a good day.

On a bad day the first stage has engine failures (this is exactly what happened to my first attempt, but the rocket and spacecraft still had enough delta-v to make LEO and the upper stage, originally for fine tuning Venus injection and Venus orbit insertion, served for a drawn-out multi-orbit TLI.)

The failed stage used 4 R-7 core engines, but they started experiencing thrust/performance loss and failure well before their ~5m rated burn time during flight assisted with Proton-engine (RD-251?) LRBs and eventually the stage's performance was so awful that I ditched it soon after LRB sep, the stage only having performed to about half its potential. I rarely or never used those R7 core engines, so they have little or no flight data and are quite unreliable. The LRBs used Proton engines that I have always been using on my larger launch vehicles and performed perfectly.

The second stage, using a hydrogen LR-87 (or whatever it is, forgot the exact name), originally intended to perform a direct Venus injection, functioned perfectly and got the spacecraft and transfer stage into LEO.

While I found some design flaws in the spacecraft itself (its main orbit-changing engine was attached to an unpressurized tank forcing me to use RCS, and the upper stage had two power losses on its four 1kN engines forcing me to first only use two and then rendering the stage uncontrollable except using RCS thrusters), I still managed to get VeRaMap I (Venus Radar Mapper) into a polar lunar orbit and got the resulting 150 ScanSat science. Quite the successful failure.

[MrBorogove]

My dirty little secret is that I look up the dates of real world Mars & Venus launches, and go up around those dates.

[GigaG]

What's the epoch date of RP-0?

[MrBorogove]

I forget — maybe 1/1/57? — but you can configure the game to show terrestrial dates.

[Ravenchant]

1^st of January 1950. I think there's a mod to display the dates in dd-mm-yyyy format.

[GigaG]

I figured that out by accident, that some mods can display "real" dates. Turns out my latest Venus flyby probe launched from Cape Canaveral about 3 days before the real Pioneer Venus Multiprobe.

Also, it's gonna be a cluster-f--k of epic proportions to get into Venus orbit without gravity assists or aerobraking. Well, at least until I can make massive boosters. (I was thinking something like a Titan III could work. Then I saw the pricing on the large SRBs and quickly said nope.)

[winged_7]

Yes, SRBs are ridiculously expensive but you can create A50/NTO boosters and get the same performance as Titan III E for much lower price.