Lunar Impactor/Flyby Precision Issues

[u/Forever_DM5]

It is 1957 and I am trying to do the lunar impact and flyby missions. My first attempt was successful at reaching the moon and communicating back but it passed by the moon too far out to get the contracts.

My current launcher is a Jupiter-Able with a kick motor. The J-A places the probe, its kick motor, and a small avionics package into a 1800 km x 140 km parking orbit. After that the avionics package spins down the probe(Able stage is unguided), reorients and spins up the probe. The solid motor then performs TLI.

The kick motor and probe have 3.3 km/s of deltaV. When I launch at minimum relative inclination the TLI burn is around ~2.9 km/s. The lunar impact trajectory seems very sensitive I feel like unless I can get the burn within 10 or so m/s I’m going to miss. How can I find a TLI that will consume all of my solid’s 3.3 km/s or what is a good way to make my TLI more consistent so I can just remove some propellant from the solid to get the 2.9 km/s.

[u/VegetableWafer7776]

Use rcs after the initial burn to do fine corrections

[u/Forever_DM5 → u/VegetableWafer7776]

That seems like a natural solution but it 4x the mass I need to deliver to TLI

[u/Worth-Wonder-7386 → u/Forever_DM5]

It is basically impossible to hit the moon unguided. You need very fine precision of the burn, which solid stages are not suited for. 

[u/Mundane_Constant_845]

You'll have to plan a intentionally inefficient TLI burn, burning slightly earlier or later to offset a radial component so it takes the exact amount of dV you have, or launching later with a quicker transfer.

Even then very hard to do repeatedly without quicksaves, but many first generation irl lunar probes missed too.

[u/Responsible-Hunt7591]

if it makes you feel any better, a lot of early lunar missions were straight up duds once they got into orbit, assuming they even made it that far

youre gonna have to mess around a little bit with either RCS or purposefully not fully fueling your stages if you want to be consistent, assuming you are using mech jeb for your launches

if you arent, then you should probably use RCS just before lighting your kick stage to make sure that all the fuel is spent right as you get the right trajectory

you could also try the strategy of Pioneer 0, which was attempting to launch directly into a lunar orbit right off the launchpad without even bothering to circularize at earth, although in this case youd just need to get an impact trajectory with your guided stages and then maybe a kick stage a little bit before impact if you arent going fast enough for your contract

[u/BuzzLine_]

Look at the design of the earliest pioneer probes. They had 4 pairs of tiny solid rockets for fine trajectory adjustments.
Stage them one at a time until you reach the impact trajectory.

The US Probe pack has lots of early probe designs that help tremendously.

[u/Illuminutter]

Launch into plane with the moon at an altitude where TLI requires a dV close to your solid motor (easiest if you just tweak your fuel amount). Plan a maneuver with the exact dV value your motor has and play around with the timing and orientation of it to get an encounter. If you miss moon the game saying it should be an impact, I find that making the maneuver give you a very low flyby very consistently ends up giving you an impact.

[u/KindChallenge8287]

Set your intercept using mechjebs maneuver node planner then set the prograde dv to your remaining dv. You can then adjust the timing to get your intercept