Interplanetary transfer with low TWR

[Sandermatt]

I really enjoy the engineering part of the game, but suck at piloting (even with mechjeb). Currently I struggle to do a proper interplanetary transfer to Eve. I tried both, mechjeb and the online calculater of the apropriate angle and deltav. My problem is the rocket has low twr (0.24-0-26) and somewhat more than 10 min burn time. I tried to do it with multiple burns during multiple circulations, but this costs a lot of time and makes hitting time windows hard. Do you have any advice on how to do it?

[Hour Hero Yes]

My interplanetary probe design uses 1 nuclear engine for most maneuvering, but when it comes time to hit a planetary transfer window, I have 2 liquid fuel engines strapped to the side that I turn on to accelerate the process. They get turned on after the encounter has been hit.

[RoboRay]

Periapsis kicks, where you burn for a couple of minutes each time you pass through the proper ejection angle on subsequent orbits, is the way to go. Interplanetary transfer windows in KSP are days wide, so taking a couple of hours to split your burn across multiple orbits should not be causing you to miss your transfer window... as long as you aren't making your last orbit with apoapsis out past the Mun. With an Ap near Minmus, your orbit will take a week and that does cause problems with phase angle timings.

So, split your burn up across multiple orbits, but stop when your Ap rises to the Mun's orbit. From there, you want to do all of the rest of the transfer in one long burn. Unless you planned ahead and started your kicks a week before you reached the right phase angle, of course.

[Skorpychan]

Add another nuclear engine. That's what I did for my Duna mission. It's a bit less efficient due to the extra hardware, but it pretty much halves the burn times.

Otherwise, put your mission on a diet, and remove stuff you won't need.

[numerobis]

An orbit around Kerbin is about a half hour; windows tend to be open for days without major change in cost. So burn half of the node, then plan a second to perform the other half.

[antbin]

You can do it! Learning to pilot with low TWR lets you get away with some crazy engineering.. For a TWR of 0.05ish multiple passes are needed.

Using Protractor Mod, you can choose the right size of boosting orbits by counting how many degrees / day your planetary intercept window moves, then boosting only until your "time till Periapsis" is the right number of days. That way your ship will swing around for the escape burn right on time!

[Sandermatt]

An orbit around Kerbin is about a half hour; windows tend to be open for days without major change in cost. So burn half of the node, then plan a second to perform the other half.

The problem is the highly eliptical orbit just before the last burn.

[Francesco]

The problem is the highly eliptical orbit just before the last burn.

if you put your apoapsis at about the Mun's height or below (11400 km), it won't take many hours to travel back to your Kerbin periapsis for the final burn, so you should still be within the right transfer window.

having already spent about 860 m/s or so, there will be only 200 m/s left for the last burn to transfer to Eve.

[karolus10]

It's useful to split the burn between few orbits before main burn. After You get into escape vector You will had a lot time to do the rest .

[Sandermatt]

if you put your apoapsis at about the Mun's height or below (11400 km), it won't take many hours to travel back to your Kerbin periapsis for the final burn, so you should still be within the right transfer window.

having already spent about 860 m/s or so, there will be only 200 m/s left for the last burn to transfer to Eve.

OK, I managed to do the transfer, it might not have been the most efficient transfer (as I only guessed, when the angle was right) but it worked. Thanks all of you guys for your help.

[RoboRay]

The problem is the highly eliptical orbit just before the last burn.

Well, like I said above, don't make a highly elliptical orbit that takes a week to complete if your transfer window is going to close in a couple of days.

The ÃŽâ€v difference between reaching the Mun's orbit and reaching the edge of Kerbin's SOI is less than 100m/sec. That's really not enough to worry about.

OK, I managed to do the transfer, it might not have been the most efficient transfer (as I only guessed, when the angle was right) but it worked. Thanks all of you guys for your help.

Great! With some practice, you'll find it easier to manage. Just keep at it.

[Frederf]

Given proper scheduling ti should be possible to any impulse at Pe over any number of orbits provided enough lead time is given. It could even be a sort of optimization to break up your X dV burn into the maximum number of parts. The only limit I can think of is the SOI range of Kerbin and any Muns that would get in the way (very rude).

If you need 1000dV in 200dV chunks then there should be a look up table that says

burn 1 80x300 period 2h16m

burn 2 80x750 period 3h50m

burn 3 80x1450 period 7h10m

burn 4 80x4500 period 1d10h

burn 5 final trajectory

total additional time 1d18h37m

Then you'd just have to find out when your simple impulse burn would be and start the whole dance that much sooner. With flexible dV per burn you could make the periods nice round numbers and even dodge moons.

[Sauron]

Bear in mind that your window as displayed by something like protractor is fairly narrow. In practice, missing your launch window by an orbit or two (or even a day or two) doesn't really impact your delta-V requirements that much--it's a question of a few tens of M/S to ~100 depending. Unless you're flying an incredibly lean mission and are an awesome pilot, it doesn't really matter too much as long as your ejection angle is correct.

For what it's worth, I personally consider a TWR of .25 to be my lower limit, and I've not had any problems yet.

[Frederf]

Alternately you can extend your time in the impulse window by picking a higher parking orbit. If your acceleration is 2m/s/s and you need 3000m/s then you obviously need 1500 seconds burn. There is a parking orbit that provides 1500s of alignment to prograde +-5 degrees.

[SRV Ron]

So long as you are not trying to use ion engines, you should have no problems using the low powered engines for interplanetary exploration. It is just a matter of properly splitting up the several minute first encounter burn which should be done while you are in solar orbit. The needed fine tuning correction burns should be quite short. Try for a high orbit capture since those burns will be short. You can lower the orbit with additional short maneuvers or try for an aerobraking maneuver on Eve, Jool or Duna. Ike, or the moons of Jool can be used to slingshot you to lower orbit using shorter low power burns.

If you are trying for a Mun or Minmus encounter, set up a maneuver to get you into higher and higher orbits. Then the planned encounter burn will be a lot shorter and more accurate.

[flatbear]

I have this problem with trying to get to duna.  I have a single nuclear engine with a ttw ratio of 0.12 trying to expend 1750 m/w delta v.  how do I split this up.  Do I just burn a minute before and after then stop and try again when I come around again?  The problem is that when I burn before and after the manuver node, the rest of my orbit around kerbin changes

 

[Geschosskopf]

On 8/3/2013 at 3:33 PM, Sandermatt said:

I really enjoy the engineering part of the game, but suck at piloting (even with mechjeb). Currently I struggle to do a proper interplanetary transfer to Eve. I tried both, mechjeb and the online calculater of the apropriate angle and deltav. My problem is the rocket has low twr (0.24-0-26) and somewhat more than 10 min burn time. I tried to do it with multiple burns during multiple circulations, but this costs a lot of time and makes hitting time windows hard. Do you have any advice on how to do it?

Well, #1 would be to build ships with higher TWR.  I find about 0.75 to be the minimum acceptable for a 1-burn transfer because the burn is then only about 6 minutes, which is the max you should ever burn in LKO.  High TWR is pretty easy to obtain these days, what with 3.75m parts.  It might, of course, require breaking a single, massive ship into a flotilla, but that's way more advantageous in many ways than not, so you might as well.  But anyway, a single, short burn is the preferred method because 1) it's the most efficient, and 2) it short :).  You can do this sort of burn with both LV-Ns and chemical rockets.  With ships like this, however, you usually have a tight dV budget. so you care about the efficiency (plus the short burn time).

At the other extreme are things with low TWR.  There's usually the the trade-off of high TWR and low dV or high dV and low TWR.  If you have both low TWR and low dV, then redesign your ship .  But anyway, with low TWR, you have a choice of doing the burn in multiple passes or starting from a very high, circular orbit and doing the whole burn in 1 go.  Neither is as efficient as a single, short burn in LKO (plus of course they take longer) but if you have low TWR, you really shouldn't be worrying about efficiency because you've got gobs of dV to waste.  Again, if you have both low TWR and a tight dV budget, you've got a problem.

In general, it's more accurate to do multiple burns than it is to start from a high orbit.  This is because it takes less time to do an ellipse than it does a circle with the same Ap.  Thus, you can start on the best day of the overall transfer window instead of waiting days to weeks to make a big orbit of Kerbin.  The main trouble you can have with the elliptical orbit is getting an unwanted Mun intercept that will totally hose you up, so be careful about that.

Edited July 24, 2016 by Geschosskopf

[technicalfool]

That's a heck of a necro, there.

@flatbear, feel free to ask your question in a new thread. I'll lock this one so it can't rise from the dead again.