Lowest functional TWR

[1of6Billion]

I just built a large ship to cruise around the solar system. With it's 3 LV-N's it has only a Trust to Weight Ratio of 0.06

When doing a burn, the 'spent dV' indicator (Mechjeb) is going up much faster than the 'needed dV' (Navball) is dropping. So, I'm guessing my ship has a lower-than-functional TWR.

Does anyone have some info on what is the bottom line here?

[capi3101]

There isn't a true "bottom line" for TWR once you're in space, other than "is the craft capable of making the maneuver you need it to make". For a ship with a TWR as low as the one you've got, I'd suggest using the "periapsis kick technique" to raise your apoapsis in stages (basically, you keep your periapsis low - say between 70-72 km around Kerbin and make a couple of burns (say three to four minutes each) to gradually stair-step your apoapsis until it's where you want it. If you're trying to go interplanetary with a craft like that, you'll want to do your kicks in advance of your transfer window, of course.

Gotta ask - how many engines do you have and how much mass are they pushing?

[impyre]

I just built a large ship to cruise around the solar system. With it's 3 LV-N's it has only a Trust to Weight Ratio of 0.06

When doing a burn, the 'spent dV' indicator (Mechjeb) is going up much faster than the 'needed dV' (Navball) is dropping. So, I'm guessing my ship has a lower-than-functional TWR.

Does anyone have some info on what is the bottom line here?

The "dV needed" assumes an "impulse" maneuver... which is one that happens all at once. Obviously this is impossible, but for most cases where burn time is less than 45 seconds or so (depending on orbit shape and semi-major axis when the burn is started) this is a pretty close approximation. Close enough that you don't notice the difference. For vehicles with such a low TWR, it's not uncommon that maneuvers take much much longer. The problem with this is the loss of advantage gained from the oberth effect... as you burn, you raise your apoapse... and the periapse will move with the vehicle (meaning that you pretty much stay at the fastest point in the changing orbit, burning more efficiently). If your TWR is very low, you will pass your periapse (and maneuver node) while burning... and more of the fuel will have to be spent constantly correcting for your ship being where it's not supposed to be (if you'd completed the burn in an instant you'd be much higher than you currently are). In effect, you're fighting against orbital mechanics instead of working with them.

- - - Updated - - -

As noted earlier, the only real solution to this is to implement the maneuver in increments... maybe 30seconds to 1 minute per burn. In my opinion, the best solution is to simply have enough TWR to complete maneuvers in *no more than* two "kicks"... it gets a bit tedious otherwise. You also have to consider the potential fuel cost for correcting mistakes (more kicks = more chances to mess up).

[Red Iron Crown]

Any TWR is workable if you are patient enough, it's more a matter of personal tolerance. Personally I find it tiresome to play with TWRs less than 0.2, and even that is lower than I prefer.

You can check out some precomputed periapsis kicking at the link in my signature. Even so, that only allows splitting of the first 950m/s or so of a burn from Kerbin, the rest must be done in one single burn.

[Laie]

When doing a burn, the 'spent dV' indicator (Mechjeb) is going up much faster than the 'needed dV' (Navball) is dropping.

I did some experiments, but my notes were scribbled on a piece of paper that has long gone lost. The rule of thumb I remember is that the initial Kerbin-TWR when going to Jool should be about 0.2; any more than that is convenient but not really necessary. If TWR drops below 0.15, adding more (nuclear) engines will usually save fuel despite the higher weight.

I also found that maneuver nodes aren't straight pointers. I haven't figured out the algorithm behind them, but they are updated during the burn and change direction and size to make up for errors. My attempts to outsmart the maneuver node have been inconclusive (the outcome was all wrong and I couldn't figure out any relationship between my doings and what happened).

To rephrase what Impyre said: what really matters isn't how much time you take, but how much your trajectory bends while doing the burn. If your maneuver is prograde only, but the maneuver marker on the navball is 30 degrees off from prograde for most of the burn, you will have severe losses.

Edited April 6, 2015 by Laie

[1of6Billion]

snip...

Gotta ask - how many engines do you have and how much mass are they pushing?

Upgraded to 4 LV-N's pushing 305T, TWR =0.08

Launch to 600KM for maximum time warp. It does the burn in time, but after that it's not even near an intercept (with Moho).

I rely on Mechjeb for launch windows, doing multiple burns will be quite tricky. I think I'll just rethink my Grand Tour strategy.

Edited April 6, 2015 by 1of6Billion

[impyre]

I know this wasn't your original plan, but I recommend the "wagon train" method I like to use. If carefully planned, it's almost as efficient as using one large vehicle. Basically, send several modules instead of just one. Good TWR's will be easier to obtain, payloads will be easier to control in terms of attitude, smaller launches are more manageable, and failure of a single module doesn't wreck the mission (assuming you send spares). The overall fuel cost is still related primarily to TWR (which is itself often related to engine mass), payload mass, and destination. These can be kept the same regardless of how many launches you use. On my last mission to Duna, I basically sent up everything I needed to build a large orbital station around Duna, with some spares. Refuelling, life support, science, return capability, etc. I plan on leaving everything else (including spares) for future missions... I've even considered refuelling all the modules and sending them on to Jool direct from Duna. That way when I get around to Jool missions, I'll only need to send crew and life support (and a couple of modules filled with equipment specific to the Jool system).

I know this isn't really a "solution" to your problem, but it's a decent enough workaround I think.

[Captain H@dock]

Launch to 600KM for maximum time warp.

You can time warp from the KSP (tracking station), so this isn't a good reason for being at 600x6000. But 600x600 will be more forgiving for low TWR.

However, this is only interesting IF you have refueled your ship once you've parked it at 600x600. Else, stay lower (see oberth effect).

I rely on Mechjeb for launch windows, doing multiple burns will be quite tricky. I think I'll just rethink my Grand Tour strategy.

Shouldn't be a problem. Periapsis kick is really your only option here, though a multi kick like in Red Iron Crown signature might sound complex. You could start with a simple two part burn (1 kick, then the final burn) :

I'm assuming mechJeb tells you when to burn / places the burn node for you?

So pick that departure node, right click, and hit 6 times the "-" button. This should move your burn 6 orbits (6 x 1h13mn18s) earlier. Remember what the burn dV value is.

Then change this node so that it is only making your orbit a 600x6123 km orbit (it should only be a prograde burn, nothing else, and it should require about 519 dV : that's 11mn at 0.08 TWR)

Do this burn. At the end of the burn, focus on the Ap Value rather than the node details. Try to hit 6123 km*.

Then this burn is over, create a new node at your Pe, and make it the initial value - 519 (which you've already done). Fiddle around with the Blue component, and you should get your encounter back. This new burn should be also in the 500ish range, which means if you made the first one, you'll make this one.

So that's a very rought way of doing a single Pe kick (I'd be more careful of restoring the normal component of the initial burn, but i'm assuming ejection inclinaison angle is very small). It gives you the concept, but uses tricks to make it simple to execute. Namely:

- The intermediary orbit period is a multiple of your parking orbit, so you can just use the "-" button. That's why I picked 600x6123, it's (7:19:46 which is very close to 6 x 1h13mn18s). (I use this tool to come up with orbital periods: http://files.arklyffe.com/orbcalc.html)

- A 600 x 6123 orbit takes around 7h, which means the Pe you're setting 7h in advance will only be 1 degrees off compared to the one required for your final maneuver node. That's close enough.

With a .08 TWR, there's no real way around Pe Kicks, but once you've made this crude version, you'll probably feel more comfortable making a "real" one.

* Actually, as indicated by Red Iron Crown in his thread, try to make the sum of your Pe + Ap be 600 + 6321. But that's hard to do in real time in the final second of the burn.

Edited April 6, 2015 by Captain H@dock

[impyre]

I thought this image was a good illustration of the fact that how much fuel you burn and which way you're pointing aren't the only important things... where you are in your orbit is very important. This is from http://flyonbudget.one-giant-leap.info/index.html chapter 8 (which also discusses the issue of extended burns). This is why you either need high enough TWR or to perform the burn in "kicks" and probably does a better job showing it than I can do with words alone. The real question is: are you working *with* your momentum or against it?

[Laie]

Upgraded to 4 LV-N's pushing 305T, TWR =0.08

Launch to 600KM for maximum time warp. It does the burn in time, but after that it's not even near an intercept (with Moho).

Uh-oh. Low TWR to Moho is gonna be fun. The first time I tried that, I zipped past at insane speed -- I suggest you have a look at this post: http://forum.kerbalspaceprogram.com/threads/61478-Oh-bugger-Injection-burn-at-Moho?p=835667#post835667

The gist of it: when going to Moho, one can save a lot of fuel if one does not care about Moho's position at first, but just launches into Moho's plane from Kerbin (window comes up twice a year). Then waits in this transfer orbit for Moho to come around for a rendezvous. Given Mohos short orbital period, you won't have to wait long.

[NASAHireMe]

I'll be honest, this mission won't work. Even with 4 nukes, your TWR is too low. Unless you pull off a crazy number of Moho gravity assists to 'slow' you down, you won't be able to decelerate enough to capture into a Moho orbit. Even if you begin burning retrograde at max thrust from the moment you enter Moho's SOI, you won't be able to decelerate before you zip out of Moho's SOI again.

I'd recommend building another Moho craft that has a higher TWR for the Moho capture burn. Use an Eve gravity-assist (or several) to sling you into an orbit that crosses, and is in the same plane as, Moho.

[Yakky]

I'll be honest, this mission won't work. .... Even if you begin burning retrograde at max thrust from the moment you enter Moho's SOI, you won't be able to decelerate before you zip out of Moho's SOI again.

Still could work if you start burning before you enter Moho's SOI. Basically you are going to equalize your orbit to nearly match Moho's, then drift into its SOI at a slow rate of closure.

Think of it as docking with Moho. You make Moho your target, then when you get close (but still outside the SOI), you burn retrograde to your target velocity indicator on the nav ball in order to slow your closure rate, same as for docking. Remember the distances will still be huge compared to a normal docking, so don't cut your closing speed too much -- just enough so that you can successfully finish the job once you enter the SOI.

Edited April 8, 2015 by Yakky

[Norcalplanner]

I was asking a similar question in a different thread. OhioBob posted some results that showed how .3 is pretty much the sweet spot for interplanetary burns: http://forum.kerbalspaceprogram.com/threads/114358-Looking-for-info-on-TWR-gravity-losses-steering-losses?p=1813824&viewfull=1#post1813824

[Daid]

Just a theory, you could be losing some dV due to wobbling engines?

[panzer1b]

My personal hard limit is roughly a 10 minute burn from LKO to jool. If it takes more time then this, i need more engines.

Anyways, for pure space, even a .00001 TWR will still work, its a matter of how much you are willing to wait for burns, and there are also certain maneuvers you just CANNOT do with a TWR lower then x. For example most braking burns (aerobraking aside) require a minimum TWR, or else you will just fly past the targeted planet and fail to actually get into a orbit. But for general purpose maneuvers such as raising/lowering orbit, circularizing, ect, its a matter of how long you are willing to wait.

Im not exactly impatient, but i dont use autopilot mods, and i would very much rather do something else then spend all my time flying doing super long burns. This pretty much rules out ion engines 100% for me, as given i rarely touch craft below 10t, dV or not, im not about to spend 60 minutes burning to jool.