Friendly Reminder

[Kerbin vonKerbal]

That just because you pack lots of gas, you'll still run out if your engines don't put out enough thrust to move very efficiently. i.e there's a point where you just start canceling out your dV with lack of TWR. 

My duna space station experienced this today. Lots of fuel but only 4 nukes. Oh well.

[netbumbler]

Lol yeah my first few attempts to get off Eve I had plenty of dV, but the TWR was insufficient to the task.  I would just sit there with my awesomely engineered 8000 m/s burning up on the surface.

[max_creative]

I agree! TWR is still important!

Delta-V is too though! A good balance between TWR and Delta-V is important for things such as Tylo or Eve landings and returns. 

[cubinator]

If you're already in orbit, you could get by with a large Kerbodyne tank and a single Ant engine if you wanted to, it'd just take a lot of really long burns. Launching and landing is a different story, though.

[Kerbin vonKerbal]

10 hours ago, cubinator said:

If you're already in orbit, you could get by with a large Kerbodyne tank and a single Ant engine if you wanted to, it'd just take a lot of really long burns. Launching and landing is a different story, though.

I mean yeah, but if the TWR is stupid low, all the fuel is gonna be spent doing a burn that accelerates at .05m/s, consuming more fuel than acceleration is output. If that makes sense. 

[tseitsei89]

43 minutes ago, Kerbin vonKerbal said:

I mean yeah, but if the TWR is stupid low, all the fuel is gonna be spent doing a burn that accelerates at .05m/s, consuming more fuel than acceleration is output. If that makes sense. 

No. It doesnt make any sence because it doesnt work like that... High TWR os only really needed on takeoff and landing. Once in orbit you can use any twr. Sure high twr makes it easier and faster to plan and execute burns but they are totally doable with very low twr also. Just harder to plan in advanca and take longer to execute

For real life example see how ion engines work. VERY VERY low thrust but lots of dv and they can get really far once put in space by more powerful engines...

[T-Bouw]

No, I must agree with the high-enough-TWR-camp.
If it's too low, you can't expect to get accurate orbit-changes, as they cannot be executed in a timely fashion.

[THX1138]

14 minutes ago, tseitsei89 said:

No. It doesnt make any sence because it doesnt work like that... High TWR os only really needed on takeoff and landing. Once in orbit you can use any twr. Sure high twr makes it easier and faster to plan and execute burns but they are totally doable with very low twr also. Just harder to plan in advanca and take longer to execute

For real life example see how ion engines work. VERY VERY low thrust but lots of dv and they can get really far once put in space by more powerful engines...

 

8 minutes ago, T-Bouw said:

No, I must agree with the high-enough-TWR-camp.
If it's too low, you can't expect to get accurate orbit-changes, as they cannot be executed in a timely fashion.

I think you're both right, depending on the situation. It depends what you need to accomplish.

[Carraux]

I once lost a Duna mission because of a TWR too small. The deceleration burn took much longer than traversing the SOI.

[eddiew]

Delta-v is delta-v, regardless of how many engines you have. There are no drag losses in a vacuum, so you can take as long as you like over burns. 3km/s in the tank is the same whether you have a 0.01 TWR or a 1.0. As to whether you have time to wait around for 3 days while you shed speed and insert into orbit, that's another matter  

What you do find, is that every ton of fuel you add creates less available delta-v than the previous ton, because it has to move all the other tons of fuel in addition to the vessel's dry mass. But the problem isn't TWR per se. If you add more engines, you will still have the same delta-v available (slightly less, even). Meaning you need to add both engines and the additional fuel to move those engines. Which gets harder and harder as you get heavier  

8 minutes ago, Carraux said:

I once lost a Duna mission because of a TWR too small. The deceleration burn took much longer than traversing the SOI.

Therein lies the true problem of low TWR  

*edit* I love that the forums now turn TWR into an acronym tag... that's very rookie friendly ^^

Edited June 23, 2016 by eddiew

[Carraux]

I never ever made that mistake again. 

[magnemoe]

Low TWR makes exit burn far more complex in that you have to burn in multiple round and capture burns less efficient as you have to start braking well before low attitude.
Worst of all trying to do an duna burn over 15 minutes from low orbit will introduce errors.

[KerikBalm]

4 hours ago, Carraux said:

I once lost a Duna mission because of a TWR too small. The deceleration burn took much longer than traversing the SOI.

What kind of crazy inefficient transfer did you take there?

Anyway, you can perapsis kick with no loss in efficiency up to the edge of kerbin's SOI. From there its only a couple hundred m/s to duna...

A jool or moho transfer on the other hand..... yea, then low twr can significantly increase the dV needed.

In the case of moho, I sometimes start burning with ion engines well before hitting moho's SOI.

Moho's gravity is pretty small anyway, so not much oberth from that... most of the oberth just comes from getting close to the sun as you approach moho (or most transfers... some may dip inside moho's orbit before the encounter.. even though thats less efficient)

[wumpus]

Unless you are fighting gravity pulling you into an atmosphere or lithobraking, TWR doesn't matter at all.  The big catch is that you might need to perform a mangalyaan maneuver (often called "periapsis kicking" in KSP).  The problem is that you likely want exactly what the maneuver node thinks you are going to do (an instantaneous burn at a certain position).  Burning too far before and after periapsis means that you are expanding your orbit without all of your power going into increasing your apoapsis and may well be losing your obereth effect as well.  A mangalyaan maneuver just means firing closer to periapsis as possible, and taking a number of orbits to do it.

Doing this with stock KSP can be somewhat tricky (although those funny +/- buttons on the maneuver nodes should help, they scroll forward an extra orbit.  I can't say I've bothered with them), and of course this won't work at all once you hit escape velocity.  Hopefully your trajectory will be sufficiently flat at PE so you can burn whatever is needed to go from your escape to your transfer delta-v.

(does anybody have a link to the picture that shows the "concentric" ellipses that shows what this really is?)

Here's Red Iron Crown's guide to planning such a trip:

 

[magnemoe]

6 hours ago, Carraux said:

I once lost a Duna mission because of a TWR too small. The deceleration burn took much longer than traversing the SOI.

How do you manage? at normal speeds it a day to pass trough the SOI, even using orion engines and 20km/s it still takes hours. 
Ion engines at Moho can have this problem as its SOI is small and speeds high. 
pretty common at Gilly as SOI is tiny you tend to pass in in a few minutes

[Corona688]

20 hours ago, cubinator said:

If you're already in orbit, you could get by with a large Kerbodyne tank and a single Ant engine if you wanted to, it'd just take a lot of really long burns. Launching and landing is a different story, though.

But the ant engine is really inefficient, and the kerbodyne tank so heavy that you don't gain much by using a tiny engine, you'd be better off with the highest possible ISP.

[cubinator]

1 hour ago, Corona688 said:

But the ant engine is really inefficient, and the kerbodyne tank so heavy that you don't gain much by using a tiny engine, you'd be better off with the highest possible ISP.

True, but with the amount of fuel in the Kerbodyne tank you'd get enough delta-V to go pretty far, at the cost of an incredibly minute TWR.

[wumpus]

9 hours ago, Carraux said:

I once lost a Duna mission because of a TWR too small. The deceleration burn took much longer than traversing the SOI.

After carefully checking the subway map, I'm shocked that this possible:

delta-v Duna-Transfer: 130 m/s (assuming you PE-kicked all the way to Minmus and maybe beyond).  Hopefully you didn't take an Eve slingshot due to lack of TWR (the capture delta-v goes up, and optimizing the capture is hard).

delta-v Duna-capture: 250 m/s (and with less Obereth than Kerbol).

I still think you could have gotten away with a "set focus" and adjust the PE to be within Duna's atmosphere, but it looks possible.  Might take a few save/reload sequences to get 120+ m/s of aerobraking out of the Duna atmosphere, and that would *still* qualify as "lost a mission" to plenty of people.

Ion engines can of course make the flight, but it does take more delta-v (not a problem for such craft).  You just have to do more to match your velocity with Duna on the way.

[Carraux]

Sorry for the confusion, probably in that short form I told it wrong.

My ship got enough delta v to make the burn for getting into orbit of Duna so I tried. But the burn took longer than expected so that the burn was in no way finished as my ship passed PE. This was way back, 0.23 or so. And one of my first Duna visits, and one of my first interplanetary ships. So I got into Duna's SOI much too fast with a ship overfueled and underpowered.

Today I this would be no problem at all, for me but hey... everybody starts as a beginner... 

17 hours ago, KerikBalm said:

What kind of crazy inefficient transfer did you take there?

Anyway, you can perapsis kick with no loss in efficiency up to the edge of kerbin's SOI. From there its only a couple hundred m/s to duna...

I took a direct transfer from LKO (~150km) to Duna...

Wouldn't do this that way today but at those days...

Edited June 24, 2016 by Carraux

[Carraux]

17 hours ago, wumpus said:

Unless you are fighting gravity pulling you into an atmosphere or lithobraking, TWR doesn't matter at all.  The big catch is that you might need to perform a mangalyaan maneuver (often called "periapsis kicking" in KSP).  The problem is that you likely want exactly what the maneuver node thinks you are going to do (an instantaneous burn at a certain position).  Burning too far before and after periapsis means that you are expanding your orbit without all of your power going into increasing your apoapsis and may well be losing your obereth effect as well.  A mangalyaan maneuver just means firing closer to periapsis as possible, and taking a number of orbits to do it.

Doing this with stock KSP can be somewhat tricky (although those funny +/- buttons on the maneuver nodes should help, they scroll forward an extra orbit.  I can't say I've bothered with them), and of course this won't work at all once you hit escape velocity.  Hopefully your trajectory will be sufficiently flat at PE so you can burn whatever is needed to go from your escape to your transfer delta-v.

(does anybody have a link to the picture that shows the "concentric" ellipses that shows what this really is?)

Here's Red Iron Crown's guide to planning such a trip:

 

I got the problem Red Iron crown describes...

Anyway, you are right about TWR doesn't matter at all. At least if you are looking from an engineer's (or rocket scientist's) point of view. But for a pilot, maneuvering a massive ship with very low TWR can be tricky. Up to the point were fate knocks on your door...

Since then I take care that my ship has sufficient TWR...

[KerikBalm]

19 hours ago, wumpus said:

a mangalyaan maneuver (often called "periapsis kicking" in KSP).

Umm, that probe was launched in 2013... perapsis kicking has been a thing long long before that probe.

Why is is now the "mangalyaan maneuver" with KSP terminology being implicitely wrong.

I'll acknowledge asparagus staging as an (almost completely) KSP specific term, but perapsis kicking... that's just what it is.

[wumpus]

3 hours ago, KerikBalm said:

Umm, that probe was launched in 2013... perapsis kicking has been a thing long long before that probe.

Why is is now the "mangalyaan maneuver" with KSP terminology being implicitely wrong.

I'll acknowledge asparagus staging as an (almost completely) KSP specific term, but perapsis kicking... that's just what it is.

The Mangalyaan maneuver was actually performed by the Mangalyaan spacecraft to visit Mars (2013).  Asparagas staging dates from at least 1998 (just for the name) and the concept dates back to at least 1953 if not 1947 (unfortunately, I don't think anybody has been able to make it work).

 

[KerikBalm]

Yes, the concept for asparagus staging has been around for quite a while... but references to asparagus staging outside of KSP, particularly pre-KSP, are exceedingly sparse... to the point where I'd say it wasn't commonly accepted terminology.

It seems asparagus-stalk was used just once in one book.

My dad used to work with spy satellites in the US airforce... from one of those windowless buildings with lots of dishes for satellite uplinking... he was involved in planning many of the orbital maneuvers... he had never heard of asparagus staging when I mentioned it to him.

He immediately knows what I mean when I say periapsis kicking (although he may prefer the term perigee... but perigee appleis only to earth orbit).

Periapsis kicking is just a plain descriptive term... its something you couldn't trademark, (like one could and did trademark "snickers", but one couldn't trademark "chocolate bar")... asparagus staging... not so much.

"The Mangalyaan maneuver was actually performed by the Mangalyaan spacecraft to visit Mars (2013)"

Yea, so?since when does the most recent craft to do it gets the maneuver concept named after it? or are you saying that this was the first instance of it actually being executed? ... because it wasn't

For example, this periapsis kicked its way to a gravity assist from the moon:

https://en.wikipedia.org/wiki/PAS-22#Rescue_of_satellite

Has the term "Mangalyaan maneuver" gained currency, or did you make it up? this is the first time I've heard it... and I already knew about the probe (as evidenced by my previous message mentioning the mission).

Why should it be the Mangalyaan maneuver and not the PAS-22 maneuver? why shouldn't it just be peirapsis kicking - a generic descriptive term?

Edited June 24, 2016 by KerikBalm

[Bill Phil]

On 6/23/2016 at 3:31 AM, T-Bouw said:

No, I must agree with the high-enough-TWR-camp.
If it's too low, you can't expect to get accurate orbit-changes, as they cannot be executed in a timely fashion.

Technically true. Gravity losses can result in a loss of delta-V due to low TWR, however they usually aren't large. It just takes a very long amount of time, and eventually you'll just have to stop burning and wait an orbit.

[tg626]

High TRW=rapid acceleration 

High ISP=more miles per gallon, so to speak.

DeltaV=how much faster or slower you go, conveniently measured in the same units as your speed.