TWR in Vacuum?

[manskal]

I was just wondering if having a really crappy TWR in vacuum really has a huge detrimental effect on the craft (IE reduced dV, loooooong burn times etc) or is it just negligible? What about landing on airless moons (Mun, Minmus)? Is having optimal TWR there as important as having optimal TWR on Kerbin?

[Sof]

Long burn times for heavy ships. Otherwise it isn't all that noticable if you're just going for a small lander.

The lower TWR are the better ones to use in space as they tend to have a higher ISP, and that gives better fuel economy which equates to higher dV.

You just need to make sure your TWR is bigger than 1 for taking off. That of course will depend on the size of the world you land on.

[Xaiier]

Delta-V and TWR are (somewhat) directly interchangeable, less TWR will net you more DV, but it will take longer to do so. The only negative is if it is too low and your ship takes far too long to complete a burn, then it starts to become inefficient.

In order to land, you need at the minimum a TWR higher than 1, though somewhere around 2 is usually what a good lander has. TWR is directly influenced by weight (duh, it has weight in it) so on lighter gravity bodies you can get more TWR out of less thrust, and vice versa for heavier bodies.

[Uberick]

TWR doesn't reduce your efficiency in space, your burns will take LONGER, but they won't require any additional delta V. As a general rule, the most efficient engines(delta v), produce the lowest thrust(Nukes, Ion, etc).

For interplanetary travel this doesn't pose too much of a problem, unless you hate really long burn times(my ion probes have burn times of 2-5 hours). However, if you are trying to land on(and take off from) a planet, you WILL need sufficient TWR to escape its gravity. The wiki has a handy list of gravity values for each planet, and the mod Kerbal Engineer will calculate your TWR on each specific moon or planet to see if you can actually escape the gravity well.

[manskal]

One more question,

Is it possible to land on the Mun (or any airless body) using low thrust engines like the ant? I don't intend on taking off though.

Edited July 17, 2013 by manskal
Spellnig error

[SunJumper]

Without trying to use the ion engine, I believe any engine can get you landed on the Mun.

[Sof]

One more question,

Is it possible to land on the Mun (or any airless body) using low thrust engines like the ant? I don't intend on taking off though.

If your rocket is small enough to have a TWR of > 1 with that engine.

It is probably feasible to build crafts that can survive landing with a TWR less than one.

[AfailingHORSE]

One more question,

Is it possible to land on the Mun (or any airless body) using low thrust engines like the ant? I don't intend on taking off though.

depends on the moon/planet's gravity and the weight of your vessel.

You can land on at least Minmus, Gilly, and Ike with just ion engines, so if your weight isn't too large, then you should be able to land on the Mun with just an ant engine.

[Moon Goddess]

Don't get your TWR too low in vacuum.

I thought it was ok to dock as much stuff as I wanted to my Jool mission as long as I had enough Delta-v.

My TWR got down to .01 and it was over an hour burn. Well that means you start 30 minutes before right... well when you're in LKO, 30 minutes before Peri is Apo, burning there to raise your apo is not efficient at all and I'm not a good pilot at all, I wound up in the atmo and had to restore quicksave.

[leopardenthusiast]

Depends on the size of your lander. I've put tiny little probes on the Mun with just an OKTO, some folding solar panels, an Oscar-B tank, an Ant, and four tiny little landing gears, but I tend to ride my transfer stage most of the way to the surface. You'll NEVER be able to put a 3-man pod on anything but Gilly unless you use ridiculous amounts of Ants, though.

[Eric S]

TWR doesn't reduce your efficiency in space, your burns will take LONGER, but they won't require any additional delta V. As a general rule, the most efficient engines(delta v), produce the lowest thrust(Nukes, Ion, etc).

Actually, too low of a TWR can reduce your efficiency by reducing the amount of benefit you get from the oberth effect, but it takes an extremely low TWR for this to become a major factor.

[CCKinnison]

Moon Goddess: I've had that problem (not that extreme, but still). My process is to break it into multiple burns, about 2 minutes each side of Periapsis. For perfect transfer windows, this is obviously non-optimal, but my kerbals are both patient, and opportunists.

[Vanamonde]

Fully loaded, my interplanetary ships run at 1/3rd of a G or less. Fuel efficiency is how you move big loads long distances.

[Sof]

Moon Goddess: I've had that problem (not that extreme, but still). My process is to break it into multiple burns, about 2 minutes each side of Periapsis. For perfect transfer windows, this is obviously non-optimal, but my kerbals are both patient, and opportunists.

The Protractor mod suggests to burn so that you have completed 2/3rds before you pass the periapsis.

This is obviously for one burn, but it means it is better to do more burning before periapsis than after, so you would probably be better off doing 3mins before and 1min after or similar.

If you are taking more than 2 passes, then your TWR is probably too low.

[TheHengeProphet]

Everybody already answered the question about fuel efficiency in space, so I'll provide an answer for the other question.

Yes, it is entirely possible to land "safely" with less than 1 T:W, assuming you have a shallow enough approach, and burn close enough to periapsis so you don't drift too far past it, you can essentially get nearly zero vertical speed, and just kill your horizontal speed for a safe landing. I would not suggest this, however, as it is NOT easy...

[spikeyhat09]

TWR is only important when lifting off a planet/moon. there, the more thrust you have, the less fuel you submit to gravity. in space however, it makes no difference. you will just have to burn for a while.

if you think about it, its simple. its a thrust to WEIGHT ratio. there is no weight in space (with the exception of artificial gravity designs, which have been proven to work in ksp).

the thrust to weight ratio depends on the gravity of whatever planet you are on.

Edited July 17, 2013 by spikeyhat09

[Scottiths]

Don't get your TWR too low in vacuum.

I thought it was ok to dock as much stuff as I wanted to my Jool mission as long as I had enough Delta-v.

My TWR got down to .01 and it was over an hour burn. Well that means you start 30 minutes before right... well when you're in LKO, 30 minutes before Peri is Apo, burning there to raise your apo is not efficient at all and I'm not a good pilot at all, I wound up in the atmo and had to restore quicksave.

Actually what you do in this scenario is Pe kicks. Burn for 5-10 minutes to raise your Ap then stop. Come back around again and burn 5-10 minutes at Pe to raise Ap more. It takes a few orbits, but you can certainly still do it. Just not all in one burn anymore.

[bloodgusher]

Actually what you do in this scenario is Pe kicks. Burn for 5-10 minutes to raise your Ap then stop. Come back around again and burn 5-10 minutes at Pe to raise Ap more. It takes a few orbits, but you can certainly still do it. Just not all in one burn anymore.

Sometimes you will find that you can't get enough delta-v in one pass to go from just under escape velocity to the desired exit velocity. In this case you will waste fuel as you will not get the benefit of the oberth effect for all of your burn. You would need a very low TWR for this to be an issue, for example it may happen when using ion engines to get a heavy craft to the outer planets.

[Moon Goddess]

Actually what you do in this scenario is Pe kicks. Burn for 5-10 minutes to raise your Ap then stop. Come back around again and burn 5-10 minutes at Pe to raise Ap more. It takes a few orbits, but you can certainly still do it. Just not all in one burn anymore.

But when I'm trying to raise my AP from LKO to Jool won't I miss my window if I go to many of those orbits? after all the orbit is going to take longer and longer each lap.

[Van Disaster]

Start preparing your burn before the window, so you're already in an elliptic orbit when it arrives. The tricky bit is not being at the far side of the orbit when it arrives, but you don't have to be *that* accurate with windows, although if your parking orbit has a gigantic period it might get uncomfortable. Half the time as long as it's in roughly the right quarter I'll launch to a planet - y last Duna mission went all the way inwards to Eve's orbit ( not orbiting Eve! ) to catch up with Duna.

That is obviously horribly inefficient, but it was just to make the point.