Nukes and their TWR

[maceemiller]

Hi all. So im currently building a Jool 5 Challenge craft and decided to use Nukes to get there with however I struggled to get anywhere near the TWR I thought I'd need so decided to do some testing. I launched a small probe with a heavy fuel tank and a nuke. KER claimed I had a TWR of 0.35 however once in orbit I fired my nuke at a manoeuvre node and off my probe went as normal?

Is there something im missing here as I believed TWR had to be over 1?

[Temstar]

Your probe went off as normal? What does that mean? Can you see your AP increasing at all?

If you AP is not increasing and the nuke engine looks like it's firing then most likely some part of your ship is blocking the exhaust. You don't need TWR over 1 for orbital maneuvering, even a tiny TWR will move your AP.

[maceemiller]

4 minutes ago, Temstar said:

Your probe went off as normal? What does that mean? Can you see your AP increasing at all?

If you AP is not increasing and the nuke engine looks like it's firing then most likely some part of your ship is blocking the exhaust. You don't need TWR over 1 for orbital maneuvering, even a tiny TWR will move your AP.

Spoiler

 

As normal meant everything went as well as expected. Maybe I was reading too much into what KER said and trying too hard to number crunch I'd forgotten the obvious! Thanks for the quick reply.

[Hannu2]

You need TWR of over 1 when you lift from surface. But when you are on orbit you do not have to fight against gravity. You push forward and even extremely low TWR can accelerate your ship to high velocities. Probes with a real ion engine can have TWR of 1/100000 compared to g and they are capable of high dv interplanetary travels from LEO (actually much more capable than probes with traditional thrusters).

However, very long burns are difficult to time. There are not suitable tools in KSP or any mods. My thumb rules are:

1. stage (+possible SRBs):  dv 1500-2000 m/s, TWR (at start) 1.3 - 1.5.

2. stage: dv 1500-2000 m/s, TWR (at start) 0.7-1.0.

At this point ship is on low Kerbin orbit. If my destination is Mun or Minmus, it is typically 100 km and 300 km for interplanetary operations.

3. stage: dv depends on target and mission profile, typically from 1000 m/s (Mun) to 5000 m/s (Moho), TWR 0.25-0.35.

The lower TWR you have in ejection burns the larger errors you have, if you do not calculate exact trajectory and time burn with separate software. Therefore I use 300 km orbit and TWR around 0.3. The higher parking orbit the less errors you get but you have to use more dv because you do not get Oberth effect. And the higher TWR the shorter and more accurate burn but payload ratio is hit by heavy engines.

[Warzouz]

3 hours ago, maceemiller said:

Is there something im missing here as I believed TWR had to be over 1?

Back beta 0.9, I did a tylo landing with a nuke powered lander. It was 14 tons with 2 LVN. The TWR before deorbiting was around 0.95 (Tylo-related). That's not an issue in the begining. You first need to reduce your horizontal speed, the gravity will pull you down to tue surface but in the begining the pull is quite manageable. When the pull start to increase, you already cancelled a good part of your horizontal speed and your ship is lighter. your TWR raise above 1 and you can land.

To do that, you can't follow retrograde, you're velocity will increase too much and you won't be able to cancel it. You'll have to do an unefficient landing : you start burning retrograde but as you vertical velocity start to rise, you must conteract it by raising AoA. Vertical velocity must be kept under control (less than 200m/s) while you cancel horizontal speed.

I don't have the lander precise specs, but I think that the TWR was 1.25 when I landed. When reorbiting, I burnt 6000m/s in total, way more than the 2270 x2 I should have. BTW, I also landed a TWR=3 probe using exactly 2267m/s.

The issue with this design is that LVN engine weight was risen to 750kg more since 1.0. and physicless parts have mass now. Building such a LVN powerd ship is now quite hard and much more massive. It may not even be practical.

[WhiteKnuckle]

6 hours ago, maceemiller said:

Is there something im missing here as I believed TWR had to be over 1?

TWR = Thrust to Weight Ratio

Weight varies by the local gravity.

Having a TWR of 1 or greater simply means you have enough thrust to lift your weight on a given body. A ship that has a TWR of 1 on the Mun will have enough thrust to lift the weight of the craft in the Mun's gravity well, but that same ship will weight more on Kerbin, and since it's thrust hasn't gone up it's TWR will go down.

I believe KER will let you change bodies, so you can see if your ship has sufficient TWR on the moons you plan on landing on.

 

 

Once in a stable orbit, your TWR can be any number, and the ship will move. A higher TWR will simply mean it accelerates faster.

 

 

[RizzoTheRat]

KER can also give you a calculation based on atmospheric or vacuum, if you've accidentally left on atmosphere you're very different TWR reading, especially with nukes which are rubbish in atmosphere.

I quite often haul stuff around with nukes at around 0.2 TWR, as others have said TWR>1 is only needed if you're suborbital.

[maceemiller]

I guess Im so wrapped up in trying to build this rocket i took what KER had said and wasnt thinking correctly. Makes perfect sence now so thankyou for all your comments. Might have a break from building and just have some fun

EDIT: so I did have some great fun tonight. My friend came round with both his kids, a 6 year old girl and a 9 year old boy. The lad took an instant interest in KSP and within 1 hour had almost made LKO his own game has been saved and ready for his return.

@SQUAD will be happy to know the next generation of the real world's gamers, scientists, pilots or engineers are already loving your game.

 You have probably created a benchmark, not just in gaming but for peoples futures. That is simply stunning....

Edited February 12, 2016 by maceemiller