Can a Terrier lift a 5.25 ton craft to orbit from Duna?

[JoeSchmuckatelli]

I've started playing again after a hiatus.  Never tried to land on Duna much less take off.  At my tech level, do I need something like Reliant?

[Streetwind]

Rather than tell you the answer, I'm going to tell you how to discover it yourself without even needing to go to Duna.

In the editor, build the craft you hope to launch from Duna, or a sufficiently similar stand-in. Now, click on the stage that has the engine in it to expand its view. You should be able to see its TWR there. Finally, go to the dV app in the toolbar on the bottom right. Make sure it is set to sea level. In the dropdown menu at the top, select Duna.

Your stage info will now show you Duna-relative TWR at Duna's sea level. If this number is greater than 1, you can launch with your current engine. If you then set the dV app to vacuum and refer to the Community dV Map for a typical cost to launch into low Duna orbit, you can check if you have enough dV as well.

[JoeSchmuckatelli]

 

7 hours ago, Streetwind said:

n the dropdown menu at the top, select Duna.

Your stage info will now show you Duna-relative TWR at Duna's sea level. If this number is greater than 1, you can launch with your current engine. If you then set the dV app to vacuum and refer to the Community dV Map for a typical cost to launch into low Duna orbit, you can check if you have enough dV as well.

Holy vrapnalls!  I did not know that existed. 

 

Thanks for teaching this kid how to fish! 

[Fraktal]

For a Mk1-3 pod, an X200-16 fuel tank and another 1-2 ton of parachutes, landing gear, solar panels, antennas and a science payload, two Terriers are more than enough on Duna with 2000-something delta-V. Two Thuds are overkill, giving around 5 TWR for slightly less delta-V.

Edited December 11, 2020 by Fraktal

[jimmymcgoochie]

With around 1/3g gravity and so little atmospheric pressure that engines operate at effectively vacuum efficiency, Duna is pretty easy to take off from. A Terrier will be enough for a small lander, usually 1.25m and single occupant, or you could build something larger in a two-stage configuration with drop tanks/boosters for the landing.
If one Terrier isn’t enough and you have the Dart aerospike engine unlocked (assuming career/science mode) use that instead of multiple Terriers- it’s more efficient and produces the thrust of three Terriers for the weight of two plus it’s the same diameter and only slightly longer.

[JoeSchmuckatelli]

I'm... flabbergasted that after so many, many hours of playing this game, that I never knew this.  That I could have been predicting / know my Lander's abilities - and built more efficient craft - is... well I'm flabbergasted.

 

As is - I've just brut forced stuff to the moons and back - which wasn't that costly in time invested or materiels - but going for my first interplanetary... 

 

Anyway - thanks all for the help!

 

[AHHans]

29 minutes ago, JoeSchmuckatelli said:

I'm... flabbergasted that after so many, many hours of playing this game [...]

Well, I don't know how many hours in game it took me before I learned that Kerbals on EVA have suit-lights. But I do know, that I got really good at finding my way around a spacecraft in pitch-black darkness.

I guess we all have similar stories to tell.

[king of nowhere]

4 hours ago, AHHans said:

Well, I don't know how many hours in game it took me before I learned that Kerbals on EVA have suit-lights. But I do know, that I got really good at finding my way around a spacecraft in pitch-black darkness.

I guess we all have similar stories to tell.

it took me dozens of rescuing stranded kerbals before discovering that the jetpack can move in six dimensions. i only knew the wasd commands, but not the shift and ctrl. my rendez-vous manuever was trying desperately to move by the incoming ship and grab a ladder while it zipped past the equatorial plane

EDIT: while my brother didn't see the twr factors in the VAB and was calculating everything with a spreadsheet

Edited December 12, 2020 by king of nowhere

[Spricigo]

On 12/11/2020 at 4:08 AM, Streetwind said:

Rather than tell you the answer, I'm going to tell you how to discover it yourself without even needing to go to Duna.

Excellent.

Nonetheless, I will point that a Terrier can indeed lift a 5.25t craft from Duna, only because I'd like to raise the question: Is the terrier the best option for this task?

We can discard the option for anything heavier because either the Terrier have better ISP or the extra weight  negates the Isp advantage. Ants and Spider being so weak would require impractical clustering to compete and the puff is just terrible in so many ways. But Spark, Twitch and Cub lower weight may still give you an edge, it boils down to how much of those 5.25t is fuel and if that is enough to provide the necessary deltaV   (Understanding mass fractions, perhaps the single most important insight when it comes to building rocket stages )

 

In any case, I took  a look at the stats of craft with 5.25t more 1-2 of those engines:

•   1xTwitch, 80kg TWR1.02  Isp 289s
• 1xSpark, 130kg TWR 1.25 Isp 312s
•  1xCub, 180kg TWR1.99 Isp 308s
• 2xTwitch, 160kg TWR2  Isp 289s
• 2xSpark, 260kg TWR 2.44 Isp 312s
• 2xCub, 360kg TWR4 Isp 308s

For comparison, Terrier, 500kg, TWR 3.36 Isp 327s. 

I'd take a closer look before deciding.

 

[JoeSchmuckatelli]

5 hours ago, Spricigo said:

Excellent.

Nonetheless, I will point that a Terrier can indeed lift a 5.25t craft from Duna, only because I'd like to raise the question: Is the terrier the best option for this task?

We can discard the option for anything heavier because either the Terrier have better ISP or the extra weight  negates the Isp advantage. Ants and Spider being so weak would require impractical clustering to compete and the puff is just terrible in so many ways. But Spark, Twitch and Cub lower weight may still give you an edge, it boils down to how much of those 5.25t is fuel and if that is enough to provide the necessary deltaV   (Understanding mass fractions, perhaps the single most important insight when it comes to building rocket stages )

 

In any case, I took  a look at the stats of craft with 5.25t more 1-2 of those engines:

•   1xTwitch, 80kg TWR1.02  Isp 289s
• 1xSpark, 130kg TWR 1.25 Isp 312s
•  1xCub, 180kg TWR1.99 Isp 308s
• 2xTwitch, 160kg TWR2  Isp 289s
• 2xSpark, 260kg TWR 2.44 Isp 312s
• 2xCub, 360kg TWR4 Isp 308s

For comparison, Terrier, 500kg, TWR 3.36 Isp 327s. 

I'd take a closer look before deciding.

 

Thanks for the insight.  My problem is that I only have a vague understanding of how isp works.  TWR above 1 - that I get.  But as someone who legit failed calculus 3 times in college (I'm nothing if not persistent)' running the numbers' is difficult for me.  I've a story /visual driven brain - and I've yet to find a tutorial that really helps me to picture what is going on when I look at isp numbers. 

[Streetwind]

2 hours ago, JoeSchmuckatelli said:

I've yet to find a tutorial that really helps me to picture what is going on when I look at isp numbers.

The most important takeaway is: Isp is measured is seconds because one of the ways of looking at it is simply the duration the engine can run at a specific amount of thrust with a specific amount of propellant. Which amounts those are aren't important. It just means that between any two engines with the same thrust and the same amount of available propellant, the one with 10% higher Isp will run 10% longer before consuming all the propellant.

An example is a Terrier and a Reliant. In a vacuum, they have 345s and 310s Isp, respectively. Throttle the Reliant to 25%, and both engines will produce the same 60 kN of vacuum thrust. Now mount them on the same tank.

Because the Terrier has 11.3% more Isp, it will burn 11.3% longer even though it runs at the same thrust and has the same amount of propellant available. So in a world where the Reliant drains the tank and flames out after 100 seconds, the Terrier will keep going for another ~11 seconds longer because it is more efficient. You can do this experiment in game and verify that's how it works.

But if even that mental image isn't really doing it for you, don't worry. You don't need to get hung up on Isp too much. It's not what really determines your available dV; it's just a small contributing factor. The real centerpiece of it all is the mass fraction.

 

Edited December 12, 2020 by Streetwind

[JoeSchmuckatelli]

5 hours ago, Streetwind said:

The most important takeaway is: Isp is measured is seconds because one of the ways of looking at it is simply the duration the engine can run at a specific amount of thrust with a specific amount of propellant. Which amounts those are aren't important. It just means that between any two engines with the same thrust and the same amount of available propellant, the one with 10% higher Isp will run 10% longer before consuming all the propellant.

An example is a Terrier and a Reliant. In a vacuum, they have 345s and 310s Isp, respectively. Throttle the Reliant to 25%, and both engines will produce the same 60 kN of vacuum thrust. Now mount them on the same tank.

Because the Terrier has 11.3% more Isp, it will burn 11.3% longer even though it runs at the same thrust and has the same amount of propellant available. So in a world where the Reliant drains the tank and flames out after 100 seconds, the Terrier will keep going for another ~11 seconds longer because it is more efficient. You can do this experiment in game and verify that's how it works.

But if even that mental image isn't really doing it for you, don't worry. You don't need to get hung up on Isp too much. It's not what really determines your available dV; it's just a small contributing factor. The real centerpiece of it all is the mass fraction.

 

Your mass fraction tutorial is stunning.  I'm gonna have to read it again - but thanks very much for the resource!