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From the specifications, how do you determine a powerful rocket?


MetricKerbalist
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Hi KSP colleagues,

I hope that I am posting this question in the correct forum.

Just a few hours ago, I discovered the website KerbalX.com.  I learned about this from this forum's The Spacecraft Exchange.  The website KerbalX.com impressed me so much that I have now joined it.

So here is my question.  I am trying to find a good heavy lift rocket so that I could go to the far ends of the Kerbol system if I chose to try.  For right now, while I am still learning KSP, I am playing in Sandbox mode, and therefore I am not trying to be efficient.  I just don't want to run out of fuel, and if I have excess fuel, then for right now I don't care.

Well, then, what is the main specification to which I should direct my attention?  Wouldn't it be delta-V, or am I wrong about that?  If not delta-V, then what?

I cannot say that I have examined this thoroughly, but I don't see that delta-V is listed on any of the summaries of the rockets in KerbalX.  Maybe it is listed, and I am not looking in the right place.  Or perhaps I should be looking for some other specification to measure the rocket's power.

Thank you.

Stanley

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36 minutes ago, MetricKerbalist said:

Wouldn't it be delta-V, or am I wrong about that?  If not delta-V, then what?

There are two main things to look for:

  • Delta-V for how far you can hope to reach: the more you can change your velocity, the bigger the differences in start/end trajectory.
  • TWR for how much payload you can hope to lift/move: the more TWR of the unloaded craft, the more extra mass you can add.

 

41 minutes ago, MetricKerbalist said:

I cannot say that I have examined this thoroughly, but I don't see that delta-V is listed on any of the summaries of the rockets in KerbalX.  Maybe it is listed, and I am not looking in the right place.

KerbalX.com doesn't list dV/TWR because it only shows what is directly readable from the craft file - dV and TWR are not saved as numbers in the craft file. The potential dV and TWR have to be calculated from the sum of the craft's fuel, mass, and the attributes of the different (sets of) engines in different stages, while adjusting for atmospheric pressure and gravity. So it is left up to the uploader to provide that information on the craft page if they so wish, in whatever form they feel is appropriate.

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41 minutes ago, swjr-swis said:

KerbalX.com doesn't list dV/TWR because it only shows what is directly readable from the craft file - dV and TWR are not saved as numbers in the craft file. The potential dV and TWR have to be calculated from the sum of the craft's fuel, mass, and the attributes of the different (sets of) engines in different stages, while adjusting for atmospheric pressure and gravity. So it is left up to the uploader to provide that information on the craft page if they so wish, in whatever form they feel is appropriate.

Hi @swjr-swis,

Hmm....  Well, that's inconvenient.

I still like KerbalX.com very much.  If they required those who uploaded files to specify dV and TWR, however, that would help KerbalX.com users.  KerbalX is a great site, but requiring this information would make it even better in my opinion.

OK.  Thank you for your instructive reply.

Stanley

 

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25 minutes ago, MetricKerbalist said:

I still like KerbalX.com very much.  If they required those who uploaded files to specify dV and TWR, however, that would help KerbalX.com users.  KerbalX is a great site, but requiring this information would make it even better in my opinion.

In basic principle I agree with you - yes, it would be even better if it was able to show those numbers in some meaningful way that would be helpful for a potential downloader to use in their particular context.

It is however not a trivial bit of calculation to code, not by a long shot, not even if we could reach a clear consensus of The One Context Above All Other Contexts to choose for the one set of numbers to show (you've already encountered this issue yourself, having to switch the in-game readout between settings, to get the more relevant numbers depending on what you want to know at the time). Keep also in mind it's a volunteer-run site paid for by its owner and a few fan contributions, which can only fund so much hosting capacity. Current processing loads are already straining its capacity at times.

So all things considered and within the hosting limitations available, KerbalX gives priority to easy uploading and lots of freedom on how you wish to present your craft, powerful searching on an extensive list of fixed attributes of craft, and easy downloading so it can be quickly tested in your own game. "Let the sharing site do what it does best, and the game what it does best."

 

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Hi @swjr-swis,

Yes, everything you said makes good sense.  Making suggestions as I did is easy.  You have convinced me, however, that carrying out such suggestions is difficult.

At this point, then, I rest content that KerbalX.com is a great site.  I am glad that I joined, and I look forward to making use of it.

I appreciate your getting back to me.  Thank you.

Stanley

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On 6/24/2021 at 4:53 PM, MetricKerbalist said:

Hi KSP colleagues,

I hope that I am posting this question in the correct forum.

Just a few hours ago, I discovered the website KerbalX.com.  I learned about this from this forum's The Spacecraft Exchange.  The website KerbalX.com impressed me so much that I have now joined it.

So here is my question.  I am trying to find a good heavy lift rocket so that I could go to the far ends of the Kerbol system if I chose to try.  For right now, while I am still learning KSP, I am playing in Sandbox mode, and therefore I am not trying to be efficient.  I just don't want to run out of fuel, and if I have excess fuel, then for right now I don't care.

Well, then, what is the main specification to which I should direct my attention?  Wouldn't it be delta-V, or am I wrong about that?  If not delta-V, then what?

I cannot say that I have examined this thoroughly, but I don't see that delta-V is listed on any of the summaries of the rockets in KerbalX.  Maybe it is listed, and I am not looking in the right place.  Or perhaps I should be looking for some other specification to measure the rocket's power.

Thank you.

Stanley

The best I can recommend is that you look for payload to LKO in tonnes, assuming that the creator provided that info. You're going to have to design the rest of your own mission anyway.

 Otherwise you can guesstimate the payload fraction of a lifter by its pad mass, assuming it's all liquid fueled. An average lifter should be able to orbit 10-15% of it's launch mass as payload, a really excellent one might hit 25%.

 This number goes down when solid boosters are used.

TL/DR: If you want to launch really big payloads, find a really big rocket that's not a SSTO.

HTHs,

-Slashy

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3 hours ago, GoSlash27 said:

[Stuff]

3 hours ago, Zhetaan said:

[Spit take]

 Slashy!  When did you get back?  It's nice to see you around.

 

 

Anyway, to avoid going too far off-topic:

On 6/24/2021 at 5:53 PM, MetricKerbalist said:

Well, then, what is the main specification to which I should direct my attention?  Wouldn't it be delta-V, or am I wrong about that?  If not delta-V, then what?

Everything in rocketry is a trade-off, so different needs emphasise different things at different times.  Having 70,000 m/s of delta-V on the pad is worthless if the rocket lacks the thrust to lift off.  Slashy is correct in that payload fraction is a good choice for getting things into orbit.  'Bigger = more powerful' is really not applicable here, but to get you started, it's better than nothing until you get more of a feel for it.

And definitely stay away from SSTOs for now.  They offer intriguing possibilities, but none of the order that I think you're looking for presently.

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This is obviously a very open-ended question, but as a rule of thumb I would have the first ~2,000 m/s of dV that my stack has at a TWR of >1.4, and the next 1,000 m/s or so at a TWR of >1. That should be plenty to get you to LKO if you follow an efficient ascent profile.  After that, unless I plan on landing anywhere, I find that a TWR of 0.2-0.4 is sufficient to do all necessary orbital maneuvers without it becoming too tedious. Making a transfer  stage with Nervs that has that sort of TWR and upwards of 5km/s dV is pretty straightforward, and with that much dV you can get to pretty much anywhere and back without getting fancy about gravity assists.

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