Jump to content
  • 0

Ion probe launches


Go to solution Solved by Snark,

Question

So I've been setting up more and more relays, and my Kerbin SOI is just blanketed with radio comms, much like Earth. However, everytime I try to design a long-distance probe, to start setting up my extra-Kerbin relay network, I run into the problem of my ion powered probes having virtually zero thrust. Raw stats of my midrange relay however is a mass of slightly under 4.5 tons, and 11,142 m/s delta v. TWR is ridiculously low at a mere 0.05.

 

My basic (mid-range) relay is a HECS2, powered by 2 1x6 solar panels and 4 RTG's. Communications covered by an RA-15 relay dish, an HG-5, and a Communotron-16 S for local coverage. Primary propulsion is covered by two PB-X750 Xenon tanks and a single Dawn engine. I'd have a screenshot, but my recently installed EPL mod has a panel that doesn't go away so I'm deleting that mod to get a proper screenshot.

 

Is this the major issue with ion probes? Because running a burn of three or four hours with a x4 timewarp doesn't really feel enjoyable to get a relay probe from Kerbin SOI on it's way to Duna before accounting for reverse thrust to stay in orbit after arriving. I have little experience outside the Kerbin SOI and what experience I do have is primarily using considerably larger (and more) engines

Link to post
Share on other sites

15 answers to this question

Recommended Posts

  • 1
  • Solution
16 hours ago, Somtaaw said:

My basic (mid-range) relay is a HECS2, powered by 2 1x6 solar panels and 4 RTG's. Communications covered by an RA-15 relay dish, an HG-5, and a Communotron-16 S for local coverage. Primary propulsion is covered by two PB-X750 Xenon tanks and a single Dawn engine.

First... why all the multiple antennas?  The RA-15 has a power so much insanely higher than the HG-5 that adding the HG-5 doesn't really do anything.  And given that you have the RA-15, why do you need the Communotron-16 S at all?  Seems to me that the RA-15 is the only antenna you need.

Anyway, on to your problem.  You're way over-killing the dV requirements, and ions aren't really needed to go to Duna.  With a good transfer window, you only need ~1050 m/s (plus a small maneuvering reserve) to go from LKO to Duna orbit, if you're aerobraking on arrival.   You need only ~1700ish m/s to do it even if you're not doing any aerobraking.

So, how much ship do you need to do that?

Let's say you put together a craft consisting of the RA-15 (0.3t), plus a HECS2 (0.2t), plus a 0.5-ton LFO tank (0.5625t), plus a Spark engine (0.1t), plus a couple of the folding solar panels (0.025t each).  You're looking at a total satellite mass, with full tank, of 1.21t, of which 0.5t is fuel.

Given the Spark's vacuum Isp of 320, that gives you a dV of just under 1700 m/s... which coincidentally is about what you need to go from LKO to low Duna orbit without aerobraking.  Toss in some aerobraking, and that's enough to get you to your destination right there.  You could move up to a 1-ton fuel tank instead of 0.5-ton, to give yourself some extra breathing room:  that would boost your dV up to 2600 m/s, which would give you a big safety margin, allow for suboptimal transfer window, and still keep the total ship mass to under 2 tons.

13 hours ago, bewing said:

IIRC, it takes 12 ion engines to put a 1G acceleration on a 2.5 tonne mass

Side note... it's physically impossible to get 1G acceleration with ion engines, even with an infinite number of them.  The engine itself masses 0.25t, which sets an absolute upper limit of just over 0.8g even if it didn't have to deal with any mass for fuel tanks, probe core, solar panels, or anything else.

Link to post
Share on other sites
  • 0

Ion engines have very low thrust for KSP (it's actually literally thousands of times more powerful than real ion propulsion). They are mostly intended for very light probes or vessels which don't end up having days-long burns. Giving them more thrust would make them extremely overpowered, right now they are "balanced" (as balanced as a thing having 4200s of Isp can be).

Link to post
Share on other sites
  • 0

So a 4.5 ton "probe" is probably far too large for a single ion engine then? I'll probably have to start redesigning then, maybe stripping the thruster section into it's own module and just making the probe purely payload with nothing but RCS.

Link to post
Share on other sites
  • 0

It's just a generic mid-range communication relay satellite. I have to use the community Commnet link I just found out about an hour ago to double check my dish ranges, but this particular variant should be going into polar orbits on Duna and Dres, with possible additional clones moving into inclined solar orbits for extra relays.

 

It's just spending the multiple hour long burns to go from LKO to destination that bothers me, the actual flights are fine but while I'm thrusting I can't turn timewarp high enough to bother.

Link to post
Share on other sites
  • 0

Duna? You underestimate what small LFO engine can do - Ant is one of my favorite engines; and since 1.2, Spark get Isp buffed (300->320), which make it also a decent choice.

And just to make sure - you didn't plan to capture into equatorial orbit and then go to polar, did you?

Link to post
Share on other sites
  • 0

IIRC, it takes 12 ion engines to put a 1G acceleration on a 2.5 tonne mass -- and that takes a lot of EC. So yes, ion engines just take a while. Start your burn, and then go take a shower or do the dishes or something.

Link to post
Share on other sites
  • 0

Ion drives are just fantasies and false promises, even when you have the patience. They excel in very specific use cases, but not much else.

From Kerbin orbit to Mun/Minmus, you just need a lot of patience and pre-planning on where/when you will finally intercept the Mun's orbit. You have enough attempts for periapses kicks in this situation, it just uses more RL time.

From Kerbin orbit to Duna/Eve, when you reach almost escape velocity you will only have one periapses kick left. Extremely low thrust can only get you that far in 6minutes time. Probably enough for the 100m/s extra that you need.

From Kerbin orbit to Jool, you need some +1000m/s extra after escape velocity. No way Ion drives are going to get you that fast in 6minutes...so what happens is you lose out on the Oberth effect badly(the extra needed dv becomes much more than just a mere 1000m/s). Which is fine since Ion drives have a lot more dv going for them, but what is the use of it all when you have to use most of that extra dv?

From interplanetary to Moho, Ions are great. Moho doesn't offer a lot of Oberth anyway, so you can just start slowing down like halfway into it's SOI(and solar panels are super charged there). This is completely different from Eve again, where you have heaps of dv and one chance to capture into orbit(cautious aero capture might be better)

All in all, if the mission requires mostly deepspace burns,moons/eelloo intercepts and the like, then Ions are advantageous. If the mission needs launching, landing, quick captures, then nukes are a pretty good compromise.

Link to post
Share on other sites
  • 0

I don't think you need more than the ra-15 for everything,  maybe wrong.

Also I tend to only use ions on quad couplers (4engines)cos anything less is painful.

Gigantors/large batteries should be good out to duna but beware of the dark side luke or the force will definitely not be with you :wink:

Edited by Palaceviking
Link to post
Share on other sites
  • 0

Honestly, I'd say the answer is you're carrying too much fuel. An ion probe with 3.5km/s dV in LKO can go pretty much anywhere. 11km/s is nuts. Cut fuel, replace with more engines and batteries/panels.


Alternatively, use one of the small chemical engines; an Ant, Spider cluster, Spark, or even a Terrier for large probes (4.5t is probably too small to warrant that). The TWR I use for orbit work depends on what kind of burn I'm expecting to do. Something like circularizing a moderately flat suborbital trajectory (e.g. 100x0 -> 100x100) can easily be done on 0.05, but if I'm trying to execute a major burn (e.g. going interplanetary) I wouldn't consider anything under 0.2 for apsis kicks or 0.5ish for doing it in one shot. Low TWRs are a pain.

Link to post
Share on other sites
  • 0

All of the above is good advice...... but

This is one of the few chances you'll get to use ion engines properly in ksp, so I'd go with it just for the experience and learning along the way. 

Only problem is if you don't go polar you'll be doing a large part of your capture burn in darkness,  but problems call for solutions :wink:

Link to post
Share on other sites
  • 0
On 12/19/2016 at 0:06 PM, Snark said:

First... why all the multiple antennas?  The RA-15 has a power so much insanely higher than the HG-5 that adding the HG-5 doesn't really do anything.  And given that you have the RA-15, why do you need the Communotron-16 S at all?  Seems to me that the RA-15 is the only antenna you need.

Anyway, on to your problem.  You're way over-killing the dV requirements, and ions aren't really needed to go to Duna.  With a good transfer window, you only need ~1050 m/s (plus a small maneuvering reserve) to go from LKO to Duna orbit, if you're aerobraking on arrival.   You need only ~1700ish m/s to do it even if you're not doing any aerobraking.

Habit of using extra antenna from my time using RemoteTech, and I instinctively assumed that having a long range dish meant utterly worthless for close-range, so for anything that isn't in Kerbin/Mun/Minmus area, I always triple antenna. Long range antenna to actually reach Kerbin, an omni wire antenna for general use, and possibly a short- to mid-range dish for use if I'm orbitting high enough up the omni has issues.

 

So from your well-explained answer, I am most definitely over-engineering my probe, time to strip her down and probably tear off the useless HG-5, I'll probably keep the Communotron as a back-up just in case the standard Relay shifts more RemoteTech.

 

On 12/20/2016 at 9:24 AM, Palaceviking said:

All of the above is good advice...... but

This is one of the few chances you'll get to use ion engines properly in ksp, so I'd go with it just for the experience and learning along the way. 

Only problem is if you don't go polar you'll be doing a large part of your capture burn in darkness,  but problems call for solutions :wink:

Ah, well that's why my little relays rely on RTG's, one of these days that's going to bite me on the butt if I ever pickup one of the mods that make RTG's actually run out but my relay satellites run a loose 1:2 RTG to Solarpanel ratio. Really helps out for the longer darksides, not to mention a single RTG is enough to keep a probe powered up long enough to activate a non-ion engine which generates electricity to keep everything else running. Without using something like NearFuture Electrical, it's just too hard to cram enough EC onto a probe to rely purely on panels for power.

 

 

Thanks for the answers, time to go and let Jeb fill his junkyard with what we rip off the old mark 1 relays before calling them mark 2's :D

Link to post
Share on other sites
  • 0
11 minutes ago, Somtaaw said:

Habit of using extra antenna from my time using RemoteTech, and I instinctively assumed that having a long range dish meant utterly worthless for close-range, so for anything that isn't in Kerbin/Mun/Minmus area, I always triple antenna. Long range antenna to actually reach Kerbin, an omni wire antenna for general use, and possibly a short- to mid-range dish for use if I'm orbitting high enough up the omni has issues.

Yup, makes perfect sense in RemoteTech... just not in stock CommNet, where (to use RemoteTech terminology),

  • every single antenna is an omni
  • every antenna has the exact same electricity cost to keep it turned on, which is zero
  • except for the HG-5, every relay antenna is a fixed dish that doesn't snap off in atmosphere

There's basically never any reason in CommNet to have more than one antenna, unless 1. you're combining multiple antennas to extend your range, or 2. your long-range antenna is one of the deployable ones that snap off in atmosphere, and you need to aerobrake or something, so you have a shorter-range-but-sturdier one for that purpose, or 3. you haven't climbed the tech tree to the long-range relay dishes yet, so you have a short-range relay plus a long-range non-relay.

11 minutes ago, Somtaaw said:

I'll probably keep the Communotron as a back-up just in case the standard Relay shifts more RemoteTech.

Seriously doubt that's gonna happen at this point.  RemoteTech is cool, but it's also at a complexity level that puts it way out of range of the stock game-- I'm really skeptical that Squad would ever go that route.  If they wanted RemoteTech-style complexity, they would have implemented it that way from the get-go.  (It's not like they didn't know about the existence of RemoteTech when they built their own CommNet, so it's clear that going a different, simpler route was a conscious decision on Squad's part.)

On the other hand, it's not like having the Communotron on there is hurting you much, it's not exactly a heavyweight part.  :)  It just doesn't actually do anything, is all.  (In this case, it actually, literally doesn't do anything-- i.e. doesn't boost your main antenna's range even 0.000000000000001%-- because the 16 S, unlike its deployable cousin, isn't combinable.  But even if it were, its contribution would still be negligible.)

Link to post
Share on other sites
  • 0
6 minutes ago, Snark said:

Seriously doubt that's gonna happen at this point.  RemoteTech is cool, but it's also at a complexity level that puts it way out of range of the stock game-- I'm really skeptical that Squad would ever go that route.  If they wanted RemoteTech-style complexity, they would have implemented it that way from the get-go.

On the other hand, it's not like having the Communotron on there is hurting you much, it's not exactly a heavyweight part.  :)  It just doesn't actually do anything, is all.  (In this case, it actually, literally doesn't do anything-- i.e. doesn't boost your main antenna's range even 0.000000000000001%-- because the 16 S, unlike its deployable cousin, isn't combinable.  But even if it were, its contribution would still be negligible.)

Okay, okay... I'll keep the Communotron in case I ever decide to start using RT again instead of just using the stock commnet >.< I'm a great believer in enjoying unmodded games, and Squad is great for taking popular mods and making them stock.

Because nothing is ever truly as annoying as updating the game, and loading up a save to see endless summarized "Craft X had modded parts nolonger available and was removed" and you went from a huge space fleet and industry to nothing..... and you've both forgotten to do a backup save recently and no longer have any older backups either >.<

 

But thanks for the probe redesign suggestions, what would you suggest for a longer ranged concept of the same general concept (as close to stock only as possible), orbit to be no closer than slightly inside Jool. I design my probes to network (RT experience), so I not only drop relays into target planetary orbits, but additional relays that copy planetary orbital information but are offset 45-90 degrees.

Link to post
Share on other sites
  • 0
39 minutes ago, Somtaaw said:

Because nothing is ever truly as annoying as updating the game, and loading up a save to see endless summarized "Craft X had modded parts nolonger available and was removed" and you went from a huge space fleet and industry to nothing..... and you've both forgotten to do a backup save recently and no longer have any older backups either >.<

Yeah, I hear ya on that one.  :)

My own coping strategy for that particular form of heartbreak is:  I just schedule my careers so that I always start a new one when there's a major KSP update.

I know that there are players out there who like to have very long-running KSP careers, which span years and multiple major KSP versions, but that's not me-- I'd get bored, playing that way.

My own play style is to start a new career, picking a particular set of mods, house rules, and an idea of what I consider "done" right from the start.  Then I play it for a few months, until my self-imposed "done" condition, at which point I mothball it and start a new career (typically with a different set of mods and "house rules" than the previous one).  That's how KSP stays fresh for me over a period of years.

39 minutes ago, Somtaaw said:

what would you suggest for a longer ranged concept of the same general concept (as close to stock only as possible), orbit to be no closer than slightly inside Jool.

When I want to scale up slightly from the above:  try an RA-100 antenna sitting on a 1.25m probe core, atop a 2.5m fairing, atop an 8-ton LFO tank with Poodle.  That ends up having scads of dV, and is easy to stick on top of a 2.5m get-to-LKO stack.

Or, if you're running SpaceY (as I do), which has some really nice interstage adapters:  instead of an 8-ton LFO with Poodle, I like to use the short, fat 4-ton LFO, on top of a 2.5m-to-1.25m interstage adapter, with Terrier.  Still has lots of dV, is smaller and lighter, and still fits neatly on top of a 2.5m stack thanks to the cool SpaceY interstage adapter.  Love those things; such a simple and elegant solution to a really common problem with rocket design.  They really ought to be stock, IMHO (and I don't say that about many mod parts).

39 minutes ago, Somtaaw said:

I design my probes to network (RT experience), so I not only drop relays into target planetary orbits, but additional relays that copy planetary orbital information but are offset 45-90 degrees.

I simply don't bother with that, at all.  There's simply no practical point.  Building precise constellations of satellites in synchronized orbits is fine, if you enjoy doing it for its own sake (i.e. the challenge of doing that is fun gameplay for you)-- but I don't find that fun, at all.  Probably because it's technically impossible to get an exact synchronization, and I'm OCD enough that being almost-but-not-quite exact would bug me, and using a cheat to edit them into precision goes against my grain.  And in any case, "precisely align orbits to each other" isn't a challenge that I personally find interesting at all, so I don't do it.

Instead, I take the exact opposite approach.  I don't align comsats at all.  I just spam them in various random orbits, and rely on there pretty much always being some random path back to KSC.  If I don't have enough coverage, spam some more.  It works well.  (Small comsats are so cheap to launch, they might as well be free.  A single run-of-the-mill "science data from space around the Mun" that takes ten seconds to complete using an existing satellite will pay for many comsats.)

In the case of solar relays:  I don't bother launching dedicated ones.  "Launch a new station on a solar orbit" is one of my contract cash cows (they're incredibly lucrative, and very quick to slap together)-- just fire-and-forget.  So as soon as I've climbed the tech tree to have the RA-100 antenna available, I just make a point of sticking one on every throwaway "launch a new station on a solar orbit" ship that I have, and that gives me all the solar network I need.

Bearing in mind that solar-orbit relay satellites are basically pointless.  For all practical purposes, every planet has direct LOS to Kerbin at all times.  No, occlusion by the sun itself isn't something you have to worry about, ever.  It never happens to any significant degree.  Further discussion of why-you-don't-need-solar-relays is here, if you're interested:

 

Link to post
Share on other sites
This thread is quite old. Please consider starting a new thread rather than reviving this one.

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Answer this question...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...