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What are the most efficient/cost friendly methods to high Delta-V?


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As someone who's built rockets before plenty of times but hasn't played the game for too long, I was wondering if people had a way to build high Delta-V without having to slap something costly like the Mammoth or having to use a bunch of boosters such as Clydesdales to lift heavy rockets into orbit. Like I've built plenty of high Delta-V rockets before; around 10-11k Atm / 15-16k Vac but they always ended up being the most bulky overpriced rockets you could find.

My question is what are your methods to building high Delta-V rockets like that but without the bulk/cost. (Obviously having at least a decent TWR too). It just seems rather way too bulky/costly when building to make anything proper back in career.

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44 minutes ago, eM-Krow said:

without ... having to use a bunch of boosters

If you're going to build cheap but effective rockets, you need to use plenty of SRBs in your first stage to get you out of the lower atmosphere. 'Moar boosters' exists for a reason. Everything beyond that is mostly optional.

Beyond boosters, the next thing you need is effective staging. You need to have drop tanks for your liquid fuel that empty quickly, and you need to get rid of them. For example, VoidSquid's rocket would work even better if he had LF tanks on top of his SRBs that drain in the same time that the SRBs burn. Then drop them.

Once you are out in vacuum, your Isp gives you your engine efficiency, so Isp divided by cost tells you the bang for the buck of your engines.

And if you really want true efficiency, then start looking at spaceplanes. Especially spaceplanes that drop their airbreathing engines. That's what gets you your best mass fraction to orbit.

And if you want to do better than that, you need in-orbit refueling.

And if you want to do even better than that, you need a craft that can mine, refine, and refuel itself.

 

 

 

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13 minutes ago, bewing said:

VoidSquid's rocket would work even better if he had LF tanks on top of his SRBs that drain in the same time that the SRBs burn. Then drop them.

I don't know... the idea(s) here: start with cheap boosters up to approx. 12km / 400-450 m/s vertical velocity. Let the skipper bring the rocket close to circularization (avoiding space junk). From there on, two stages with nuclear engines, having 8300 and 8900 m/s, respectively, following the old guideline of stage dv equals roughly ten times the numerical ISP of the engine (800 m/s here, both), thus having an acceptable thrust while at the same time giving a good amount of dv.

And this was a more or less quick build, back the days of 1.7.3, didn't put too much efforts into further optimization / even more dv, just did this for a contract.

  

13 minutes ago, bewing said:

You need to have drop tanks for your liquid fuel that empty quickly, and you need to get rid of them

Yeah, this would be probably what I'd do for even more dv, avoiding to carry the weight of big half-empty tanks.

Edited by VoidSquid
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@bewingWell I don't exclusively mean no boosters, but I mean to stay away from the massive ones, 140 tons as a booster seems quite a large amount if you ask me. As for that ISP by cost that seems something I should really keep track of. Also @VoidSquid that cost for Delta-V is mainly exactly what I'm trying to get to.

I think mainly the main issue I'm also having where I felt like I needed to use larger rockets is that I'd have trouble having a decent boost from Kerbin to Orbit. Some of my cheap ships could get out with around 6000 Delta-V at the cost of TWR but it makes that difficult to work with rough adjustments. Those I just really used for Comm triangles.

As for that about fuel tanks, I'm not too sure how I'd exactly go about dropping tanks per priority and keeping a rocket stage intact.

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Make sure that you intelligently manage your mass fractions, and downsize your vacuum engines. As a rule of the thumb, in space you trade TWR for dV. It's most obvious in the nuclear and ion engines, but it applies to chemical engines too - and it counts even when the more powerful vacuum engine has better Isp, because the additional dry mass of the engine is usually going to negate that advantage. And then, it's going to sap dV from every stage below it, too. Example: I use a Terrier, not a Poodle, to land 2.5m sized landers on the Mun. And the vessel has more dV because of it.

 

Edited by Streetwind
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@StreetwindIf that's the case then is there a recommended threshold I should try to be in for weight/dV to maximize the output to each stage? Like per say a terrier would be useful as a VAC engine on spaceships but if the ship is capable of carrying the extra mass, a NERV might be worth it? And usually I try to keep my TWR within a certain range per stage so around like 1.5 min and 1.6 max generally between stages, but I'm not sure of the reliability of that or if those values are really optimal either.

Edit: I just read that somewhat fully. Gonna need to play with that a bit. Ty for the post reference.

Edited by eM-Krow
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I wrote some code in kOS a while back to do the flying to orbit, so I have a consistent basis for experimentation as to how to get to orbit cheapest. You could do some experimentation yourself in MechJeb.

I have three different rocket profiles:

  1. Boosters + LFE at liftoff, separate trans-orbital stage
  2. Boosters + LFE at liftoff, no separate trans-orbital stage
  3. Boosters only at liftoff, LFE stage, trans-orbital stage

Of those, for most rockets #1 is least expensive. #2 is good for a bulky, draggy payload where much thrust is necessary to gain the first 20km altitude (but using a payload fairing and one of the other types is usually a better choice). #3 is good for small, cheap rockets where the cost of radial decouplers would be a significant fraction of the launch cost.

When using method #1, for a 75km orbit I usually aim for about 1200-1400 dV in the trans-orbital stage (plus what's necessary after circularization, of course), with a TWR not too much above 0.8. I then try to have about 1,400 dV in the LFE main stage (leaving 600-800 for the boosters), with a liftoff TWR around 1.0-1.2. The boosters provide the surplus thrust at liftoff, to a total of about 1.8-2.0 TWR.

The most expensive part of a launch is the liquid fueled engine in the main stage; this method allows you to use a smaller, cheaper engine than with a two stage to orbit design.

Example design: payload is two Large Holding Tanks full of ore (combined mass 34T; 34.7T including pod and nose cone). The trans-orbital stage is a Skipper with Rockomax-32 and Rockomax-16 tanks plus a 2.5m reaction wheel, the LFE main stage is a Mastodon with Delta-Deluxe wings, a Kerbodyne converter (the new one, with fuel) and a Kerbodyne S3-7200, and the boosters (which fire simultaneously) are 3 Stompers (I forget if they're from Making History or from the Missing History mod). The liftoff TWR of just the Mastodon is 1.07; with the boosters the total is 1.98 (in vacuum), with a total craft delta-V of 3,531 and total cost of $51,478 (of which $10,150 was the payload). My code put it into orbit with 169 m/s dV remaining, for 3,362 m/s expended.

Example:

Spoiler

1nIJqAn.jpg

 

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22 minutes ago, eM-Krow said:

I try to keep my TWR within a certain range per stage so around like 1.5 min and 1.6 max

For me the actual acceleration value m/s² is more important than the TWR, a vessel in LKO might have a low TWR there, in a 1000 km Mun orbit the TWR would be way greater, the actual acceleration though remains the same. For nuclear powered stages, I'm quite happy with some 2.5 m/s².

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Quite old but still good tutorial:

 Mind you, I don't agree 100% with the guide. But its a good starting point to start to figuring out your own preferences.

On 10/8/2015 at 5:00 PM, Norcalplanner said:

Almost always use gimbaling LFO engines.

And that...well, I know all those words, but put then together and the resulting sentence don't make any sense for me (a glance at my crafts make evident why).

Anyway, I can't consider it "an error" since apparently gimbals are the cheaper source of control, whatever the reason people want it in launch vehicles. But it really bother me that it kinda contradict with the recommendation of SRBs.

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the best way to go everywhere is to mine your own fuel in space. minmus is the ideal place, with its low gravity it is very cheap for a mining vessel to land and take off. nowadays all my missions only have enough fuel to go to minmus, where they are refueled. it would be evencheaper to send fuel from minmus to LKO and only send rockets big enough to get there (smaller rockets, and all the extra fuel spent to send from minmus is free) but it would take many more manuevers to keep a refueling station and make regular trips on it.

really, that's the only way to get really high deltaV at low cost. the rocket equation is a harsh mistress. with a few tons of fuel and a terrier engine you can get 3 km/s in orbit with a reasonable payload, and it takes a reasonably small rocket to lift that. but if you want more, you need a lot more fuel (or you need a bit more fuel and another stage, with another engine). and to carry that extra weight to orbit you need more fuel on the second stage too. but then your engine isn't powerful enough to lift this, so you need a bigger engine. and then you need to make the first stage proportionally bigger. there's no way around that.  if you want a lot of deltaV, you need a big rocket. you can optimize your engines and staging all you want, but in the end, but at best you'll end up with a slightly less big rocket.

i mean, the real space agencies are all studying ways to mine fuel from the moon. and every plan for a mission to mars entails getting fuel out of mars atmosphere. and doing it in ksp is much easier than in real life.

aside from that, i can't add much. my ideal TWR (measured on kerbin, because what you really want is acceleration) are 1.5 at launch, 1 for the second stage, and 0.5 in orbit. the last one is a compromise between saving on engine mass and making manuevers in a reasonable time. you could get away with less if you don't need to rocket brake on landing, but getting out of kerbin's SOI with 0.1 TWR is booooring.

 

 

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7 hours ago, Spricigo said:

Quite old but still good tutorial:

 Mind you, I don't agree 100% with the guide. But its a good starting point to start to figuring out your own preferences.

And that...well, I know all those words, but put then together and the resulting sentence don't make any sense for me (a glance at my crafts make evident why).

Anyway, I can't consider it "an error" since apparently gimbals are the cheaper source of control, whatever the reason people want it in launch vehicles. But it really bother me that it kinda contradict with the recommendation of SRBs.

It's fun that the guide is still useful for folks.  The advice on LFO engines could perhaps be better stated as "if you're going to use LFO engines, use one with a gimbal if you can".  Back when I wrote this almost five years ago, there was some debate regarding LVT-30s vs. LVT-45s and clusters of Sparks vs. one larger engine, and this was meant to address that.  If you look at some of the examples later in the thread (if the links even still work), you can see designs where there's a single LFO engine core for control, with four or more SRBs for cheap lifting power.  

Edit: There have been a lot of balance passes since I wrote the guide, so some of the specific part choices might not be the most accurate.  But the major gist of things remains: use SRBs heavily; minimize the use of parts which don't contribute to delta V; burn your engines a long time to maximize return on investment; and keep the initial TWR of your upper stages and moon landers lower than you might think.

Edited by Norcalplanner
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9 hours ago, eM-Krow said:

If that's the case then is there a recommended threshold I should try to be in for weight/dV to maximize the output to each stage? Like per say a terrier would be useful as a VAC engine on spaceships but if the ship is capable of carrying the extra mass, a NERV might be worth it?

The issue with calculating that is that it's a question with multiple unisolated variables. You can at best narrow down the value with empirical testing, but you'll probably find that whatever value you determined won't be consistent across vessels.

For me personally, I don't bother trying to calculate. I just compare two or three engines during construction. Whichever gives me the combination of dV and TWR (across all stages) I like best, gets the job.

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11 hours ago, Norcalplanner said:

If you look at some of the examples later in the thread (if the links even still work), you can see designs where there's a single LFO engine core for control, with four or more SRBs for cheap lifting power.  

I go it.

When I say it don't make sense for me, that is because the way I design my launch vehicles. I don't need the central gimbaled engine since my rockets have a tilt set in the launch pad to start the gravity turn automatically. It don't seems to be "standard procedure" for others and I'm well aware most people don't share that (quite strong) bias.

In any case, nice to read about the reasoning behind that part. 

 

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besides the swivel/reliant, i'm not aware of any other case where you have two similar engines, one with gimbal, one without. so, in most cases you won't worry about gimbaling anyway, because there is no choice. most of my launches use either the skipper or the bobcat (i seem to prefer payloads in that weight category), both of those engines are unique in their thrust. there is nothing else coming close. so, if i want that specific thrust, and i don't want to use clusters of engines (which often ruin the aerodinamics) then i don't have any choice.

in the case of the swivel/reliant, if you want more control, you are probably better off adding a reaction wheel. you get extra control, but it's much lighter. and if you put it in the last stage, you also can recover it afterwards. it's a bit more expensive, though.

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8 hours ago, king of nowhere said:

... and i don't want to use clusters of engines (which often ruin the aerodinamics) then i don't have any choice.

I have to disagree. That is not lack of choice, it's your preferences to not take a choice in particular. 

Also, the Kickback have a saying about the alleged uniqueness the Skkiper have in terms of thrust. As the BACC for the Bobcat. And a quite strong voice if the subject is 'cheap and cheerful'.

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