Standard Lift Subassemblies

[Wcmille]

I have a number of subassemblies rated to deliver payload of different masses to Kerbin orbit. That way, I just design a payload, pick the launch vehicle, and I'm done. I am using 3500 dV and 1.2 TWR as my standard benchmarks. If you are doing this, what specifications are you choosing? Why?

[Rocket In My Pocket]

-Cue cheesey commercial music, pan in on an elderly grandmother type pulling rocket shaped cookies from an oven-

Here at RIMPs old time home grown country rockets we want your family to feel like part of our family, so every launcher is hand made with love. Sure it might take a little longer than your "big box stores brand name launchers" but we think you'll agree it's worth it.

Our rockets are made with love, passion, and a commitment to quality.

Edited January 4, 2018 by Rocket In My Pocket

[Cpt Kerbalkrunch]

1 hour ago, Rocket In My Pocket said:

-Cue cheesey commercial music, pan in on an elderly grandmother type pulling rocket shaped cookies from an oven-

Here at RIMPs old time home grown country rockets we want your family to feel like part of our family, so every launcher is hand made with love. Sure it might take a little longer than your "big box stores brand name launchers" but we think you'll agree it's worth it.

Our rockets are made with love, passion, and a commitment to quality.

Mmm. I can smell those warm rockets from here. Is that a Mammoth?

[OhioBob]

I use to use standard subassemblies, but I stopped doing that.  That was back in version 0.90 before the current aerodynamics, so any standards that I used are no longer applicable.  Now when I build a launch vehicle I try to follow these guidelines as closely as possible:

Second (upper) stage:  propellant mass = 1 x payload mass, TWR = 1 to 1.2
First (lower) stage:  propellant mass = 2 x payload mass, TWR = 1.3 to 1.4

where "payload mass" refers to everything that sits on top of the second stage, including decouplers, fairings, and maybe even a third stage.  This configuration will usually produce a delta-v of around 3400 m/s depending which engines are used.
 

Edited January 4, 2018 by OhioBob

[steuben]

I'm usually a STP dv of 4kps, and a STP TWR around 1.5. It gets me to LKO with juice to spare regardless of ascent profile... which more of the time sucks.

[Tyko]

2 hours ago, OhioBob said:

I use to use standard subassemblies, but I stopped doing that.  That was back in version 0.90 before the current aerodynamics, so any standards that I used are no longer applicable.  Now when I build a launch vehicle I try to follow these guidelines as closely as possible:

Second (upper) stage:  propellant mass = 1 x payload mass, TWR = 1 to 1.2
First (lower) stage:  propellant mass = 2 x payload mass, TWR = 1.3 to 1.4

where "payload mass" refers to everything that sits on top of the second stage, including decouplers, fairings, and maybe even a third stage.  This configuration will usually produce a delta-v of around 3400 m/s depending which engines are used.
 

Hmm...I've been a lot more successful with launch TWR of 1.4 - 1.5. It seems like a higher TWR reduces my overall DV budget to orbit and that kind of makes sense because I'm not spending as much time in my initial ascent off the pad where at 1.2 I feel like I'm wasting a lot of fuel doing just slightly more than hovering.

Is my understanding incorrect? Why would I choose a lower TWR? The lower would allow for a lower engine weight, but increase the amount of fuel I'd need for that initial ascent, wouldn't it?

[Snark]

I tend to build most of my ships from scratch every time, for a couple of reasons:

• Different mission = different requirements = different ship needs.  Using a standard boost stage can result in fitting square pegs in round holes.
• I just really like building ships. 

That said, I do occasionally make one or two custom boost assemblies, usually for the repetitive part of mid-career where I'm grinding contracts to make enough cash to upgrade the science facility and I just want to blow through a lot of missions as quickly as possible.

A fairly common one I'll use for mid-sized rockets looks like this:

That's a Skipper in the middle, with two 4-way-symmetry groups of Thumpers around it.  Staging is set up thus:

• First stage:  All 8 Thumpers activate.  The four Thumpers at NW, NE, SW, SE are set to 100% thrust.  The four Thumpers at N, S, E, W are set to some lower thrust, which is calculated to give the overall ship a TWR of 2.0.  (So it varies with total ship mass, but they usually end up somewhere in the 60-90% range).
• Second stage:  Jettison the four Thumpers that are at 100% (since they burn out first).
• Third stage:  Jettison the remaining Thumpers and activate the Skipper.

This is a cheap, efficient, low-part-count booster that works pretty well for a wide variety of mid-sized ships.

I also have a somewhat larger one for bigger ships, that's basically the same as this one except that it uses a 32-ton tank instead of a 16-tonner, has a Mainsail in place of a Skipper, and uses Kickbacks instead of Thumpers.  Same idea, works the same way, it's just scaled up.

[Tyko]

To add on regarding standardization - In my current build I've been following more of a ULA Atlas V model in which I keep the same core and vary the number of boosters to accommodate different mission requirements. It's worked pretty well.

[OhioBob]

1 minute ago, Tyko said:

Hmm...I've been a lot more successful with launch TWR of 1.4 - 1.5. It seems like a higher TWR reduces my overall DV budget to orbit and that kind of makes sense because I'm not spending as much time in my initial ascent off the pad where at 1.2 I feel like I'm wasting a lot of fuel doing just slightly more than hovering.

Is my understanding incorrect? Why would I choose a lower TWR? The lower would allow for a lower engine weight, but increase the amount of fuel I'd need for that initial ascent, wouldn't it?

I don't judge the effectiveness of my launchers based on delta-v.  I look at cost per ton of payload.  For any given engine, loading it up with as much fuel as it can carry will get the most payload to orbit for the least cost.  Yes, the gravity losses will be greater, and I'll expend more delta-v getting to orbit, but I don't care about that.  Generally speaking, low TWR yields the best cost efficiency, at least when using liquid fueled rockets.

[Geonovast]

I tend to build everything from scratch.  Building the stuff is half the fun of the game, why would I want to streamline that!?

I also tend to launch big, which makes my rockets work a bit differently from most I see on here.  I don't have any numbers, because honestly I rarely look at the weight of the payload and I just wing it.  Having KER tell me TWR and dV has helped a lot, but even before then I would usually build it big enough just by guessing.

Most rockets I see have the lifter on the bottom, payload on top.  Standard rocket stuff.

I put the payload in the middle, and surround it with nacelles.  Probably less effecient aerodynamically, but I think it's more stable, easier to strut, and looks more sciencefictiony.

 

Should be noted I do not play career.

Edited January 4, 2018 by Geonovast

[XLjedi]

I have to admit, I spend very little time in the VAB nowadays.  

I think the first time I put chutes on boosters to recover them and they failed to work as intended...  it just turned me away from rocket design.  (Yes, I have since heard there are mods to address this.)

 

 

 

 

 

[OhioBob]

There are few members of this forum whom I respect more than @Snark.  He's quick to respond to questions and always provides good quality answers and advice, with which I almost always agree.  However, these "best TWR" threads crop up frequently and Snark and I always post very different recommendations.  I always advocate for the low TWR approach, while he routinely suggests a much higher TWR, around 2.  But there's a very good reason why we seemingly disagree.  Snark prefers and makes extensive use of solid-fueled boosters, while I prefer liquid-fueled boosters.  That makes a big difference.

My way of lowering the TWR of a liquid-fueled booster is to pack on as much propellant as its engines can reasonably lift (or by using smaller or fewer engines).  This is not an option with SRBs.  An SRB is a self-contained unit that has a finite amount of propellant.  We can't add more propellant to an SRB, we can only add more SRBs.  This means that SRBs by there very nature are going to have a high TWR.  Since we can't lower the TWR by adding more propellant, the only way to decrease the TWR is by thrust limiting.  I never advocate thrust limiting unless the TWR is just going to be ridiculously high if we don't.  I believe that if you have the thrust, use it.  

So when making a first stage by clustering together SRBs, I agree with Snark that a high TWR is probably the way to go because it's going to reduce the gravity losses.  But when using a liquid-fueled first stage, decreasing engine size and/or adding propellant, thereby lowering TWR, is going to get your payload to orbit for the least cost (even though the delta-v requirement may be greater).
 

[Snark]

14 minutes ago, OhioBob said:

However, these "best TWR" threads crop up frequently and Snark and I always post very different recommendations.  I always advocate for the low TWR approach, while he routinely suggests a much higher TWR, around 2.  But there's a very good reason why we seemingly disagree.  Snark prefers and makes extensive use of solid-fueled boosters, while I prefer liquid-fueled boosters.  That makes a big difference.

My way of lowering the TWR of a liquid-fueled booster is to pack on as much propellant as its engines can reasonably lift (or by using smaller or fewer engines).  This is not an option with SRBs.  An SRB is a self-contained unit that has a finite amount of propellant.  We can't add more propellant to an SRB, we can only add more SRBs.  This means that SRBs by there very nature are going to have a high TWR.  Since we can't lower the TWR by adding more propellant, the only way to decrease the TWR is by thrust limiting.  I never advocate thrust limiting unless the TWR is just going to be ridiculously high if we don't.  I believe that if you have the thrust, use it.  

So when making a first stage by clustering together SRBs, I agree with Snark that a high TWR is probably the way to go because it's going to reduce the gravity losses.  But when using a liquid-fueled first stage, decreasing engine size and/or adding propellant, thereby lowering TWR, is going to get your payload to orbit for the least cost (even though the delta-v requirement may be greater).
 

Well, sure, SRBs and liquid rockets have different strategies, no argument there.

Not quite sure how what you're saying connects to what I posted, though.  You'll note that at no point in my post did I even slightly make any suggestion about what TWR is "right" in general, or indeed is "right" for anything other than this booster.

This is not a "what is the best TWR" thread.  This is a thread that's simply about "what standard lift subassemblies do you use."  And that's a subassembly that I use.  I happen to mention its TWR only in the context of explaining how I choose where to set the thrust limiters, which I include simply because it explains what I'm doing and would be relevant to anyone using this booster.

So sure, I'll agree that if someone built a completely different booster with a completely different design using completely different fuel and engines, it would work differently.  But it's not, and I'm only talking about this-booster-right-here-whose-mass-is-75%-SRB, so I'm a little confused...?

 

[OhioBob]

43 minutes ago, Snark said:

Well, sure, SRBs and liquid rockets have different strategies, no argument there.

Not quite sure how what you're saying connects to what I posted, though.  You'll note that at no point in my post did I even slightly make any suggestion about what TWR is "right" in general, or indeed is "right" for anything other than this booster.

This is not a "what is the best TWR" thread.  This is a thread that's simply about "what standard lift subassemblies do you use."  And that's a subassembly that I use.  I happen to mention its TWR only in the context of explaining how I choose where to set the thrust limiters, which I include simply because it explains what I'm doing and would be relevant to anyone using this booster.

So sure, I'll agree that if someone built a completely different booster with a completely different design using completely different fuel and engines, it would work differently.  But it's not, and I'm only talking about this-booster-right-here-whose-mass-is-75%-SRB, so I'm a little confused...?

 

You're correct that this is not a "best TWR" discussion per se, but it is part of the discussion.  The OP said that 1.2 TWR is one of his standard benchmarks and asked what specifications we use.  So it's not just about subassemblies, it's also about what benchmarks/specifications we use, of which TWR is a big part.

I made a post in which I said my preferred liftoff TWR is 1.3-1.4.  That was followed a couple posts later by Tyko asking me why, to which I explained my philosophy on why I believe low TWR equates to low cost.  In the middle of that you made a post in which you provided a design that sets the TWR to 2.0.  I figured that some people might find it confusing why one poster is advocating for low TWR, while another posts a design using high TWR.  So I just explained the difference.  The specifications that we choose and why (which was the OP's question) can vary depending on design strategy.
 

Edited January 4, 2018 by OhioBob

[Snark]

1 minute ago, OhioBob said:

I made a post in which I said my preferred liftoff TWR is 1.3-1.4.  That was followed a couple posts later by Tyko asking me why, to which I explained my philosophy on why I believe low TWR equates to low cost.  In the middle of that you made a post in which you provided a design that sets the TWR to 2.0.  I figured that some people might find it confusing why one poster is advocating for low TWR, while another posts a design using high TWR.  So I just explained the difference.  The specifications that we choose (which was the OP's question) can vary depending on design strategy.

Ah, okay.  I didn't actually read the thread at all.  Since it was just a "hey, what do you use," I just skipped to the end and said "I use this". 

[Tyko]

4 hours ago, OhioBob said:

<SNIP>

This means that SRBs by there very nature are going to have a high TWR.  Since we can't lower the TWR by adding more propellant, the only way to decrease the TWR is by thrust limiting.  I never advocate thrust limiting unless the TWR is just going to be ridiculously high if we don't.  I believe that if you have the thrust, use it. 

A workaround I've used for this is to mount supplemental liquid fuel tanks on top of my SRBs with Fuel Lines or adjusted fuel priority to make sure they drain by the time the SRBs are expended

Edited January 4, 2018 by Tyko

[SnakyLeVrai]

I have standardized all my launchers in subassemblies too, I tend to like the classical look of it so, no weird looking launchers with unaerodynamic payload for me... They are all equipped with chutes and / or spare fuel for recuperation (I use Stage Recovery), even the 2nd stages in the rare situations I use them - most of my launchers have one main core with boosters all around. I can lower the launch costs dramatically this way: I usually recover 75 to 80% of the launcher's cost.

TWR is around 1.4 at takeoff, including payload of course. I keep lowering the thrust until Max Q is passed, so TWR stays around 1.4, and then I let it go up naturally as fuel is burned. I put thrust to the max usually above 30km when air is thinner. 

For my medium-class launchers (the "Creamy Goodness" family), I've simply made an adapter to fit a 3.75m size fairing for the bulkier payloads. They take care of most of my launches, and I do a lot of them.

This, plus a strategy based on ferrying tourists to the Mün, Minmus, just outside Kerbin's SOI, rescuing Kerbals and using the tourists cruises to train them to up 3 stars... That's my main income, this way I can finance just about everything I need. They paid for the mining equipment I put on Minmus and the Mün, the orbital fuel depots, the space tankers, ... Even when I go over the top for the base building contracts, I'm still making money  In short, base building anywhere in the Kerbol system is not so expensive anymore...

(yeah I know, the names... well, Kerbals like 2 things in life: rockets, and food... it seemed only fair that rockets get the name of food... I need to rename the Minus family though - how about Peach Pie?)

 

[shdwlrd]

I do have some standard designs that I use for more routine missions like satellite placement and crew rotations and resupply. I tried to keep a TWR to no more than 1.3 and a DV of 3700. I usually use MJ for launches and 3700dv is usually enough to get to a orbit of 80-85km with fuel to spare to de-orbit the 2nd/3rd stage. I don't play career, I just do it to keep my airspace(?) clear of debris.

Edited January 5, 2018 by shdwlrd

[TheFlyingKerman]

I am lazy so I use about 10 lifter designs from 4 families for pretty much all my launches. I use SRB heavy because they are cheap.

For my pure SRB rockets the TWR at takeoff is around 1.5.

For my mixed SRB/LF rockets the TWR is around 1.7-1.8, but goes down to 1.0-1.2 when the SRB's are dropped.

[XLjedi]

@SnakyLeVrai  I am very impressed with that array of rockets!   

You may just have renewed my interest in the VAB.

[SnakyLeVrai]

Thanks @XLjedi

The VAB is a good building tool. As much as I appreciate the SPH for particular payloads, like vehicles or other things that have a horizontal plane as reference, the VAB does a great job for "cylindrical" shaped designs. I find rockets pretty efficient for catapulting stuff to orbit quickly (under 4 minutes I'm good and done) and easily. Building space planes takes too long for me, I find them impractical, mostly due to the fact that the cargo bays are very, very narrow and the time to orbit is like forever. It sucks because I like the concept.

Some of my medium class launchers don't appear on this illustration. Alongside the Creamy Goodness series, I have the Moka series & Chestnut Cake series, which have the same cargo capacities but come in 3.75m parts. They are so much more expensive that I don't use them, even if they are, too, recoverable.
See, with an average 75% recovery value, when you still lose 25% of the launcher's cost, it might as well be 25% of a cheap rocket than 25% of a very expensive one... That's why I made the Creamy Goodness I, II & IIIR-S3 version, for "size 3", with a 3.75m adapter with adequate fairing. I can lift bigger payloads for a fraction of the cost of an immense launcher. And it's recoverable.

 

[Wcmille]

On ‎1‎/‎4‎/‎2018 at 12:13 PM, OhioBob said:

... I look at cost per ton of payload. ...

What cost/ton are you experiencing with your present designs?

[OhioBob]

21 hours ago, Wcmille said:

What cost/ton are you experiencing with your present designs?

I don't remember.  I just recall doing a study of it a couple years ago in which I concluded that cost per ton of payload was at its lowest with a launch TWR somewhere around 1.2 or 1.25.  Increasing TWR above that reduced the delta-v to orbit, but increased the cost per ton.  I think delta-v efficiency topped out with a TWR of about 2.  I eventually settled on my current design guidelines because they're easy to remember, the designs fly very nicely, and I think it's reasonably efficient.  But I stopped paying attention to what the actual cost per ton is.
 

Edited January 8, 2018 by OhioBob

[Dfthu]

I love my lifters, most of the time I can fit my payload on. Although when my payloads get too big I custom make them.

[Tyko]

For those using standard lifters - What do you do if your payload falls between two of your standard lifters?

I usually use the bigger lifter - KSP doesn't model this well, but IRL it would be cheaper to use an over-spec'd lifter that's already designed versus spending the time / money to modify it. In-game it saves ME a ton of time, but feels a bit wasteful.

I'm just wondering if others go the same route or have a better solution.