ISP and TWR

[MPDerksen]

I've asked this before, but cannot find the reply.  Sorry for that.

Thus far I mostly build my rockets based on dV goals.  But I also seem to burn much more to attain orbit than published (yes, my gravity turns are within margins).  I don't know how to select the right engine/fuel for a particular goal.  I don't know when to use SRBs vs. some asparagus-staged LF engines.  Stuff like that.  While this is mostly true for launching off Kerbin, I think the same issue is true for my landers that need to escape Mun, or Duna etc.  

Without becoming a huge math lesson, could someone break this down for me?

Michael

[Aegolius13]

1 hour ago, MPDerksen said:

I've asked this before, but cannot find the reply.  Sorry for that.

Thus far I mostly build my rockets based on dV goals.  But I also seem to burn much more to attain orbit than published (yes, my gravity turns are within margins).  I don't know how to select the right engine/fuel for a particular goal.  I don't know when to use SRBs vs. some asparagus-staged LF engines.  Stuff like that.  While this is mostly true for launching off Kerbin, I think the same issue is true for my landers that need to escape Mun, or Duna etc.  

Without becoming a huge math lesson, could someone break this down for me?

Michael

So you're saying you're measuring delta-v as shown on your craft, before vs. after launching, against what the chart says?  The ISP of your engines will not affect  "efficiency" by that metric, because it's already baked into the delta-v numbers.  High TWR will, however, generally lower the delta-v taken to get to orbit, by reducing gravity losses.

The (slightly oversimplified) way I like to think about it is: your delta-v supply is based on how you build the rocket, and your delta-v consumption is based on how you maneuver.

Does that mean TWR is better than ISP?  Well, no (or at least not for that reason), because "delta-v to orbit" is not a terribly useful metric.  You could build a rocket with atrociously low ISP (all SRBs and Puffs, let's say), which might  have a typical delta-v to orbit figure, but it would probably still be a lousy rocket because you'd have to build it huge due to all the fuel you're burning. 

Perhaps a more direct way to look at it is to figure out what you need to accomplish your intended mission (delta-v, parts, thrust, etc), figure out what the appropriate rocket would look like in orbit, and then build to suit.  If you're on a career game, it might make sense to optimize by cost, which often means low-tech parts like SRBs and Reliants.  If money is no object and you want to build a small, sleek rocket, you might end up with expensive parts like Vectors.There's no categorical best answer, since after all, it's an open-ended game.

All that said, as a very general rule, ISP is relatively less important in earlier stages of a rocket, and more important in later stages.  You can get away with an inefficient, heavy booster at launch because you don't need to consume fuel moving it around the solar system.  But in an upper stage, every bit of weight matters a lot.  You also inherently need a higher TWR at launch than when you're near or in orbit, since you have to fight gravity losses and want to quickly get high and fast.  So people often prioritize thrust in lower stages.  This is all true in the real world as well, which is why we tend to see lower ISP, high thrust engines at launch (think shuttle SRBs or the Saturn V first stage), and more efficient engines on later stages (e.g., hydrogen-burning J-2 or RL10).

[VoidSquid]

9 hours ago, MPDerksen said:

I don't know how to select the right engine/fuel for a particular goal.

You can classify engines in roughly two categories, vacuum and ASL=at sea level, the former have a poor ISP at ASL and a high ISP in vacuum, the latter have a good ISP ASL but their vacuum ISP is not much greater than their ISP at sea level. In general, ASL engines have a much higher thrust than vacuum engines.

For reaching orbit, my first stage when starting is usually SRBs until about 12-15km altitude, next stage are liquid based ASL engines like the Reliant or Skipper or Mainsail, depending on the thrust needed. In space, best use vacuum engines like the Terrier or Poodle. If you need a lot of dV, use atomic engines like the Nerv, which have a very high ISP but not much thrust.

SRBs are in general cheaper than liquid based engines plus tanks, if possible, I prefer simple SRBs over asparagus staging.

Edited August 21, 2019 by VoidSquid

[Zhetaan]

19 hours ago, MPDerksen said:

I don't know how to select the right engine/fuel for a particular goal.

@Aegolius13 did a good job of explaining the rationale; I'll just give you some basic categories.

As a very rough thumb rule, engines with an atmospheric (not vacuum, for once!) I_sp of less than 250 seconds are usually vacuum engines.  Engines with atmospheric I_sp greater than 275 are usually good sea-level lifting engines, and engines with atmospheric I_sp between 250 and 275 can fit either role, albeit not necessarily so well as more specialised engines.

To refine that, if an engine has a vacuum (note that this time, it's vacuum, not atmospheric) I_sp greater than 320, then it is probably a good choice for space.

If you want to pick an engine with an eye specifically for launching, then try for one that has an atmospheric thrust of greater than 100 kilonewtons (kN).  The Thud is arguably the worst choice that still follows this rule, the Rapier is a niche engine, and the Rhino is actually best in vacuum (you can tell because its atmospheric thrust is about 60% of its vacuum thrust, whereas most launching engines have between 85% and 90% of their vacuum thrust at sea level), but it has good properties on the ground, too.  The others all are good choices for launch stages.

SRBs don't follow the I_sp rule:  they have low I_sp in any situation, and are best suited to the launch stage.

Don't attempt to use jets in space.  You can try, but you'll be disappointed.

That's not to say that many of the remaining engines do not have specific niche applications that defy these rules, but the rules will probably get you to your destination with a minimum of trouble.

Concerning thrust-to-weight ratios, it depends on the celestial body.  More is often better, but when an atmosphere is involved, you need to consider drag (usually not a problem unless your rocket is not streamlined) and heating.  For rockets launching from Kerbin, I typically design for a TWR of between 1.3 and 1.6.  You can get away with anything from 1.2 to 2.0 provided you design the rocket well.  TWR values under 1.2 take too long to get up to speed (and they waste too much fuel fighting gravity during that time), and values over 2.0 tend to get too hot in the lower atmosphere.

For airless bodies, it's simpler:  I generally try for a TWR of more than 2 for the body I'm landing on.  Remember that a rocket weighs less in weaker gravity, but an engine's thrust is constant for a given throttle setting:  a TWR of 1 on Kerbin can be 6 on the Mun.

20 hours ago, MPDerksen said:

I don't know when to use SRBs vs. some asparagus-staged LF engines.

SRBs are terrible engines insofar as efficiency is concerned, but they are the cheapest engines available.  Use them in the launch stage, either on their own or as a low-cost way to increase your rocket's TWR without resorting to a complete redesign.

When you have a choice of engines, the usual method is to burn your least efficient engines first and go in order from there, and SRBs are no exception.

When designing your initial launch stages, don't neglect the Twin Boar.  It is also extremely cost-effective for the thrust that it delivers, and unlike SRBs, it isn't utter rubbish in the upper atmosphere.

[tcdonald]

Generally

For launch: TWR>ISP

Engines: all the rest not mentioned below

 

For space maneuver: ISP>TWR

Engines: Terrier, Ant, Poodle, Rhino, Nerv. Their thrust increase in vacuum as well

 

SRB: for create a rocket looking like real one. Only improve your TWR during launch

asparagus-staged booster: better than SRB but requires more time and fund to build

[Aegolius13]

2 hours ago, tcdonald said:

For space maneuver: ISP>TWR

Engines: Terrier, Ant, Poodle, Rhino, Nerv. Their thrust increase in vacuum as well

I'd add the Spark to the list of good vacuum engines, though it can be good in both roles.  While its ISP is not top-tier, it's great for smaller payloads. And in relatively TWR-intensive vac applications like lander, a cluster of Sparks might beat out a Terrier.

 

Making History engines: Cheetah and Wolfhound are vacuum engines.  Kodiak, Bobcat, Mastodon are launch engines.  The Skiff is a bit of a tweener; kind of a beefed-up Swivel.  The Cub is usable in both roles if you want a radial engine.

[MPDerksen]

Great answers.  Thanks.

Follow up clarification:  When building a rocket, how do I evaluate it’s ISP and TWR?  Specific example, I’m prepping for my first mission with a rover under a lander.  I want to construct a lifter to get it into LKO.  Some of my designs take off and I get heat build up, others seem to accelerate VERY slowly and I burn for a VERY long time before getting up to orbit. 

It would just be nice to have some guidelines for basic construction.  Getting there.

[HebaruSan]

10 minutes ago, MPDerksen said:

Follow up clarification:  When building a rocket, how do I evaluate it’s ISP and TWR?  Specific example, I’m prepping for my first mission with a rover under a lander.  I want to construct a lifter to get it into LKO.  Some of my designs take off and I get heat build up, others seem to accelerate VERY slowly and I burn for a VERY long time before getting up to orbit. 

That info should be in the tooltip of the engine in the parts list (hover it and/or right click), and also in the expanded UI of the staging list if you right click a stage.

Edited August 25, 2019 by HebaruSan

[Aegolius13]

22 hours ago, MPDerksen said:

Great answers.  Thanks.

Follow up clarification:  When building a rocket, how do I evaluate it’s ISP and TWR?  Specific example, I’m prepping for my first mission with a rover under a lander.  I want to construct a lifter to get it into LKO.  Some of my designs take off and I get heat build up, others seem to accelerate VERY slowly and I burn for a VERY long time before getting up to orbit. 

It would just be nice to have some guidelines for basic construction.  Getting there.

The mod Kerbal Engineer Redux is great for seeing stats these stats, both in the VAB and in flight.  The newish stock Delta-V tools button is helpful too in the VAB, and lets you check ISP at any given altitude, but I tend to still use KER.

In flight, you can right click an engine and see its ISP at that particular moment.  

When you take off, you can also get a rough idea of your TWR by looking at your g-meter.  A rocket with a TWR of exactly 1 would hover, and show 1g.  A rocket with 1.5 TWR would show 1.5g, and so on.  

Edited August 26, 2019 by Aegolius13
Removed last sentence

[Zhetaan]

22 hours ago, MPDerksen said:

When building a rocket, how do I evaluate it’s ISP and TWR?

I_sp is dependent on the engine, and it is part of the tooltip for the engine in the VAB editor window where you select it.  In fact, both the I_sp and thrust are available in that information window without needing to right-click.  Note that I_sp is also affected by the presence of an atmosphere:  the values given in the VAB represent a range from sea level on Kerbin to vacuum.

TWR is dependent on the rocket, the engine, and the gravity of the celestial body.  Since fuel mass decreases as the rocket flies, TWR changes in flight.  I do not know whether the current version of KSP has a TWR readout to go with the delta-V readout; I use Kerbal Engineer for that information.  MechJeb has it, too.  For a stock answer, @Aegolius13 has it right; the gee readout on the right side of the navball will tell you the thrust-to-weight ratio.  That works because the readout already accounts for the mass of the rocket, and since a standard Kerbin gravity is one gee, you have to divide the gees by one to convert it to a TWR readout (i.e. it's the same).

It's of limited use:  it only works once you're in flight and it's only accurate when you're pointed up.

It seems flippant, but you'll figure out whether your rocket has enough thrust to fly by whether it falls to the pad and explodes, so that's another way.

23 hours ago, MPDerksen said:

Specific example, I’m prepping for my first mission with a rover under a lander.  I want to construct a lifter to get it into LKO.  Some of my designs take off and I get heat build up, others seem to accelerate VERY slowly and I burn for a VERY long time before getting up to orbit.

You're contending with atmospheric drag.  You get more drag with more thrust, but you also get more heating.  You can alleviate that somewhat by using less thrust, but then you spend more time and fuel fighting gravity because you take a while to get to orbit.

23 hours ago, MPDerksen said:

It would just be nice to have some guidelines for basic construction.  Getting there.

The standard advice applies:  make your rocket long and pointed.  Put the fins on the bottom should you need them.  Taper the size changes; don't have any flat faces pointing into the wind.  Put everything that's not a fuel tank or a pointed pod in a fairing.  Keep the fairing as close to the original diameter of the rocket as possible.

For a rover, that often means that the best thing is to make it as small as possible.  If it can fit in a service bay, then it won't need a fairing.  Don't feel attached to the idea of a mobile base; a metal plate on wheels with a solar panel, a battery, and a command seat will work as a rover.  Begin with that and then, once you have both a payload that works and a lifter than puts it in orbit, try scaling it to larger rovers.  Eventually, you'll reach a point that you need to consider skycranes and underslung rovers, but work out how to fly a normal rocket payload before you start on the exotic stuff.

[AHHans]

3 hours ago, Zhetaan said:

I do not know whether the current version of KSP has a TWR readout to go with the delta-V readout;

It does! And since 1.7 the window to select the celestial body and atmospheric height in the VAB/SPH also works on (my) Linux. Very useful to figure out if your rocket will be able to take off from Eve!