Mun Lander: Terrier v. Nerv

[Baker]

I think I have misunderstood something about ISP and hope you can help. I'm trying to refine my multi-hop Mun lander by replacing the Terrier engine with a recently discovered Nerv.  I was hoping replacing the Terrier with the Nerv, a much higher-ISV engine, would enable my hopper to stop at more Mun locations before returning. But something's not right.

My hopper is a command pod, some sciency things, a T-400 fuel tank, and an engine. Because the Nerv's ISP is so much higher, I would expect it to either offer more thrust at the same burn time or the same thrust with longer burn time. But this is not proving to be so. Here is what the same craft looks like with Terrier v. Nerv engines:

Engine	Delta-v	ISP (vacuum)	Thrust	TWR	Burn
Terrier	1932 m/s	345s	60 kN	8.02	113s
Nerv	1064 m/s	800s	60 kN	5.19	118s

 

The Nerv craft seems to be more efficient (higher ISV). But it has the same thrust and burns for basically the same amount of time. What am I not understanding about ISP?

 

[Superfluous J]

Your understanding of ISP is fine (other than calling it ISV ) but you're missing the fact that the NERV is HEAVY. It's just not suited for a small lander. The Terrier is the right tool for that job.

Also, just in case you don't know, the NERV doesn't need oxidizer so if you are carrying that in your tanks you're carrying dead weight.

[Rocket In My Pocket]

I'll add one more thing about the NERV, it makes for a terrible rescue lander engine as well. Lol.

Almost fried the Kerbal I was going to rescue when she approached the ship, never put radiators on it so she had to sit and wait a while for it to cool down enough that she could approach.

It's also tall and awkward to place. @5thHorseman explained the rest. NERV's are mainly for long range missions to other planets.

[Baker]

OK everything you said @5thHorseman explains the TWR. But not the burn time. A higher ISP (not ISV! lol) engine should burn longer at the same thrust, right? Otherwise what exactly does ISP mean?

In any case I take your point that Nerv is probably not right for a lander. This other question is academic.

[merlinux]

46 minutes ago, Baker said:

OK everything you said @5thHorseman explains the TWR. But not the burn time. A higher ISP (not ISV! lol) engine should burn longer at the same thrust, right? Otherwise what exactly does ISP mean?

In any case I take your point that Nerv is probably not right for a lander. This other question is academic.

yes but as @5thHorseman told you the nerv is heavier and your craft as well, even more if your carrying oxidizer    all this extra weight is giving you a loss in Dv.

nerv are a lot more effcient for heavier craft that dont need a high TWR.

And as a side note, before squad nerfed the nerv some years ago, I had many landers using it, its only drawback was its lenght It was by far the best engine in the game and the fact that it was used in landers might have contributed to the nerfing. It was always meant to be an interplanetary engine by squad.

 

Edited June 2, 2019 by merlinux

[bewing]

46 minutes ago, Baker said:

engine should burn longer at the same thrust, right? Otherwise what exactly does ISP mean?

Isp is a measurement of efficiency. It tells you how fast you will burn up all your fuel. Or, as you say, how much longer it will burn for the same thrust. But if you replace a tiny lightweight engine with a heavy one in the same design, then that difference in mass changes the whole calculation.

 

[cantab]

As mentioned, the Nerv burns only liquid fuel. Which means that if you replace a chemical engine and change nothing else, then the Nerv has less than half the propellant available that the chemical engine had, and it's stuck pushing the oxidizer as dead weight! No wonder that doesn't work well!

At the very least you need to remove the oxidizer from the fuel tanks. Better is to use liquid-fuel-only tanks, but in stock the range is limited.

[HebaruSan]

3 hours ago, Baker said:

A higher ISP (not ISV! lol) engine should burn longer at the same thrust, right? Otherwise what exactly does ISP mean?

Higher ISP means higher exhaust velocity (ISP is just exhaust velocity divided by 9.81 m/s/s). Higher exhaust velocity means more thrust per molecule of propellant, so if ISP increases while keeping the thrust the same, then fewer molecules of propellant must be going out the back. So I'd say that your statement is correct in the abstract, holding constant all sorts of things that aren't constant in this example.

[Zhetaan]

14 hours ago, Baker said:

I think I have misunderstood something about ISP and hope you can help. I'm trying to refine my multi-hop Mun lander by replacing the Terrier engine with a recently discovered Nerv.  I was hoping replacing the Terrier with the Nerv, a much higher-ISV engine, would enable my hopper to stop at more Mun locations before returning. But something's not right.

My hopper is a command pod, some sciency things, a T-400 fuel tank, and an engine. Because the Nerv's ISP is so much higher, I would expect it to either offer more thrust at the same burn time or the same thrust with longer burn time. But this is not proving to be so. Here is what the same craft looks like with Terrier v. Nerv engines:

Engine	Delta-v	ISP (vacuum)	Thrust	TWR	Burn
Terrier	1932 m/s	345s	60 kN	8.02	113s
Nerv	1064 m/s	800s	60 kN	5.19	118s

 

The Nerv craft seems to be more efficient (higher ISV). But it has the same thrust and burns for basically the same amount of time. What am I not understanding about ISP?

 

Respectfully, I disagree with the others; a Nerv-powered lander can work just fine, but you have to take into account its particular issues.  Mass is part of it.  Heat is another.  The length of the engine is yet another.  But you also need to load it with the correct fuel.

 

My apologies to those who commented before if I repeat anything, but in the interest of gathering these ideas into one place:

 

First, I_sp is a measure of efficiency, as others (@bewing) have said.  It is not the only way, and it is not the most intuitive way.  The most intuitive way is exhaust velocity:  an engine that propels its exhaust at a higher velocity imparts more momentum to that exhaust (and, because of conservation of momentum, to the rocket, too) than an engine that propels its exhaust at a lower velocity.  The convenient thing about I_sp and v_exh is that they directly relate:  in other words, if you have two engines and one has double the I_sp, then it also has double the v_exh.

Please note that efficiency does not necessarily relate to thrust.  Because of the way engines are made, there's often a trade-off between thrust and efficiency, but this is not always necessarily so.  For example, the ion engine accelerates its exhaust to ludicrous speed which makes it extremely efficient, but it does so almost a molecule at a time, so its thrust is terrible.  A solid rocket booster, on the other hand, comparatively vomits its propellant onto the pad nearly all at once and thus achieves high thrust, but at such low velocity that the only real advantage is that SRBs are cheap.  On the gripping hand, Project Orion is both efficient and high-thrust; please never mind the fallout.

 

Second, you are correct that at the same thrust, the higher I_sp of the Nerv gives a longer burn time for the same amount of fuel.  We can work that through the calculation and figure out how much fuel is involved:

We can consider that thrust from an engine is essentially constant regardless of the mass of the rocket (unlike delta-V).  The engine exhaust velocity is also constant, and that is directly related to I_sp (thanks to @HebaruSan):  For the Terrier, it is 3383.29 m/s, and for the Nerv, it is 7845.32.

Thrust is a force, which traditionally is defined as mass times acceleration (that's effectively Newton's Second Law), but acceleration is velocity divided by time, which means that we can define thrust in terms of velocity and still get a valid answer.  Specifically, thrust, the force, is equal to velocity times the quantity of mass divided by time--which we can call mass flow rate.  In other words, thrust results from the engine exhaust velocity times the rate of fuel flow.  Tabulated, these give us the following values:

Thrust / v_exh = M_flow

Terrier:  60000 N / 3383.29 m/s = 17.734 kg/s
Nerv:  60000 N / 7845.32 = 7.648 kg/s

Fuel flow rate times total burn time gives the total mass of fuel consumed:

Terrier:  17.734 kg/s * 113 s = 2 tonnes
Nerv:  7.648 kg/s * 118 s = .9 tonnes

The FL-T400 tank holds two tonnes of LFO, divided up as .9 tonnes fuel and 1.1 tonnes oxidiser.  In this case, using the same tank for your Nerv engine starved it of more than half of its potential fuel load.  I could use the rocket equation to see, given your delta-V values, whether you unloaded the oxidiser from the tank, but you really ought to have used Mk1 Liquid Fuel Fuselage tanks for a Nerv-powered lander.  It's the exact same tank but in a liquid-fuel-only variant; if you had used that, you would have had more than double the burn time and much more delta-V.

Specifically:

2 tonnes LF * 7.648 kg/s = 262 seconds, approximately

5.19 TWR / 60 kN = 11560 N weight
11560 N / 1.63 m/s² Mun surface gravity = 7092 kg mass
(You didn't drain the Oxidiser!)

Assuming 2000 kg liquid fuel:

ln (7092 kg / 5092 kg) * 7845.32 m/s = 2599 m/s delta-V

We can call this approximately 2600 m/s of delta-V.  It's only 700 more than you'd get with the Terrier, so there's an argument to be made about adding another fuel tank rather than switching to a Nerv, but there's an equally valid argument for using the Nerv and keeping the lander relatively small.

 

Replace the FL-T400 with a Mk-1 fuel fuselage.  Used in this way, your lander will be perfectly serviceable.

Edited June 2, 2019 by Zhetaan