NERVs at Eve Sea Level?

[Zosma Procyon]

How much thrust will a NERV engine make at sea level on Eve? I know it is a tiny number, but I'm considering sending a boat base to Eve, and obviously jet engines don't work there. If the majority of the hull space was liquid fuel, I figure a boat with 6 or so NERV engines could make slow progress across the exploidium seas. The only other kind of stock boat I've heard of is an overly complicated paddle wheel design that I don't feel like reproducing.

[Mad Rocket Scientist]

It's not worth it since Isp scales with altitude, not just thrust. i.e. a NERV burns as much fuel ASL as in vacuum, and only produces 0.3x the thrust. The Vector has the best ASL Isp, with the aerospike a close second. The best option would to make a electric propeller if you're not worried about speed.

Edited August 17, 2018 by Mad Rocket Scientist

[Ultimate Steve]

I agree with @Mad Rocket Scientist. NERVs generate little if any thrust at Eve sea level and because of that would get a really low specific impulse. The electric propeller option is great if slow, but unfortunately you'd need a way to attach it which is fairly difficult.

I agree you're going to want Vectors, Mammoths, or Aerospikes. They get decent isp at ESL. The problem I can foresee is how you are going to refuel it in the middle of the ocean...

One thing I did on Eve, while not on water, was a transportation craft that could refuel itself and used 2 vectors and a mammoth. It is called the ESREP, Evian Self Refueling Exploration Plane, basically an airliner for Eve.

 

Unfortunately my only experience with interplanetary boats is a tiny submarine on Laythe that I haven't used yet. Good luck!

[qzgy]

1 hour ago, Mad Rocket Scientist said:

It's not worth it since Isp scales with altitude, not just altitude.

Huh? Do you mean both ISP and thrust scale with altitude, not just thrust?

 

As for a boat, why not make a RTG powered electric propeller?

Edited August 17, 2018 by qzgy

[Mad Rocket Scientist]

2 minutes ago, qzgy said:

Huh? Do you mean both ISP and thrust scale with altitude, not just thrust?

As for a boat, why not make a RTG powered electric propeller?

Thanks for catching that.  I can't write at the end of school days and I'm too lazy to proofread.

EDIT: I probably made a markup vs. margin mistake too. Oh well.

Edited August 17, 2018 by Mad Rocket Scientist

[qzgy]

1 minute ago, Mad Rocket Scientist said:

Thanks for catching that.  I can't write at the end of school days and I'm too lazy to proofread.

EDIT: I probably made a markup vs. margin mistake too. Oh well.

No problem. Happens to the best of us. Don't blame you.

[wumpus]

9 hours ago, qzgy said:

Huh? Do you mean both ISP and thrust scale with altitude, not just thrust?

As for a boat, why not make a RTG powered electric propeller?

Fuel flow rate should be constant regardless of atmospheric pressure.  I'd think that exhaust velocity varies with atmospheric pressure, thus reducing Isp at significant pressures.  In any case the amount of delta-v per amount of fuel is significantly reduced in atmosphere.

Note that this is completely different from the pre-release model where Squad would increase fuel flow to directly decrease Isp while leaving thrust constant.  The current model is more or less how it really works (although don't expect miracles from nozzle simulation as it passes through the atmosphere).

[Mad Rocket Scientist]

3 hours ago, wumpus said:

Fuel flow rate should be constant regardless of atmospheric pressure.  I'd think that exhaust velocity varies with atmospheric pressure, thus reducing Isp at significant pressures.  In any case the amount of delta-v per amount of fuel is significantly reduced in atmosphere.

Note that this is completely different from the pre-release model where Squad would increase fuel flow to directly decrease Isp while leaving thrust constant.  The current model is more or less how it really works (although don't expect miracles from nozzle simulation as it passes through the atmosphere).

Prop flow remains constant, but Isp varies with pressure. Ve varies with Isp.

Thrust is Ve*prop flow, so thrust also decreases.

[Snark]

On 8/16/2018 at 9:30 PM, qzgy said:

Huh? Do you mean both ISP and thrust scale with altitude, not just thrust?

Correct.  If you run a vacuum engine like the LV-N in a high-atmospheric-pressure situation, both the thrust and the Isp go into the toilet.  It keeps eating just as much fuel, but you get less oomph out of it.  So if you're picking an engine to use at Eve ASL, you want the one with the highest Isp at that pressure, which means an LV-N would be one of the worst possible engines you could use.

Note that this wasn't always the case.  It used to be that KSP used an unrealistic model where the Isp stayed constant and only the thrust went down, i.e. the engine simply used fuel more slowly, but was just as efficient.  So if you read old, old posts (or watch really old videos), that could confuse you.    But that behavior got fixed many KSP versions ago-- I forget exactly which, but if I had to take a wild guess I'd say it would be in KSP 1.0.

[EDIT]  Correction, thanks to @Laie for pointing this out in his post, below.  Yes, the older KSP used an unrealistic model that was the "other way around" from how it is now, but I incorrectly stated above how it was the other way around.  The Isp changed, just as it does today.  It's the result of the lowered Isp that they had backwards.  The correct thing to happen (as they do now) is that when Isp gets worse, the rate of fuel consumption stays the same but the thrust goes down.  What they used to do, back in the day, was that the thrust stayed constant but the rate of fuel consumption went up.

Edited August 20, 2018 by Snark
Thanks, Laie :-)

[FinalFan]

On 8/16/2018 at 10:45 PM, Ultimate Steve said:

I agree with @Mad Rocket Scientist. NERVs generate little if any thrust at Eve sea level and because of that would get a really low specific impulse. The electric propeller option is great if slow, but unfortunately you'd need a way to attach it which is fairly difficult.

I agree you're going to want Vectors, Mammoths, or Aerospikes. They get decent isp at ESL. The problem I can foresee is how you are going to refuel it in the middle of the ocean...

One thing I did on Eve, while not on water, was a transportation craft that could refuel itself and used 2 vectors and a mammoth. It is called the ESREP, Evian Self Refueling Exploration Plane, basically an airliner for Eve.

 

Unfortunately my only experience with interplanetary boats is a tiny submarine on Laythe that I haven't used yet. Good luck!

Wow, that's impressive!  But what about using such a huge atmospheric craft was good enough to outweigh the drawbacks for you?  Was it just the fact that it has enough delta-V to make orbit on Kerbin? 

[Teilnehmer]

 

Let’s see the Nerv’s .cfg file:

atmosphereCurve
{
  key = 0 800
  key = 1 185
  key = 2 0.001
}

The extrapolation rules used in KSP imply that the engine’s I_sp is 0.001 s for any atmospheric pressure greater than 2 atm. The thrust is, therefore, 75 mN (milliNewton). Not exactly zero, but can be considered zero for all practical cases.

[Ultimate Steve]

8 hours ago, FinalFan said:

Wow, that's impressive!  But what about using such a huge atmospheric craft was good enough to outweigh the drawbacks for you?  Was it just the fact that it has enough delta-V to make orbit on Kerbin? 

Well, that was the original idea:

That's the core engine with all its tons of fuel, but it's also dragging two nuclear stages. It was supposed to SSTO but it didn't really work out, so I went with plan B, strap boosters to it.

And then I had enough fuel in orbit to bring it to Minmus to refuel. After I filled up there, I sent it to Eve.

You'll notice the lack of heat shields. The NERV stages burn to depletion, and that gets it into Eve orbit. The Mammoth (the two vectors at the bottom are only there for pulling up in atmospheric flight) is used to use up all of the fuel to slow it down enough so it doesn't need a heat shield. This also means that it's a glider from this point onwards.

 

So it was not an SSTO from Kerbin, although it was *almost* supposed to be at the beginning. However I don't think I really designed it with that in mind, I think I just tried to SSTO it because I noticed that it theoretically could. I designed it to be so big because first, it needed to carry ISRU, second, it needed to carry up to 16 Kerbals (this was a rescue mission, I had several Kerbals stranded), it had to use Mammoths or Vectors to get any sort of specific impulse at sea level, and it needed to have a range of at least 100 kilometers.

However, that range number is affected a lot by the starting altitude. From 1km ASL you can get over 100km out of it, if you started on Eve's tallest mountain you can go a lot further. In the end I said "I could go small or big" and went big.

Not to advertise or anything, but the craft is part of my mission report, Project Intrepid (link in sig) if you'd like to see more action, although it doesn't come in until very late and hasn't done much yet.

[FinalFan]

5 hours ago, Ultimate Steve said:

Well, that was the original idea:
[pic]
That's the core engine with all its tons of fuel, but it's also dragging two nuclear stages. It was supposed to SSTO but it didn't really work out, so I went with plan B, strap boosters to it.
[pic]
And then I had enough fuel in orbit to bring it to Minmus to refuel. After I filled up there, I sent it to Eve.
[pic]
You'll notice the lack of heat shields. The NERV stages burn to depletion, and that gets it into Eve orbit. The Mammoth (the two vectors at the bottom are only there for pulling up in atmospheric flight) is used to use up all of the fuel to slow it down enough so it doesn't need a heat shield. This also means that it's a glider from this point onwards.

So it was not an SSTO from Kerbin, although it was *almost* supposed to be at the beginning. However I don't think I really designed it with that in mind, I think I just tried to SSTO it because I noticed that it theoretically could. I designed it to be so big because first, it needed to carry ISRU, second, it needed to carry up to 16 Kerbals (this was a rescue mission, I had several Kerbals stranded), it had to use Mammoths or Vectors to get any sort of specific impulse at sea level, and it needed to have a range of at least 100 kilometers.

However, that range number is affected a lot by the starting altitude. From 1km ASL you can get over 100km out of it, if you started on Eve's tallest mountain you can go a lot further. In the end I said "I could go small or big" and went big.

Not to advertise or anything, but the craft is part of my mission report, Project Intrepid (link in sig) if you'd like to see more action, although it doesn't come in until very late and hasn't done much yet.

Thanks for all the details, and the link!  I checked it out. 

One reason I asked my question is because I was intrigued by your vessel and decided to try test-building and launching an equivalent.  It was based on eyeballing your picture and description so naturally it did not have the NERV transfer stage that you had already jettisoned.  It made orbit fine.  I have a lot less experience than you, and I'm still working on my first interplanetary experience (though I've done some "proactive asteroid intercepts"), but it seemed on paper that the core vessel itself ought to have enough to make it from Kerbin system either directly to Eve or refueling at Gilly.  (The massive fuel tanks compensating for the lower efficiency.)  Is there something I'm simply not seeing, or is yours heavier than mine, or just a thematic choice?  How many drills does your plane have?  I only put 2 large drills in mine. 

On second thought, this might be going off topic, so I think I will put this post in your Project Intrepid thread. 

[Ultimate Steve]

7 minutes ago, FinalFan said:

Thanks for all the details, and the link!  I checked it out. 

One reason I asked my question is because I was intrigued by your vessel and decided to try test-building and launching an equivalent.  It was based on eyeballing your picture and description so naturally it did not have the NERV transfer stage that you had already jettisoned.  It made orbit fine.  I have a lot less experience than you, and I'm still working on my first interplanetary experience (though I've done some "proactive asteroid intercepts"), but it seemed on paper that the core vessel itself ought to have enough to make it from Kerbin system either directly to Eve or refueling at Gilly.  (The massive fuel tanks compensating for the lower efficiency.)  Is there something I'm simply not seeing, or is yours heavier than mine, or just a thematic choice?  How many drills does your plane have?  I only put 2 large drills in mine. 

On second thought, this might be going off topic, so I think I will put this post in your Project Intrepid thread. 

The reason for the refueling is mainly so I can retruburn a lot at Eve, Eve is not kind to anything without heat shields. If I hadn't cancelled out a significant portion of my orbital velocity it would have probably burned up at Eve.

EDIT: If you can make it to Gilly, that might be more practical than Minmus. The reason I went to Minmus was mostly story, Gilly would probably have been better logistically.

Edited August 19, 2018 by Ultimate Steve

[FinalFan]

2 minutes ago, Ultimate Steve said:

The reason for the refueling is mainly so I can retruburn a lot at Eve, Eve is not kind to anything without heat shields. If I hadn't cancelled out a significant portion of my orbital velocity it would have probably burned up at Eve.

So refueling at Gilly, and then going to Eve, would have left you with too little fuel to slow down to "non-explodey" speeds? 

[cubinator]

On 8/17/2018 at 5:25 PM, Snark said:

But that behavior got fixed many KSP versions ago-- I forget exactly which, but if I had to take a wild guess I'd say it would be in KSP 1.0.

I don't remember it working like that at all, so I think it was before 0.23.0.

[Ultimate Steve]

44 minutes ago, FinalFan said:

So refueling at Gilly, and then going to Eve, would have left you with too little fuel to slow down to "non-explodey" speeds? 

No, the Gilly route probably would have been the best route (but I went to Minmus instead because I had a base there). The refueling combined with the NERV stage makes it enough to slow down to non explodey speeds, at least with the craft I designed. You can probably save mass in places to make it go farther, and there may have been some margin. I also took a rather fast transfer to Eve which increased the delta-v requirements and final velocity a bit.

[FinalFan]

1 minute ago, Ultimate Steve said:

No, the Gilly route probably would have been the best route (but I went to Minmus instead because I had a base there). The refueling combined with the NERV stage makes it enough to slow down to non explodey speeds, at least with the craft I designed. You can probably save mass in places to make it go farther, and there may have been some margin. I also took a rather fast transfer to Eve which increased the delta-v requirements and final velocity a bit.

I see.  Thanks again. 

[Snark]

18 hours ago, cubinator said:

I don't remember it working like that at all, so I think it was before 0.23.0.

No, it was definitely well after 0.23.5, which is when I began playing.

The fact that they got it the wrong way round wasn't as big a deal back then, as there was less variation between vacuum and atmospheric ISP, and the big rebalance to make some engines "vacuum engines" hadn't happened yet,  which may be why you didn't happen to notice it at the time.

[EDIT] They did indeed used to model it differently, but I wasn't thinking clearly for a moment and incorrectly stated how they had it the other way around.  Thanks to @Laie for the correction, see below.  I've updated my post above to reflect how things actually used to be.

Edited August 20, 2018 by Snark
Thanks, Laie :-)

[Laie]

On 8/18/2018 at 12:25 AM, Snark said:

It used to be that KSP used an unrealistic model where the Isp stayed constant and only the thrust went down, i.e. the engine simply used fuel more slowly, but was just as efficient.

It used to be the other way 'round: thrust remained constant but fuel consumption scaled with ISP (so your nuke or Poodle on Eve would consume an orange tank worth in a matter of moments). I think it still was that way when the Eve Rocks Challenge ran -- too much in a hurry to check right now, but wasn't that 0.90?

[Snark]

19 minutes ago, Laie said:

It used to be the other way 'round: thrust remained constant but fuel consumption scaled with ISP (so your nuke or Poodle on Eve would consume an orange tank worth in a matter of moments).

Whoops, you're absolutely right.  Brain fart on my part.    Yes, the Isp tanked, but they upped the consumption instead of lowering the thrust.  You're absolutely right.