SSTOs, how many nerves?

[Mk3 Maniac]

Dear Everyone, 

I have made an SSTO that lands on Laythe. I want to land it somewhere else in the Jool system like Val, or even somewhere like Eeloo , but the Nerve engines have to rely on the closed cycle of the rapiers The craft weighs about 300 tons. So my question is how many more Nerves does the craft need? One? Two? Three? Please give me advice oh good KSP community!

Sincerely: Mk3 Maniac

 

[IncongruousGoat]

Well, if your craft weighs 300 tons, and you want to land on Vall, then you need a thrust-to-mass ratio of at least 3.5, for a TWR of 1.5 (and having it this low will mean an unpleasant and inefficient landing). So, that means you need to be producing at least 1050 kN of thrust with your engines. Since each NERV produces 60 kN, this means 18 engines. Well, not quite. See, 18 NERVs weigh 54t, which means we have to factor the weight of engines added into our calculations. Doing some simple algebra yields a figure of 22 engines. Honestly, I think you'd be better off using the closed-cycle rapiers. 66 tons of nuclear engine are going to do horrible things to your delta-V.

[Hodo]

Nukes are horrible landing engines.  They are to heavy for the TWR.   I would suggest using Aerospikes for landing engines. While not as efficient they are more powerful and lighter.   I would also consider a lighter lander instead.  There is nothing wrong with sending it as one craft, with a lander docked inside of it.  I do that quite a bit now.   Found it cheaper and safer to just send a lander down to the surface of most moons.  

But one problem I noticed with your craft.

Your nukes are not inline with your CoM, this means the CoT will be over your CoM, causing the vessel to pitch down in space, meaning you will burn a LOT of RCS monoprop to keep the nose on target.   

[Urses]

As a though through option.

I would build this craft as a two stage vehicle. A landing SSTOplane with a NERV powered space engine tug. The whole back section docked with a senior at forward section with rapiers for landing and takeoff. NERV's for landing is to much on mass.

You can let ISRU in orbit land and mine. Go up and process the ore in orbit as example?

Funny Kabooms 

Urses 

[Razorforce7]

Solution: Drain all you vehicles fuel tanks in the VAB *cough* SPH until they match the total vessel weight when landed at Vall (or any destination)

Spawn your vehicle @ the ksc whether it be the runway or launchpad in the emptied tank configuration (the one that mimics the total vessel weight when landed at your destination, i.e. Vall)
Use the gravity cheat and change the total Kerbin gravity to 24 out of 100 (24%)

Does your vessel takeoff at Kerbin in this gravity configuration?
If yes, then it will do the same at Vall in the exact same way (albeit it terrificaly slow since valls gravity is about exactly 1/4th that of kerbin)
If it does takeoff but slowly, then, you will need more Nervas.

Eventually I have to join with Urses and recommend a two stage aproach. Even for ssto designs launched from Kerbin it will eventually be more efficient to use a 2nd stage for landing and takeoff at Joolian destinations.
This obvious and necessary aproach achieves all Jool 5 missions reported. And besides some Laythe and Bop missions nobody directly launched to both places and returned in a single stage.

 

Edited April 29, 2017 by Razorforce7

[steve_v]

10 hours ago, Mk3 Maniac said:

how many more Nerves does the craft need?

Depends on your piloting skills, SSTOs require nerves of steel, and flying them does fray mine...

OTOH, if you're talking about an engine, that would be a NERVA (or NERV, 'cause KSP is goofy). For landing, the answer is (generally): Don't. Unless you're landing on something with tiny gravity.

Edited April 30, 2017 by steve_v

[juanml82]

Uh, why do you need so much weight to begin with? An MK2 based SSTO with 4 rapiers plus 2 nervs can SSTO from Laythe and would probably work well in Vall, provided you use the rapiers for the final landing and take off (huh, which probably means packing more oxydizer than what's required for departing Laythe though). The lighter your payload, the less fuel and engines you'll need.

I'd start based on something like this

Just don't detach the rapiers and add some (draggy) landing legs to it. Take off from Kerbin, refuel in LKO (or Minmus or the Mun if you trust your astrogator skills to make your Pe once you return from Mun or Minmus coincide with the Trans-Joolian injection burn).

I'd either add the small isru equipment or refuel again in the Jool system, probably at Vall. Also, add airbrakes and landing chutes: Laythe has little land and what dry land there is, it's usually hilly, so you'll won't have much of a natural runway for a conventional horizontal landing and take off. Alternatively, you can set up the ship to land in the sea. In that case, I'd suggest you to add a bunch of verniers in the nose, to help tilt the ship upwards to help it take off from the water.

Also, that shielded docking port I have in the nose is draggy, but it stands the reentry heat rather well. A shock intake and an inline docking port are more aerodynamic, though.

[AeroGav]

20 hours ago, Razorforce7 said:

Solution: Drain all you vehicles fuel tanks in the VAB *cough* SPH until they match the total vessel weight when landed at Vall (or any destination)

Spawn your vehicle @ the ksc whether it be the runway or launchpad in the emptied tank configuration (the one that mimics the total vessel weight when landed at your destination, i.e. Vall)
Use the gravity cheat and change the total Kerbin gravity to 24 out of 100 (24%)

Does your vessel takeoff at Kerbin in this gravity configuration?

That won't work,  because Kerbin has a thick atmosphere and the NERVs will only produce 13kn or so at sea level, vs 60kn that you get in a vacuum - like you have on Vall.

 

[Cunjo Carl]

@Mk3 Maniac I'm afraid my answer is 2 Nerves. 1 for your interplanetary transfers, and the second for symmetry.

The NERVs just aren't suited for landing in gravity as high as Vall's, and by the time you have enough NERVs to do it well, you'll have so many that their Isp advantage over liquidFuel+Oxidizer engines starts becoming mute deltaV-wise! (not completely, but getting close). The 12 rapiers in closed cycle should have plenty of thrust to fill the gap (TWR on 300 tons will be .76 relative to Vall gravity .23 relative to Kerbin). Doing the first part of the gravity turn with NERVs is a given, but near the end you'll need that extra oomph.

Otherwise, 300 tons of craft would require 13 NERVs to land on Vall, and probably 20-25 to land well given the standard low SSTO maneuverability in vacuum. This would be fun, but perhaps not for the right reasons.
                  100 tons of craft would require 5 Nervs to land on Vall, and probably 7-10 to land well. Just guessing, you'll be a lot lighter by the time you get to Vall! Maybe this wouldn't be so bad after all.

That said, one vector (radially attachable) pointing down from your craft's (300ton) center of mass can turn it into a Vall VTOL. Its thrust can be controlled independently from the rest of the vehicle by right clicking it and using its thrust limiter, which can help with the gravity turn. It's touchy to fly like this in practice, but gets you where you're going without a lot of extra design effort and looks awesome. Darts (aerospikes) can do the same with better ISP for the 100 ton version.

That's my style anyways!

Good luck on your trip out to Jool's mega Minmus!

[ForScience6686]

That seems much to heavy for nerv use.  I generally stick to them for medium sized craft only.

[AeroGav]

Does the craft have ISRU ?

It is worth remembering that the craft will not be at max weight when landing, as most of the fuel will have been used getting to orbit from Kerbin and then reaching / capturing in the Jool system.

My only experience of this was my Jool 5 mission

 

4 Nervs on a very large spaceplane, but still quite a ways lighter than the op's as it doesn't carry so much fuel. TWR 1.15.    The cargo bays are mostly full with hab modules, rovers and lander cans.   I could only take off with 60% LF and 0% oxidizer, any more and we were too heavy.   Still this was enough to get us to Pol or Bop for a final refuelling stop. 

Also this was not SSTO,  the air breathing engines were on decouplers and i shed the last of them when departing Laythe, which we visited before Val.

21 hours ago, Cunjo Carl said:

That said, one vector (radially attachable) pointing down from your craft's (300ton) center of mass can turn it into a Vall VTOL

Only trouble is it has horrible ISP in Vacuum. I'd rather go with a Poodle or similar if you can work a "ski jump" style takeoff.   Then again , unless you decouple all your air breathers like i did i am sure you'll have some rapiers that can be pressed  into service,  no mass penalty as you already have them on you.

BTW the reason my plane was so large was because it was carrying a Tylo lander.   I did not send the plane itself to tylo, but it did visit every other moon.