at what altitude should i switch over to nukes from air/hybrid

[endl]

im trying to figure out how to best set up my propulsion for my space plane. atm im using rapiers which kick over to oxidizer around 35km. at this point i dont know if i need more intakes to get higher or just better thrust. either way id like to know when it makes sense to use nukes over conventional engines.

[dzikakulka]

Basically any airbreathing engine at any altitude will have better Isp than nuke (even nuke in vacuum). You should delay switch to rocket engine for as long as you can. Simpliest way is to switch to them when all jets flame out on regular ascent, but it is not optimal.

Usually what you want to do is to climb to 20-30km (highest possible altitude at which you have enough intakeair for stable burn) and then burn horizontally.... well as long as you gain speed it's good. Usually with over 2km/s you can slowly tilt up and proceed achieving orbit as usual, again, burning with jets as long as you can ofc.

PS. Nukes have Isp higher than any other rocket engine in game already at ~30km altitiude or something. Switch to them from other rocket propulsion as soon as low thrust s no longer a problem (for efficiency).

[Taki117]

The big problem with Nukes is that they are big and heavy, meaning that with a spaceplane you are better off using a smaller, lighter engine such as the 48-7S or the 24-77 engines.

[Alshain]

The toroidal aerospike makes a great spaceplane engine. It's relatively efficient in the upper atmosphere, not too heavy, and has more thrust than the 48-7S and 24-77.

[capi3101]

The short answer: when your craft stops accelerating on the jets, it's time to switch over. It's generally best to use nukes only if you plan on taking your craft interplanetary (or for that matter, Mun and Minmus); if you're just going for a Kerbin orbit, go for something light and efficient (folks have given you some good suggestions already).

[Geschosskopf]

im trying to figure out how to best set up my propulsion for my space plane. atm im using rapiers which kick over to oxidizer around 35km. at this point i dont know if i need more intakes to get higher or just better thrust. either way id like to know when it makes sense to use nukes over conventional engines.

The RAPIER isn't a nuke, it's a combo jet-rocket intended to be the only engine a spaceplane needs. It start out as jet until it gets so high it runs out of atmospheric air, then switches to rocket mode to use oxidizer instead, which should carry you the last little way ot oribt. No nuke involved.

If you're not running out of air until 35km, that's good. You can't do much better than that without obscene amounts of intake spamming. So at that point, it's just a question of whether you have enough of both liquid fuel and oxidizer to get to orbit. You should be doing over 2000m/s at 35km so when the engines switch over, point up about 46^ to raise your Ap to the desired level, then coast there and circularize. Shouldn't take but a few hundred dV to do this part.

Now, once you're in orbit, the RAPIER is a pretty inefficient rocket engine for long trips. This is when you'd use a nuke, assuming you were carrying one.

[KerikBalm]

As to what has been said - the 48-7s shines when you can get a low dry mass.

When you have jets, wings, lots of fuel tanks, etc, the added mass of a 48-7s vs a LV-909 is often not greater than the difference between 350 and 390 IPS (as a %), so for any given dv, you use less fuel (there is a difference between mass optimal and fuel optimal, and with SSTOs, I worry about fuel optimal and not mass optimal).

With stock aero, you can intake spame and get to >40km and >2000 m/s, requiring very little rocket thrust (in some cases, none, or ions alone can do it).

With FAR/NEAR, I can't seem to get much above 1600 m/s surface velocity.

[EdFred]

Done right you can fly along at 28-30km on turbojets, and push your apoapsis well up over 200km if you so desire and switch over to nukes.

[cantab]

If you have an LV-N with its position and fuel supply making it suitable to use, you should be using it any time you're on rocket (as opposed to jet) power and it gives a higher Isp than your chemical engines. The altitude this becomes the case depends on the chemical engine of course, but for an LV-T30/45 it's a mere 1000 metres or so on Kerbin.