Atomic engines and SSTO

[Kozenger]

My question is if it's practical to try and get an SSTO, around 15 to 20 tons, out of atmo with an atomic without air hogging.

I've been having problems trying to create a SSTO that uses atomic engines without air hogging or stacking. I tend to build in the 15 to 20 ton range and most of the space planes I get in atmo with the aerospike tend to have ~200 to 500m/s delta v to spare. Which is ok as long as I dock before I leave a planet to refuel but it's always tight and isn't enough for a few places. Most of my attempts to use atomic engines have had issues maintaining speed while trying to push out of atmo. In general I will be flying at around 1200 to 1600m/s and at 22k to 24k altitude but even in my smaller craft the atomic just doesn't have the umph to get me up and out before I lose too much speed.

Again this is an issue with my personal limitation of not wanting to place more then 1-2 intakes per jet engine preventing me from getting out of atmo with an atomic engine. I can air hog and get into a LKO with 3000+ m/s of delta v but I don't find that a fun way to play. Any suggestions would be appreciated as I've ran into a creative wall.

Thanks for your time and input.

Edited August 27, 2013 by Kozenger

[Dangerous_Beans]

The limit looks like the poor thrust to weight ration of the engine. We only have 60KN of thrust per nuke to work with.

I did some maths, and a 12T space plane with 2 LV-Ns would fly as a rocket pretty well, though the one i was designing had quite a low T/W at take-off, which i am unsure about.

I'll give the design a try when I get home this evening.

The design i was thinking about is, from the front: docking port, mk2 cockpit, small to mk2 adapter, mk 2 fuselage turbojet, two flt200s on the side, nukes on the end of the flts, appropriate wings and canards.

[maccollo]

Doable? Probably.

As for whether it's practical you have to remember that those engines are really heavy, they weight as much as a an Fl-t400 fuel tank. Since SSTOs are also typically light you will have to bring quite a bit of fuel to get more deltaV than you would get with a couple of Rockomax 48-7S, but then you might not have the thrust to make orbit.

[Dispatcher]

maccollo is right about the mass of an atomic engine. Its best used in a spacecraft which travels orbit to orbit; maybe even orbit to surface. But surface to orbit will require a beefy craft with engines powerful enough to compensate for the atomic engine's mass.

Consider using combinations which might include the aerospike (no vectoring), turbojets, LV 909 (relatively little mass) or even some radial 24s (yeah, I know, the 24s are not the most impressive things to see and listen to).

Good luck on that!

[Dangerous_Beans]

Doable? Probably.

As for whether it's practical you have to remember that those engines are really heavy, they weight as much as a an Fl-t400 fuel tank. Since SSTOs are also typically light you will have to bring quite a bit of fuel to get more deltaV than you would get with a couple of Rockomax 48-7S, but then you might not have the thrust to make orbit.

I agree, but you are missing the important fact that nuclear rockets are cool

Edit: I redid my numbers using LV909s, and you get better T/W, though slightly worse Dv (2000 vs 2700). You get similar results with 24-77s, and 48-7Ss give even more Dv.

As others here were saying; the smaller you make the SSTO, the larger the proportion of vessel mass that is accounted for by the engine mass. So doable but not practical.

It's a pity we don't have hybrid nuke/LOX or liquid core ones.

Edited August 27, 2013 by Dangerous_Beans

[Geschosskopf]

I agree, but you are missing the important fact that nuclear rockets are cool

But don't forget that besides their heavy weight and low thrust, nukes have very pathetic Isp's in atmospheres, only 220, as bad as the Ants. IOW, they'd probably suck your fuel away before you got to the end of the runway if your spaceplane only weighs 15 tons.

[Specialist290]

And at any rate, if you're only going to be messing around within the influence of Kerbin, the LV-N is overkill for the amount of delta-v you'd need. It's mainly intended to be used as the transfer stage on interplanetary craft.

[Kozenger]

I agree, but you are missing the important fact that nuclear rockets are cool

Edit: I redid my numbers using LV909s, and you get better T/W, though slightly worse Dv (2000 vs 2700). You get similar results with 24-77s, and 48-7Ss give even more Dv.

As others here were saying; the smaller you make the SSTO, the larger the proportion of vessel mass that is accounted for by the engine mass. So doable but not practical.

It's a pity we don't have hybrid nuke/LOX or liquid core ones.

Thank you for your information. It's appreciated.

[Kozenger]

And at any rate, if you're only going to be messing around within the influence of Kerbin, the LV-N is overkill for the amount of delta-v you'd need. It's mainly intended to be used as the transfer stage on interplanetary craft.

Well, that's exactly why I wanted to work in the atomic engine. I wanted longer range than Kerbin influence without refueling. Without the atomic engine a SSTO of that size won't get far without infrastructure already in place to refuel the ship every time it does a transfer.

[numerobis]

You can do an SSTO with 2 jets, an LV-N, and a bit of help in the form of a pair of 24-77s.

Jet up to 20km and over 1 km/s, then switch to the rockets and pitch up. Turn off the helper rockets and thrust horizontal with just the LV-N once you get to about 2 km/s.

[numerobis]

Another option is to use the jets for longer; that actually probably makes more sense if you don't have much payload. Reduce throttle by half when you get to 20km and light the LV-N -- at the same time that the jets are on. Ride that up to 23 km, throttle back to 1/3, ride up to 25 km, then cut the jets and go all LV-N, pitch up, etc. At that altitude, air should be thin enough to let your LV-N work.

[Geschosskopf]

Well, that's exactly why I wanted to work in the atomic engine. I wanted longer range than Kerbin influence without refueling. Without the atomic engine a SSTO of that size won't get far without infrastructure already in place to refuel the ship every time it does a transfer.

Hmmm.... The underlying problem with spaceplanes is that on the planets with enough atmosphere to warrant having wings at all, just getting to orbit requires as much or more dV than many transfers. So, any non-stop, round-trip spaceplane to another planet requires enough dV for at least 4 "transfers", (reaching LKO, transfer out, getting back to orbit, and transfer home) plus some extra for 2 de-orbit burns, general maneuvering, and flying around some at the destination. And if you're not going to a place where wings are useful, then why bother with a spaceplane?

Nuke-only planes work on Duna because the air is so thin and the gravity so weak. Laythe has all the same problems as Kerbin, just on a slightly smaller scale, although you can at least use jets to help there (which brings up the issue of refueling them). Eve is an order of magnitude harder than Kerbin, and I really doubt that nukes have enough thrust to overcome all the air resistance. So, unless your goal is to fly around on Duna, a nuke-only spaceplane isn't too useful.

[Kozenger]

Hmmm.... The underlying problem with spaceplanes is that on the planets with enough atmosphere to warrant having wings at all, just getting to orbit requires as much or more dV than many transfers. So, any non-stop, round-trip spaceplane to another planet requires enough dV for at least 4 "transfers", (reaching LKO, transfer out, getting back to orbit, and transfer home) plus some extra for 2 de-orbit burns, general maneuvering, and flying around some at the destination. And if you're not going to a place where wings are useful, then why bother with a spaceplane?

Nuke-only planes work on Duna because the air is so thin and the gravity so weak. Laythe has all the same problems as Kerbin, just on a slightly smaller scale, although you can at least use jets to help there (which brings up the issue of refueling them). Eve is an order of magnitude harder than Kerbin, and I really doubt that nukes have enough thrust to overcome all the air resistance. So, unless your goal is to fly around on Duna, a nuke-only spaceplane isn't too useful.

I agree. It largely isn't the best way to go about things when you consider the diversity of planets in which to visit. No single craft is. You could follow the same logical reasoning with rockets and landers. What works for Duna or Laythe probably doesn't work well for Eve. And exactly what you mentioned about the wings. Once you get to LKO the lack of atmosphere in the vast majority of places to go will make your wings more than useless. Its actually a determent to the craft because it is weight that isn't serving any purpose for the craft at that point.

To answer the question as to why bother. Because I think it's fun. It's the same reason why I mentioned that air hogging was not an option for me from the start. I can do exactly what I want to do if I air hog but I choose not to. Trying to overcome the artificial limitations that I place upon myself is a fun challenge for me. To me it's like having unlimited resources and and being asked to build a car that can fly, float on water, and do a quarter mile in 15 seconds. That's an interesting challenge. Do we have cars that can do a better quarter mile ? Do we have boats that will function better in water ? Do we have air planes that can fly better ? Absolutely, but because I have those unlimited resources at my disposal it becomes something more academic. Becomes a what if I could... ?

[Radam]

Depends on how much air intakes is airhogging for you. if you use 1 ram and 3 radial ones per engine, you can get to about 1300ms on turbos.At about 23km.

[Kozenger]

Depends on how much air intakes is airhogging for you. if you use 1 ram and 3 radial ones per engine, you can get to about 1300ms on turbos.At about 23km.

I gave some numbers in my first post regarding what I feel is air hogging and how fast/high on average I get off the jet engines.

[arq]

Generally I end up with 2-2.5 intakes/jet on my SSTO's. As for getting into orbit with only an LV-N, I have not succeeded in doing so but I haven't actually tried. Still, with 9 intakes, 4 turbojets, 2 LV-T30s, and 1 LV-N I have been to the surface of Minmus and back.

The issue with rockets for SSTO ascent isn't ISP, it's thrust. Kerbin's atmosphere at 20km is less than 2% of the surface density, which means that rockets perform with nearly their vacuum ISP (the LV-N for example has 789s at 20km). The main issue is that you typically need a rocket TWR of at least 1 to get an efficient to-orbit transfer from 25km. But suppose we could do it with 0.5: that means that you would need one LV-N for every 12t of vessel (though the LV-N eats up 2.5t of that), so you will need at least to 2 for a 20t ship if those are your only engines, but I don't know if that leaves enough mass for the rest of the craft.

One thing to consider is replacing your aerospikes with LV-T30s. the T30 has 5% less vacuum ISP (and we already established that sea-level ISP is irrelevant here) but 47% more TWR. Because of its low TWR, the aerospike is only really useful in thick air where it can take advantage of its very high ISP (even on Kerbin, it's only really useful for the first 10-15km, otherwise I use T-30s). If the T30 is overkill for your rocket then the mini-rockomax engines (the 24-77 or the 48-7s) are great options for boosting TWR for the early part of your rocket ascent, plus they add vectoring.

You need to carry all your engines with you wherever you go. Bringing a load of aerospikes to Mun or Duna is heavy. Thus it is best to use a single LV-N and let the rest of your engines have high TWR, so that you can minimize engine mass.

[Geschosskopf]

To answer the question as to why bother. Because I think it's fun. It's the same reason why I mentioned that air hogging was not an option for me from the start. I can do exactly what I want to do if I air hog but I choose not to. Trying to overcome the artificial limitations that I place upon myself is a fun challenge for me. To me it's like having unlimited resources and and being asked to build a car that can fly, float on water, and do a quarter mile in 15 seconds. That's an interesting challenge. Do we have cars that can do a better quarter mile ? Do we have boats that will function better in water ? Do we have air planes that can fly better ? Absolutely, but because I have those unlimited resources at my disposal it becomes something more academic. Becomes a what if I could... ?

Well, good luck with this challenge. I don't see a way to do it nuke-only, though.

The normal jet-and-rocket spaceplane has 2 main stages of flight. First the "jet phase", where it builds up speed and altitude, then the "rocket phase" where it goes the rest of the way out of the atmosphere and circularizes. These 2 phases require different things from the engine. In the "rocket phase", thrust is the dominant factor, but in the "jet phase", it's Isp. While nukes alone MIGHT be able to handle the "rocket phase" (although probably not lifting much more than the fuel they need for it), the real problem is in the "jet phase".

In the "jet phase", you don't need any more thrust than the bare minimum needed to move forward and generate just enough lift to climb, however slowly. Not saying you can't thrust your way through this phase, too--most folks do--but you don't have to. If you have enough wing, the wings can keep you in the air and do the actual climbing, and all the engines do is build up speed. So far, that fits nukes because they have very low thrust. HOWEVER, the less thrust you have, the longer it takes to accumulate the necessary speed and altitude to enter the "rocket phase". And the longer the "jet phase" lasts, the more fuel you need during it. This is where nukes fail miserably, because most of the "jet phase" is happening in thick air where nukes have pathetic Isp.

Nukes in atmosphere have 220 Isp and produce only 60 kN thrust anywhere. The stock Basic Jet Engine has 7200 Isp and 100 kN, so you need 2 nukes to match the thrust of 1 jet. But each nuke burns 32x as much fuel as the jet to produce this thrust, so a 2-nuke spaceplane would need 64x the amount of fuel of a 1-jet spaceplane during the "jet phase". Of course, that much fuel would add massive amounts of weight, so you'd need even more nukes just to get it off the ground, which would in turn require another huge helping of fuel, which would require yet more engines in a self-reinforcing cycle of futility. Nukes in atmosphere are just too fuel-hungry, even with wings helping them, to do the "jet phase" of flight.

[saik0]

It's possible with a 3:1 intake to turbojet ratio. My strategy was once near the max velocity I could get with the turbojets (1820 m/s at 27km) at full throttle, I pitched up, throttled back and burned both the nuke and the jets until they didnt have enough air to keep climbing. Then killed the jets and maxed the throttle on the nuke until my Ap was a the desired alt. I used MJ to control the throttle to prevent burnout. You could do it manually by watching the intake air readout in MJ or KER. I don't see how it would be possible without mods. I've done it vanilla but with a pair of Rockomax 48-7s for some extra thrust.

Here's the craft I used:

Javascript is disabled. View full album

[Kozenger]

Well, good luck with this challenge. I don't see a way to do it nuke-only, though.

The normal jet-and-rocket spaceplane has 2 main stages of flight. First the "jet phase", where it builds up speed and altitude, then the "rocket phase" where it goes the rest of the way out of the atmosphere and circularizes. These 2 phases require different things from the engine. In the "rocket phase", thrust is the dominant factor, but in the "jet phase", it's Isp. While nukes alone MIGHT be able to handle the "rocket phase" (although probably not lifting much more than the fuel they need for it), the real problem is in the "jet phase".

In the "jet phase", you don't need any more thrust than the bare minimum needed to move forward and generate just enough lift to climb, however slowly. Not saying you can't thrust your way through this phase, too--most folks do--but you don't have to. If you have enough wing, the wings can keep you in the air and do the actual climbing, and all the engines do is build up speed. So far, that fits nukes because they have very low thrust. HOWEVER, the less thrust you have, the longer it takes to accumulate the necessary speed and altitude to enter the "rocket phase". And the longer the "jet phase" lasts, the more fuel you need during it. This is where nukes fail miserably, because most of the "jet phase" is happening in thick air where nukes have pathetic Isp.

Nukes in atmosphere have 220 Isp and produce only 60 kN thrust anywhere. The stock Basic Jet Engine has 7200 Isp and 100 kN, so you need 2 nukes to match the thrust of 1 jet. But each nuke burns 32x as much fuel as the jet to produce this thrust, so a 2-nuke spaceplane would need 64x the amount of fuel of a 1-jet spaceplane during the "jet phase". Of course, that much fuel would add massive amounts of weight, so you'd need even more nukes just to get it off the ground, which would in turn require another huge helping of fuel, which would require yet more engines in a self-reinforcing cycle of futility. Nukes in atmosphere are just too fuel-hungry, even with wings helping them, to do the "jet phase" of flight.

You seem to be assuming a lot and not really understanding what the original question was. I have no problem in the "jet phase". I am not trying to use the atomic engine to get off the runway. The specific impulse of the atomic engine at 24km is almost exactly the same as it is at 100km. I do not and would not use the atomic engine until the jet engines were so starved for oxygen that they ceased to function. My problem is specifically not having the thrust necessary with the atomic engine, unless I air hog, to maintain the speed necessary after the jet engines cease to function to reach LKO at my space plane size.

[Kozenger]

Generally I end up with 2-2.5 intakes/jet on my SSTO's. As for getting into orbit with only an LV-N, I have not succeeded in doing so but I haven't actually tried. Still, with 9 intakes, 4 turbojets, 2 LV-T30s, and 1 LV-N I have been to the surface of Minmus and back.

The issue with rockets for SSTO ascent isn't ISP, it's thrust. Kerbin's atmosphere at 20km is less than 2% of the surface density, which means that rockets perform with nearly their vacuum ISP (the LV-N for example has 789s at 20km). The main issue is that you typically need a rocket TWR of at least 1 to get an efficient to-orbit transfer from 25km. But suppose we could do it with 0.5: that means that you would need one LV-N for every 12t of vessel (though the LV-N eats up 2.5t of that), so you will need at least to 2 for a 20t ship if those are your only engines, but I don't know if that leaves enough mass for the rest of the craft.

One thing to consider is replacing your aerospikes with LV-T30s. the T30 has 5% less vacuum ISP (and we already established that sea-level ISP is irrelevant here) but 47% more TWR. Because of its low TWR, the aerospike is only really useful in thick air where it can take advantage of its very high ISP (even on Kerbin, it's only really useful for the first 10-15km, otherwise I use T-30s). If the T30 is overkill for your rocket then the mini-rockomax engines (the 24-77 or the 48-7s) are great options for boosting TWR for the early part of your rocket ascent, plus they add vectoring.

You need to carry all your engines with you wherever you go. Bringing a load of aerospikes to Mun or Duna is heavy. Thus it is best to use a single LV-N and let the rest of your engines have high TWR, so that you can minimize engine mass.

Having three types of engines was going to be what I was trying next. It means a larger ship which is something I wished to avoid but like you said the thrust on the atomic alone just isn't enough, if you don't air hog, to do the trick. You make some excellent points for the T30s over aerospikes. An aerospike is enough TWR on smaller ships like mine to reach LKO. The main reason why I got into using aerospikes over T30s is the lower profile. Before I got my stick it was tough to have a strong angle of attack and not clip the engine on take off. The only issue of having the lower thrust to weight, aside from trying to get to speed to hit orbit, is longer burn times. Once you are in orbit the extra thrust to weight doesn't have as much value, as long as you have above 1 to escape from where ever you are going you can plan for longer burn times.

Thank you for the information.

[Dangerous_Beans]

Here here is my attempt; two nukes, one jet, achieved orbit with 1.3km/s Dv

Edited August 30, 2013 by Dangerous_Beans

[Dispatcher]

D_B, try . I got that advice from a forum member, whose name escapes me at the moment.

[SofusRud]

I built an SSTO with one Turbojet, two Atomics, and only 2 ram intakes. I had to use a lot of wing and it did take a long time to build up the needed speeds on the jet, but it can be done

[arq]

The main advantage of high thrust-to weight rockets on ascent stages is that it means you have less deadweight to haul through your transfers, which increases delta-v. The LV-T30 not only has more thrust than the aerospike, meaning you need fewer to get to orbit (or at least the orbital ascent can be faster, which reduces drag losses), but it weighs less which will help you get extra mileage out of an LV-N transfer stage.

[Dispatcher]

Well, its true enough that you CAN send up an SSTO using atomics. But you definitely require more delta V, as arq mentioned. In some testing I've done, an atomic is capable of launching with a total mass of about 6.5 tons in a vertical launch, but nearly a third of that mass is the engine itself. (Yes, that aproximates its TWR of 2.7) In those same tests, it turned out that both the aerospike (launching with a total mass of over 14.5 tons) and the LV-T30 (launching with a total mass of over 22.5 tons) were able to achieve escape V after converting their payloads (fuel) into thrust. In contrast, when the atomic was done converting its fuel payload into thrust, its projected apoapsis was about 185000 meters.

I guess it really boils down to what the mission is, and if you can afford it, how cool you want it to look. :-) Good luck on your project!