Jet engines on rockets; viable?

[HoY]

You can also throttle down, the engines use less intake air and fuel when throttled down.

[John FX]

I just thought of another reason, a jet launch just seems less stressfull on things. I don`t know if it is but is seems to not be as rough as a rocket launch.

Anyone else find this?

[HoY]

That's mostly because the turbo jet engines don't give their full 224kn of thrust until your at around 23km up. 20-25km is your most efficient (ISP wise) for turbojets.

They get to about 109kn each while spinning up on the pad, maybe a little higher. This helps a lot with proper planning and engine placement you can avoid going over terminal velocity by guesstimating your TWR during ascent.

[numerobis]

The velocity curve in the part.cfg specifies half thrust at 0 m/s, full thrust at 1000 m/s, half thrust at 2,000 m/s, and zero thrust at 2400 m/s.

So if you wait long enough, you will see the turbojets reach 112.5 kN. Unless you're doing a timed challenge, you're better off detaching the launch clamps as soon as you have enough thrust to hover.

The basic jets spin up more slowly, but have full thrust at a standstill, half thrust at 800 m/s, no thrust at 1km/s.

[Skorpychan]

Hmm. So, in that case, you can use basic jets like SRBs, to get heavy payloads off the ground before ditching them as the turbojets produce thrust.

How do you get your actual airspeed, though? I feel they're flaming out because I'm going too flipping fast for them to keep up.

What I really want are scramjets. That'd be fun; blast them up with SRBs to get them to speed, then they start pushing properly.

[HoY]

here, watch this from time 3 hours 39 minutes onward for this launch. that was my most perfect launch with jets ever lol.

http://www.twitch.tv/hoyin1600p/b/426419176

The first hour right before that time is all pretty much failed launches because I was still tweaking the launch stage, spent way too much time on it >.< everything after it was the trip to Tylo

If you also note, the launch went so well for me that my burn for Jool was Directly following circularization, AND I got a free 150m/s kick in the Right direction from Mun

Edited July 6, 2013 by HoY

[John FX]

Hmm. So, in that case, you can use basic jets like SRBs, to get heavy payloads off the ground before ditching them as the turbojets produce thrust.

How do you get your actual airspeed, though? I feel they're flaming out because I'm going too flipping fast for them to keep up.

What I really want are scramjets. That'd be fun; blast them up with SRBs to get them to speed, then they start pushing properly.

So a jet that has 0% thrust until about 1500m/s then building up to 100% at 3000m/s or similar?

I like.

[John FX]

Hmm. So, in that case, you can use basic jets like SRBs, to get heavy payloads off the ground before ditching them as the turbojets produce thrust.

How do you get your actual airspeed, though? I feel they're flaming out because I'm going too flipping fast for them to keep up.

What I really want are scramjets. That'd be fun; blast them up with SRBs to get them to speed, then they start pushing properly.

It seems possible to use one set of intakes and a few sets of engines. Jets for the first 500-800m/s then drop them and switch to turbojets until about 1300m/s then drop the intakes and jets and use your orbital insertion stage.

Might not be much benefit for the increase in part numbers though

[HoY]

No reason to use regular jets whatsoever. They only get another 8kn of thrust on the pad than the turbojets and they only go down from there (they are 120kn aren't they? I'm not positive anymore as I never use them)

If you don't want to watch the 5-10 min section of the stream I linked, here's the secret. For a non Reuseable hybrid. It's only slightly different for Reuseable but anyway.

Intakes over engines. If you want to get into space as efficiently as possible your absolute best (cheaty) way is to have 40-45 intakes per engine that you want to use over 40km. In the build I ended up using I had 2 jets and nowhere to install a jet in the center stack that I would be able to drop, that means I needed 40 intakes Per engine.

Launch vertically, perform a very shallow turn at about 13km and level out gradually at about 25km.

Stay nearly horizontal from 25km and only lift your nose up a few degrees if your vertical speed starts to drop below 50m/s. remember the name of the game here is to build up Horizontal speed, and inside the atmosphere a craft with the right twr in jets alone wont need to activate a rocket till its out of the atmosphere.

In my example I held this course (using mechjeb) until my apoapsis was at 80km, and periapsis was 10km. That's technically in orbit, tho not a stable one because it will come back into the atmosphere.

With a little bit of extra flight tweaking its possibly to get the same apoapsis and a periapsis of around 30km, but as you can see by the time stamp on th video, this has already taken 3.5 hours of designing and failed launches so the extra 20m/s I could have saved was justly sacrificed.

If you look closely you'll see that in using MechJeb's prevent jet flameout option as well, that will keep my engines throttled down just enough to not flameout as the air gets thin. This is doable by hand, but in all honestly I couldn't be bothered last night haha.

[Climberfx]

All these are Jet/Rockets SSTO's. Easy to get orbit with Mj, and have fuel to spend in orbit. Some can carry a cargo.

[John FX]

No reason to use regular jets whatsoever. They only get another 8kn of thrust on the pad than the turbojets and they only go down from there (they are 120kn aren't they? I'm not positive anymore as I never use them).

Thought as much. I just accept that I have half thrust on the pad and have a TWR closer to 4 than 2.

[John FX]

Ok, so I am reducing my orbital insertion stage and each time I do I end up with a greater percentage of fuel left in my orbital insertion stage. The weight loss improves my craft more than the engine and fuel did so I have removed my orbital insertion stage and go straight from turbojets to NERVA to circularize. Just on Turbojets I can get to 2300m/s Ap 82Km Pe 26 which only means I need 50m/s to stabilise my orbit.

I have 16 Turbojets with 4 ram intakes each on a craft weighing 52T. Works well. Might try shaving something until it breaks to test if I am getting near limits or not.

[HoY]

That sounds good and yeah, for every meter per second you pack on your upper stage it costs an extra 6 or 8 meters per second worth of fuel on the stsge below it, and that stage costs similarly extra on the stage below it. so on and so forth. and you lose more to gravity due to lower TWR.

Is that 52t including all the jet engines and jet fuel? Or is that the craft after it sheds the jet stages completely?

You should be able to lift that with half that number of jet engines, tho it will be slow for the first little bit. Half the engines, half the jet fuel, but keep the intakes if you have a place for them or don't mind part clipping. Could be off my tree here, but what I'm saying makes sense in my head. Not home to double check however.

Will 896kn lift 50t off the pad? I don't know the math for that calculation but when I think about it in terms of rocket engines with the same thrust it seems to me that it should work just fine.

Edited July 6, 2013 by HoY

[John FX]

That is the weight on the pad. I have about 12000m/s Dv once in LKO and down to 22T or thereabouts. I have a TWR of over 7...

So a TWR of 3.5 would. as you say lift that nicely. I notice above about 20Km it`s the intakes that decide your thrust so itwould make sense to have just enough Turbojets to get up there then loads of intakes. I already thought about reducing the number of intakes I had because I don`t think the last 30% make enough difference for the weight/cost so I will half the jets and keep the same

Ok, I went to four jets (TWR=2.49), kept the same ram intakes (64) and could be at 25% thrust at 40km. I just kept thrusting until the jets would not run any more to overcome atmospheric drag (about 68Km with MJ) Got to 96km Ap, 47km Pe 2300m/s needed 41m/s to circularise at 96km.

This also got my weight down to 36.83T.

EDIT : After launching it a few times I have discovered it suffers from fatal nose dip syndrome if you start your gravity turn either too low or too slow (which does not happen with the higher TWR) so I will get to 8km or 150m/s before turning and see how that goes. I also lowered the turn end altitude to 40km `just to see`

Seems fine so if you get your TWR below 3 start your turn later so your jets have more power to cope.

I got an Ap of 121Km, a Pe of 41Km Engines cut at 69100m, 2300m/s needing 66.2m/s to circularise at 121km or 30m/s to stabilise at 121x80km.

I think I will post this craft in the spacecraft exchange. It is a manned 12,000m/s Dv kethane, mapsat probe with pod return capability + parachute under 37T and 215 parts (53 parts and 24T to orbit)

I think I will call it Split Infinitive

Edited July 6, 2013 by John FX

[HoY]

Excellent! How much extra jet fuel do you have when you get circularized?

[John FX]

Excellent! How much extra jet fuel do you have when you get circularized?

I usually dump the engines as this is a deep space probe but there is about 9 in each so about 40l. I am trying a version with less intakes and I am getting a more circular orbit. I got 86.39kmx47.89km orbit needing 33m/s to circularise. If you keep the jet engines on it has 6256Dv and 39l of fuel left as an SSTO. The version with more intakes, although heavier, had 156l of fuel left.

[HoY]

You seem to be getting the 'less is more' thing down, you can shave off more mass by switching to all the 0.625m stack decouplers instead of the larger ones.

I think you could also get away with moving your jets to directly under the lower most fl-t800's, similarly placed like mine are on thenhexagonal strut with the intakes mashed into it, on a little decoupler. Use 2 cubic struts to either place the two engines under a single fuel tank, or what I think would work even better is switching to a 3 engine setup with only 2 fuel tanks instead of 4 engines and 4 tanks.

With the other mass you can potentially shave off up top (what exactly you need 12km/s of fuel for is your business)

There are a few other things that would work, unless your satisfied with this design in which case all is well, and it seems to work for you nicely

First and foremost, to can ditch Half of those intakes simply by using an action group to shut down 2 of the engines when you get into thinner air. When your already going horizontal you don't need a twr much higher than 1 to keep it going. My Firebat that I posted on page 1 shuts all but a single engine off to get the most use out of the intakes.

Edited July 6, 2013 by HoY

[Climberfx]

I could do with only one liquid tank. you are caring to much. Not to talk about the amount of batteries...

[John FX]

I could do with only one liquid tank. you are caring to much. Not to talk about the amount of batteries...

It started that I liked the look of the grid of panels on the core tank but found they did not power the kethane scanner during the night so I added a battery, then another, then another etc until I had nighttime power for the kethane scanner then reshaped the craft so external batteries would not fit so I went with the ring ones then changed pod and all the time kept to my design brief of 3k,6k,3k Dv stages for outward burn, *stuff* and homeward burn and return to KSC for a kerbal, all science parts, mapsat, telemachus and kethane scanner with all the lovely data (I know, way too much but that is what the brief asked for). I have already done most of the changes in the previous post as it is the logical evolution for this craft. I will probably reduce the size of the central jetfuel tank next and work on staging timing and minimising fuel wastage for the jet stages.

It looks like this now.

[Climberfx]

Ok, For energy, i prefer to use the thermo reactor, one or two is enough to made the Scanner work all night (two).

Edited July 7, 2013 by Climberfx

[Skorpychan]

I found the source of the flameout problem. I was going too fast.

I solved this by ditching the pylon jets after 100k, and sticking to the axial ones. Also, I threw another upper stage on, with a bunch more weight. THAT slowed things down nicely, so I didn't get any extraneous flameouts. I could still save some rocketry by holding onto the axial jets a bit longer, but I'm still unsure of the exact altitude for that, and air was getting thinner, so I ditched the lot early.

That pended a redesign when the upper stage fell completely apart due to default staging problems (durp), but managed to insert into a good place to get an orbit out of a rockomax skippper upper stage, which was then ditched for the previous upper stage to burn for the mun.

I'm shocked at how well it all worked. It's nearly as good as my 7x mainsail lifter rocket, but MUCH more economical, and a bunch easier to control, and has spare capacity for more cargo.

[HoY]

Climber an RTG won't power a small Kethane scanner properly. Both because the scanner uses more power than it puts out, And because the RTG doesn't work properly at time warp.

John, you can ditch ALL of those batteries and use ONE z100 radially mounted battery and one RTG mounted on opposite sides from each other, and then 2 1x6 extending solar panels.

This will save you SOO much mass you'll be able to drop a huge pile of fuel.

The secret to running your scanner on the dark side of a planet or moon is this

you don't!!

Simply orbit the body near or on the day/night terminator. Now you have solar power full time. The RTG is just onboard in case you get eclipsed or have your panels closed when your batterie dies.

You also have a command pod on the nose of this thing, would be more efficient as unmanned, and more humane lol.

[Climberfx]

Hoy, i use mine with two RTG's and in time warp, with light's on. (a lot of lights). None of Solar. And a single battery!

[HoY]

It WILL work , except the power will drain off at time warp and make little glitches in the scan resolution. With 2 RTG's it won't be as often, but its still not an ideal solution for an efficient build, as 2 RTG's weigh a whole lot more than even one solar panel that puts out 2.0 per second against the two RTG's 1.5 per second.

Don't get me wrong I Love RTG's! But for ultralight builds or where every ounce of fuel counts they are simply too heavy, and don't give enough power to rely on a single one.

[Climberfx]

Better then goes off on night man, or even goes off power! For sure!

And with you mean efficient, you will not bother in a hole because you can tell by the neighbors...

I think the word you might use is perfect. I do not need the perfect in this thing, just to know where i can have extraction.

And you are using less eight on solar, but still have a lot of battery, that too have weight. (i don't)

And with you wanna light eight, just look for my ships above...

All at the edge of efficacy.

Cheers!

Edited July 7, 2013 by Climberfx