Interplanetary SSTO with nerv engines

[jsisidore]

Can you reach ~70km using nerv engines?

edit:

Sorry. Let me reiterate myself more clearly, since I thought it was obvious that you have to use air breathing engines just to get off the ground.
Is it possible for nerv engines to give you enough thrust to leave the atmosphere past 20ish km to reach 70km orbit?

Edited June 7, 2016 by jsisidore

[YTNewneo]

Only Nerv??, no Idea, but I dont think so because of the really low atmospheric thrust and ISP

Nerv mixed with Air breathing Turbojets has been done and is possible

[Starhawk]

Darn!  I clicked the wrong one!

No, you will need other engines to get you through the atmosphere.

Happy landings!

[jsisidore]

Sorry. Let me reiterate myself more clearly, since I thought it was obvious that you have to use air breathing engines just to get off the ground.
Is it possible for nerv engines to give you enough thrust to leave the atmosphere past 20ish km to reach 70km orbit?

YTNewneo, has it been done with liquid fuel only designs if so may I have the link please?

1 hour ago, Starhawk said:

Darn!  I clicked the wrong one!

No, you will need other engines to get you through the atmosphere.

Happy landings!

Engines that require oxidizer?

[mk1980]

definitely possible, i built a spaceplane for one of the forum challenges that could get from the runway to duna and back without refuelling.

if you add ISRU equipment, it's sort of trivial actually. just a standard SSTO with a NERVA, extra fuel and ISRU stuff.

[Starhawk]

37 minutes ago, jsisidore said:

Sorry. Let me reiterate myself more clearly, since I thought it was obvious that you have to use air breathing engines just to get off the ground.
Is it possible for nerv engines to give you enough thrust to leave the atmosphere past 20ish km to reach 70km orbit?

YTNewneo, has it been done with liquid fuel only designs if so may I have the link please?

Engines that require oxidizer?

Yes, it's possible.  There are several variations on the Rapier + Nerva combo.  It can be quite tricky because the TWR of the Nerva is so low.  I find it's much easier with just a little oxidizer on board to run the Rapiers in closed-cycle mode for a short period before relying completely on the nuke.

Happy landings!

eta: And it turns out my mistaken click on the poll ended up being the right one after all.

Edited June 7, 2016 by Starhawk

[Val]

Yes, it is possible to make LF-only designs. Quite doable with Nerv/Whiplash and Nerv/RAPIER(Airbreathing only) comboes.

But in practice it's more efficient (dV vs. total craft mass) to include a little oxidizer for the RAPIERs.

A well designed craft around 36t with 2 air-breathers (Whiplash/RAPIER) and 2 Nervs should be able to get to orbit with at least a couple km dV or more depending on payload.

Barring issues with exploding landing gear, these 2 examples should still be valid for 1.1.

The first one is capable of getting to orbit even if you remove the oxidizer.

https://kerbalx.com/crafts/8018/

https://kerbalx.com/crafts/9737/

[jsisidore]

15 hours ago, Val said:

https://kerbalx.com/crafts/8018/

That's unbelievable, it looks so heavy and just two nerv engines... what are your ascent trajectories?

[jsisidore]

Never mind, I found your description, for some reason it gave me an error page the first time. I will definitely try this craft.

[jsisidore]

One question though, where 

4 hours ago, Val said:

 

One question though, no air intakes? All I see air coolers...

[KerikBalm]

Yes, I've made LF only designs in 1.1.2

I've also made an interplanetary design that can get to duna and back, but the margins are pretty thin.

Its possible to go to laythe and come back in an SSTO with no refueling... I saw it posted once... but it was a decades long mission involving many gravity assists.

[bewing]

1 hour ago, jsisidore said:

One question though, where 

One question though, no air intakes? All I see air coolers...

Engine precoolers intake air. Even in the middle of a stack.

[Signo]

Hi there, adding my 2 cents.

I love ram/nerv designs because it is easy to stretch their autonomy to the moons.

 

 

[The Optimist]

3 hours ago, jsisidore said:

One question though, where 

One question though, no air intakes? All I see air coolers...

Those are intakes, in fact.

EDIT: redundant post

Edited June 7, 2016 by The Optimist

[RizzoTheRat]

Not tried it in 1.1 but this worked well as a crew shuttle in 1.05.  The only oxidizer is in the bicoupler and the FLT-100's, adding the 100's gave a significant fuel saving as it kept the Rapiers running that bit longer, and meant it was able to land at a refuelling base on the Mun or Minmus.  Without the FLT-100's it could get to Mun/Minmus orbit but not land, with no oxidiser it can make orbit but doesn't have the fuel to go much further.  Can't remember how much dV it had once refuelled, but I think it was over 5000.  It was certainly way more than it needed to reach solar orbit and get back to Kerbin which is what it was designed to do after landing on both the Mun and Minmus.

Edited June 8, 2016 by RizzoTheRat

[katateochi]

I've been recently having a lot of success with liquid fuel only SSTO designs; I find that for a small/medium SSTOs I'm ending up with much more available dV once in orbit than I was able to get with similar sized dual fuel designs.  

This is one craft I've been using as a crew transporter to LKO (can get to a 300km orbit and rendezvous)

This is not the most efficient setup, but I was also looking for a quick ascent time. With a slightly smaller design (1 less crew compartment and no docking/maneuvering gear, but same engine setup) I was able to set a new personal record for fastest flight to orbit (reaching stable orbit in just under 6 game minutes). That beats my previous fastest dual fuel SSTO, partly due to the ascent profile which is to do a single continuous burn to stable orbit (no costing to Ap and then circularizing stage).

This design (which is not aerodynamically efficient) can reach Mun orbit and return home (https://kerbalx.com/katateochi/Retro-Fi-MkII)

 

I was really inspired to do liquid only designs because of this vid (he does also use ion engines as it's a Moho-surface and back without refueling SSTO, but similar designs without the ion engines can do Mun surface and back). That vid really made me re-think SSTO ascent profiles. 

 

[AeroGav]

 

Here's a NERV/Whiplash ship flying from 18km to orbit.   It carries 5 crew and is pretty loaded with extras like docking port, lifting engines etc.

You can see that the NERV easily produce enough power though those airliner wings aren't really suited to this kind of thing and get pretty hot.  It actually goes all the way to Minmus and back - although the fuel reserves look mostly depleted by the end of the video, the gauges only show what's in the tanks the NERV directly connect to.   On part 3 you can https://www.youtube.com/watch?v=3nI-n0J262I

 see the remaining fuel get transferred in for the interplanetary leg.   And yes, there is 55 units of oxidizer on this thing.   It's for the Vernier engines only , which enable horizontal touchdown like this - https://www.youtube.com/watch?v=w6ZxVtZswh0

The above craft can be downloaded here -

https://kerbalx.com/AeroGav/Astrojet-Citation

A "hotter" version that sheds it's airbreathing engines for extra delta-v (enough for a return trip to Duna for 5 kerbals, with a bit of flying around on the red planet).

https://kerbalx.com/AeroGav/Astrojet-Citation-X

A minimalist staged nerv/whiplash airplane (from 1.05, landing gear may need an upgrade?) 

https://kerbalx.com/AeroGav/whippynerv

....and finally, uploaded just a week ago, a rapier/nerv spaceplane that can fly from the surface of Kerbin to the surface of Duna and come back. The only oxidizer is for the lifting engines

https://kerbalx.com/AeroGav/Astrojet-Lancer

 

 

[jsisidore]

Here is an ssto I built based on Val's design, and I'm struggling with keeping altitude at 16km so I turn on nerv and get an overheating warning at 26km and at 32 they explode. 

[ineon]

Obvious question: what ascent profile are you using? (How fast are you going at various altitudes?)

Ideally at 16km you should be travelling several times the speed of sound. The complex thing with air-breathing engines is that the more air they get the more thrust they produce (more-or-less). That means that you can get a positive feedback: 

More speed -> more air -> more thrust -> more speed

but equally get negative feedback:

Less speed -> less air -> less thrust -> less speed

I am expert on planes like this but you should attempt to not climb too high until you have built up speed. Building up some speed at around sea level (maybe a bit shy of 300m/s) might get you the good push to start on the positive feedback cycle.

Some other thoughts: how heavy is your plane on the runway? I think I remember hearing a good rule of thumb for getting planes into space is to make sure that you have one RAPIER for every 15t. I assume that your plane is about 30t, but it might be worth checking.

Have you angled your wings up by a few degrees? This will give you some natural lift without giving you more drag.

Edited June 8, 2016 by ineon

[jsisidore]

53 minutes ago, ineon said:

Obvious question: what ascent profile are you using? (How fast are you going at various altitudes?)

Ideally at 16km you should be travelling several times the speed of sound. The complex thing with air-breathing engines is that the more air they get the more thrust they produce (more-or-less). That means that you can get a positive feedback: 

More speed -> more air -> more thrust -> more speed

but equally get negative feedback:

Less speed -> less air -> less thrust -> less speed

I am expert on planes like this but you should attempt to not climb too high until you have built up speed. Building up some speed at around sea level (maybe a bit shy of 300m/s) might get you the good push to start on the positive feedback cycle.

Some other thoughts: how heavy is your plane on the runway? I think I remember hearing a good rule of thumb for getting planes into space is to make sure that you have one RAPIER for every 15t. I assume that your plane is about 30t, but it might be worth checking.

Have you angled your wings up by a few degrees? This will give you some natural lift without giving you more drag.

Hey. At 10 degrees I reach 10km at 300ish m/s, then I pitch up to 15 degrees with the same speed and I do not gain any more speed, at 16km I begin to lose altitude, I think it is because of not enough air intake, as you described.

56 minutes ago, ineon said:

Have you angled your wings up by a few degrees? This will give you some natural lift without giving you more drag.

Can you explain a bit more?

[AlexisBV]

1 hour ago, ineon said:

The complex thing with air-breathing engines is that the more air they get the more thrust they produce (more-or-less).

Sadly that's the opposite of what happens in real life, since a jet engine's (net) thrust decreases as airspeed increases.

[ineon]

16 years ago, jsisidore said:

At 10 degrees I reach 10km at 300ish m/s, then I pitch up to 15 degrees with the same speed and I do not gain any more speed

Right - then what I would do is pitch up less aggressively, maybe 8 degrees. Remember: getting into space is easy, getting into orbit is hard (Really interesting read from xkcd if you have not already seen it). Gaining horizontal speed is essential with a space plane.

16 years ago, jsisidore said:

Can you explain a bit more?

 

I've attached an album to help explain. The first image is the stock Aeris. In order for the wings to produce an upwards force the whole plane needs to be pointed up and this induces drag from the other components. In the second image I have rotated the wings up by a few degrees (and translated them slightly too...). This means that the wings will produce a little upwards force even when the nose is pointing directly prograde. This doesn't need to be much - just few degrees will do it, and too much will be just as bad if not worse.

[mk1980]

that plane has more than enough air intake. don't add more intakes, they will not help. you could probably even remove the ramp intakes. the precoolers are enough to feed the rapiers.

if a slow and steady climb doesn't get you to hypersonic speeds, try an alternative ascent. that plane doesn't have much thrust at low speeds, so you'll have to make good use of the ramjet effect of the rapiers. they spool up quickly as you accelerate. 

here's what i would do with such a plane: keep it in mostly horizonal flight after takeoff, accelerate to about mach ~1.3 before you climb. climb at a decent angle - something like 15°. make sure you keep accelerating during the ascent. you want to get it to at least ~1200 m/s at around 20 km before you switch over to closed cycle mode and fire up the nukes.

you use the mk2 cockpit, which has pretty bad heat resistance. you can't really do a proper high altitude speedrun with that thing as it will explode around 1300 m/s. since you can't "milk" the rapiers fro speed, try to gain altitude instead. by the time you switch over, your (projected) apoapsis should be at the very least somewhere upwards of 30km.

 

 

 

 

[AeroGav]

19 minutes ago, jsisidore said:

Hey. At 10 degrees I reach 10km at 300ish m/s, then I pitch up to 15 degrees with the same speed and I do not gain any more speed, at 16km I begin to lose altitude, I think it is because of not enough air intake, as you described.

Can you explain a bit more?

1. The thrust output of jet engines in this game is modified by two factors  a) atmospheric air density and b) speed.    As the air gets thinner you loose power.  On the other hand, you get faster engine output increases , up until a peak, then starts to tail off again.  

There's a lookup table inside the engine's config file that tells it how much thrust to loose as air density falls off .  For the RAPIER engine it's pretty shallow up to 10km.   From 10km to 20km it looses thrust more quickly, if you start getting well supersonic at these altitudes then the thrust gain from the additional speed offsets the loss from thinner air, as does decreasing drag.  Above 20km things start weakening rapidly no matter what.

In terms of speed, the RAPIER peaks at Mach 3.75 , or 1130m/s.  If you push on to mach 4.5, or 1380 m/s, thrust falls back to 80% of maximum.  After that there's a sharp kink in the curve and the decline is rapid - thrust falls to 0 by mach 6.

2. More intake will not help.   You have 2 adjustable ramp intakes and 2 pre-coolers,  one pre-cooler is sufficient intake for a rapier and an adjustable ramp intake almost enough for 2 engines.   Lack of intakes does not cause a gradual loss of thrust, it causes the engines to splutter and surge, pop and bang.  When the engines splutter,  it only affects one engine at a time, making your craft weave from side to side.

3. You've got the right general idea in climbing up a bit before trying to go supersonic.    The exact altitude it's best to do that at depends on how much lift and drag your design has - the more lift and drag, the higher up you should be before trying it. 

For example, most of my designs are incapable of going through the sound barrier below 7.5km.     Below that altitude, drag rises faster than engine thrust  with increasing speed.         So, I pitch up to stay below 260 m/s.

However, if I go much above 12km subsonic, two problems arise.  As the air gets thinner, there is less lift, normally the speed also goes up to compensate, but if you're not getting faster, the only way to keep sufficient lift is to raise the angle of attack (nose up angle).   Beyond a certain point this leads to a lot of drag.    The other problem of course is that your engines are loosing power because you're getting into thinner air without also getting faster to give some ram air effect.      

Generally when I'm having to raise the nose 8 degrees above prograde just to get enough lift , is when it's time to go supersonic.  Drop the nose angle and light the nukes till i'm over 400 m/s, then i gently raise the nose again, and cancel the nukes.

4.

I  think you should talk in terms of Angle of Attack rather than pitch.   

Angle of attack is the difference between where the nose is pointing and your direction of travel.   So if you have the nose pointing up at 13 degrees but the prograde marker shows an 8 degree climb, this means you have an angle of attack of 5 degrees.

Pitch is just the angle of the nose.

Telling me you have 15 degree pitch could mean -

   A) you have a really overpowered ship and are climbing at almost 15 degrees, at a high airspeed and a low angle of attack, not struggling for lift at all

or B) you are  are hardly climbing but are struggling badly for lift and are wallowing along in a very inefficient slow flight regime not much above stalling speed

or C) you are actually descending at 15 degrees or more and have in fact already stalled

 

Generally I fly targeting an AoA rather than a pitch angle.

Manual flight with pitch trim

I make a few test flights to find out how much nose up pitch trim (alt + s to add more, alt + w to reduce) is needed for my airplane to settle at about a 3-4 degree AoA if left to its own devices with SAS off.

I set this before takeoff.     I also use Alt F12 to open up the debug menu, go to the physics tab, check the "display aero data gui" box.  Keep an eye on your lift:drag ratio as well as total drag and total thrust as you climb. 

I generally find that 3 degrees AoA gives highest lift:drag ratio below 160 m/s.  Above that 4 degrees,  supersonic we're talking 5 degrees, possibly more as you near orbital velocity.

Above 200 m/s I start adding more pitch up trim to keep speed below 260.   However, when the only way to keep the speed down is to increase AoA beyond 8,  it's time to go supersonic instead.  I toggle the nukes on briefly with an action group and remove the extra pitch trim, maybe even set prograde for a few seconds on SAS too.

After that you're hanging on to the tiger by the tail.  Try to level off at 18-20km to accelerate to 3.75 mach (1130m/s) , maybe even 1300 if you can do that without overheating (a shallow cilmb to 21-22km for this speed is ok).  After that trim the nose up to gain altitude without getting any faster.   When  you can no longer get a worthwhile rate of climb without loosing airspeed, toggle the nukes back on.

Note that above 20km most planes start get a bit squirrely, and tend to bob and weave.   At this point it's acceptable to just lock in a good nose angle with SAS.    SAS holds a nose angle but doesn't take account the curvature of the earth, so your nose will gradually rise.  A quick tap on the W key if it starts rising faster (say more than 1 degree above your optimal AoA) than you want.

With Mechjeb

this is the easy way.  watch this...

https://www.youtube.com/watch?v=b6Ja7B2TCmE

(note, mechjeb can't fly some planes off the runway, some of my other designs i need to take off first before engaging autopilot)

 

Finally a quick word on your SSTO

 

It looks nice,  but is there too much fuel?   Maybe that is why you can't get enough lift at only 10km, and why your are having trouble getting through the sound barrier.

 

You need one NERV for every 15 tons of weight,   and one jet engine for every 30 or so.      Therefore take fuel out of your design till it weighs less than 30 tons and see how it goes.

In a NERV spaceplane it's really hard to have more than 50% of it's mass as fuel and still be nice to fly, or even make orbit at all.     At the other extreme I've flown stuff up there with only a 30% fuel fraction and been disappointed not to have much delta V left once in orbit.

Air breathing engines are heavy btw (2 tons each) so putting them on decouplers can help if you're struggling.  If you're taking that approach it's probably best to use whiplash rather than RAPIER since whippy's only cost 2000 each instead of 6000...

[jsisidore]

1 hour ago, AeroGav said:

4.

I  think you should talk in terms of Angle of Attack rather than pitch.   

Angle of attack is the difference between where the nose is pointing and your direction of travel.   So if you have the nose pointing up at 13 degrees but the prograde marker shows an 8 degree climb, this means you have an angle of attack of 5 degrees.

Pitch is just the angle of the nose.

I was wondering how to address this effect and now I know, and this is what I really meant, that after 16km the prograde marker is falling progressively so I have to resort to switching the nukes on.

Scratch that, the marker is falling way sooner maybe at somewhere 11ish km but at 16km I barely gain any altitude.

Edited June 8, 2016 by jsisidore