Interplanetary SSTO with nerv engines

[jsisidore]

1 hour ago, AeroGav said:

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...

Thanks. You gave me a lot of info to digest.

The design weighs no more than 26 tons. I haven't weighted the rover but it should be very light.

The issue might be in that I'm not very precise at climbing so I drop speed, the SAS is somewhat stubborn in this situation.

I tried jets, around 15km is where they drop dead, but it might just be me.

2 hours ago, mk1980 said:

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.

Yea I think the climb I'm doing is too steep and I drop speed.

[mk1980]

the important thing to understand is that you don't have to climb all the time while you're still in airbreathing mode. jet Isp is ridiculously good, so you want to get as much speed and altitude in airbreathign mode as possible.

if you can't breach the soundbarrier while climbing, breach it in horizontal flight. once you are supersonic, the rapiers will pick up a lot more thrust, so you can accelerate a lot faster. find an ascent profile that works for your plane.

flying a bit in mostly horizontal direction is fine - both altitude and horizontal speed are useful, so you don't lose anything when you're not climbing. that's the fundamental difference between planes and rockets. you want to stay in airbreathing altitudes long enough to make decent use of your jets. even the fairly "thirsty" rapier is still 4 times more fuel efficient than the nerv. so every m/s of speed and every meter of altitude you can gain in airbreathing mode consumes only 1/4th of the fuel you would spend with the nervs.

[jsisidore]

3 hours ago, ineon said:

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.

I find it difficult to construct proper wings, I don't think I will be able to angle them like that. But I will definitively experiment on that.

[AeroGav]

7 hours ago, mk1980 said:

the important thing to understand is that you don't have to climb all the time while you're still in airbreathing mode. jet Isp is ridiculously good, so you want to get as much speed and altitude in airbreathign mode as possible.

if you can't breach the soundbarrier while climbing, breach it in horizontal flight. once you are supersonic, the rapiers will pick up a lot more thrust, so you can accelerate a lot faster. find an ascent profile that works for your plane.

flying a bit in mostly horizontal direction is fine - both altitude and horizontal speed are useful, so you don't lose anything when you're not climbing. that's the fundamental difference between planes and rockets. you want to stay in airbreathing altitudes long enough to make decent use of your jets. even the fairly "thirsty" rapier is still 4 times more fuel efficient than the nerv. so every m/s of speed and every meter of altitude you can gain in airbreathing mode consumes only 1/4th of the fuel you would spend with the nervs.

Yeah I alluded to this in my text - heavy post.

I usually climb up to 13km at 85% of the speed of sound, then go into a shallow dive and loose 2-3km getting through the sound barrier, and use a short burst from the nukes to help.

The first part of my launch video of a draggy, low powered (one whiplash jet, 30 ton weight) SSTM crew transporter covers this.   Skip to 7 minutes if you just want to see me going supersonic.

 

[Val]

@jsisidore, your design looks really cool.

Did you try flying my designs first? Just to get a sense of how a working design feels.

Also angling the wings up like @ineon suggests, will make a huge difference. I do it on all my spaceplanes. It reduces fuselage drag significantly. Read the section regarding Angle of Incidence (AoI) here, to understand why.

 

Most of my craft are capable of breaking the sound barrier at sea level.

I prefer it that way, because I believe ascents take shorter time that way. But I guess that's a question of personal preference and design philosophy.

[jsisidore]

what is so important about breaking the sound barrier? less drag?

[jsisidore]

5 hours ago, Val said:

@jsisidore, your design looks really cool.

Did you try flying my designs first? Just to get a sense of how a working design feels.

Also angling the wings up like @ineon suggests, will make a huge difference. I do it on all my spaceplanes. It reduces fuselage drag significantly. Read the section regarding Angle of Incidence (AoI) here, to understand why.

 

Most of my craft are capable of breaking the sound barrier at sea level.

I prefer it that way, because I believe ascents take shorter time that way. But I guess that's a question of personal preference and design philosophy.

No I haven't, yet. How will I know at what angle are my wings? Will one shift+rotate suffice?

8 hours ago, AeroGav said:

Yeah I alluded to this in my text - heavy post.

I usually climb up to 13km at 85% of the speed of sound, then go into a shallow dive and loose 2-3km getting through the sound barrier, and use a short burst from the nukes to help.

The first part of my launch video of a draggy, low powered (one whiplash jet, 30 ton weight) SSTM crew transporter covers this.   Skip to 7 minutes if you just want to see me going supersonic.

Very informative video, I was wondering why I couldn't go faster than 320m/s

Edited June 9, 2016 by jsisidore

[mk1980]

the usual SSTO jet engines (rapier/whiplash) have pretty low static thrust and really good thrust at high speeds. the sound barrier is the hardest part of the ascent because you have low thrust (due to low speed) and high drag (the sound barrier). if you can make it through it, your plane can also reach hypersonic speeds with no issues.

thrust at supersonic speeds gets A LOT higher before it starts to fall off at high hypersonic speeds, so it usually takes less time to go from mach 1.2 -1.3 to mach 4+ then it takes to accelerate from takeoff to mach ~1.2-1.3.

[Val]

1 hour ago, jsisidore said:

what is so important about breaking the sound barrier? less drag?

For me, the important part about breaking Mach 1, is that's when the engines start producing useful thrust.

And in my opinion any time spent flying at constant speed or low acceleration is wasted fuel. Very simplified, this is how I see it.

• Accelerating from 0 to 1000 m/s takes a fixed amount of fuel for a craft, if you ignore all other factors, like drag and flight profile.
  Basic rocket equation. How much fuel the craft needs to have a 1000 m/s dV?
• The faster you fly, the less wings are needed to create the required lift.
  Saves dry mass.

That's why I aim to design my crafts to be able to break the sound barrier as low and as short time as possible. To get access to the engines higher thrust and acceleration and to save fuel and dry mass.

 

1 hour ago, jsisidore said:

How will I know at what angle are my wings? Will one shift+rotate suffice?

If you have very small wings you might be able to use Shift+Rotate, but usually it is too much.

My prefered tool is Part Angle Display, but it's very much a trial-and-error process for me, so the Rotate Gizmo in non-snap mode can be used like this.

When working with wings constructed from multiple panels, I build the wings using one root panel that the other panels are attached too. That way I can rotate the whole wing as one.

[jsisidore]

Hmm, alright I was able to reach ~700m/s at 18-19 without getting hot, but then rapiers start to misbehave in a very random fashion, I go below 18 and everything seems to be normal and wham the engines switch off again, thing is they do work when I have 0 air intake so I have no clue when they are about to switch off. I did remove ram intakes and put ordinary cones for less drag. I don't think I will survive more than 800m/s at below 18km...

[Val]

7 minutes ago, jsisidore said:

Hmm, alright I was able to reach ~700m/s at 18-19 without getting hot, but then rapiers start to misbehave in a very random fashion, I go below 18 and everything seems to be normal and wham the engines switch off again, thing is they do work when I have 0 air intake so I have no clue when they are about to switch off. I did remove ram intakes and put ordinary cones for less drag. I don't think I will survive more than 800m/s at below 18km...

All stock crafts should be able to survive temperatures at 900 m/s at 10 km and 1250 m/s at 20 km.

[jsisidore]

I'm following your ascent profile (sort of) and so far I can't believe that you can juice rapiers to 29km

Quote

1. SAS on, Full throttle, Stage to activate RAPIER.
2. At 90 m/s lift nose and take-off.
3. Build up speed to 250 m/s at seal level.
4. Pitch up to 10° above horizon.
5. At 10 km pitch up to 15° above the horizon.
6. At 18 km engage LV-Ns. (Action Group 1)
7. At 29 km switch RAPIER Mode. (Action Group 2)
8. Pitch down to 10° above horizon.
9. At 35 km shut down the RAPIER (Action Group 3)
10. When AP is 45 km start following prograde marker.
11. When AP is at 81 km altitude, throttle down and coast to space.
12. Circularize at AP.

 

[jsisidore]

3 hours ago, Val said:

All stock crafts should be able to survive temperatures at 900 m/s at 10 km and 1250 m/s at 20 km.

Just made a test, at 15km and 855m/s the cockpit has exploded, so more or less you can go higher than 800km/s

correction: 875km/s

now at 825km/s

I think you need to know physics in this game...

now 800km/s

I give up.

Edited June 9, 2016 by jsisidore

[Val]

2 hours ago, jsisidore said:

I'm following your ascent profile (sort of) and so far I can't believe that you can juice rapiers to 29km

You are right. You can't, but if you switch to Closed Cycle before that, then you go to fast and explode from heat. So It's basically just the Nervs carrying it from 22-ish km to 29 km. If you are referring to my Triana Mk.2 design.

EDIT: It just did a quick test and it does seem the design has issues with exploding landing gear in 1.1.x. I'll put on some new gear and run some more tests.

 

[jsisidore]

1 minute ago, Val said:

You are right. You can't, but if you switch to Closed Cycle before that, then you go to fast and explode from heat. So It's basically just the Nervs carrying it from 22-ish km to 29 km. If you are referring to my Triana Mk.2 design.

 

I'm a bit confused, what is a closed cycle again? Just nervs in jet mode make the cockpit explode at 15km and 800m/s. And if I go higher they switch off. So what can I do?

I'll try and put rams back, maybe it will make a difference.

[Val]

9 minutes ago, jsisidore said:

I'm a bit confused, what is a closed cycle again?

Closed Cycle is what the Rocket Mode on the RAPIERs are called. 

Quote

Just nervs in jet mode make the cockpit explode at 15km and 800m/s.

Nervs don't have a Jet mode.

Quote

And if I go higher they switch off. So what can I do?

I'll get back to you when I'm done testing my changes to the Triana.

If you can share your craft, I could also take a look at it.

[AeroGav]

2 hours ago, jsisidore said:

Just made a test, at 15km and 855m/s the cockpit has exploded, so more or less you can go higher than 800km/s

correction: 875km/s

now at 825km/s

I think you need to know physics in this game...

now 800km/s

I give up.

I know you're dealing with a lot right now but at times I worry that advice i already gave isn't being taken on board because sometimes it seems you're getting confused on the basics, or that you haven't got round to reading or haven't understood all i wrote because you're asking stuff that has already been addressed.  On the other hand you made a very nice looking ship that's an absolute stunner for a "first SSTO" , so you must be pretty close to "getting it"!

I'd say you need to understand the principles of flight and know what is having an effect on what, rather than telling to follow a particular profile slavishly without understanding what's going on.

You were trying to apply Val's exact flight profile to his ship without actually having his ship.  Your ship is different, has different amounts of parasite drag, lift induced drag, different capacity for lift generation, different ability to tolerate heat.

I gave advice explaining why i raise the nose at a certain point and when i dive through the sound barrier etc. explaining the principles involved (makes for a long post) and also examples of the speeds and altitudes involved on my design, but did not tell you to try copy that exactly if not flying my ship.

Anyway, 2 issues that seem wierd

1.  cockpit blowing up at 825km/s 

that's very low speed to have stuff exploding, even the weak mk1 cockpit.  Mk2 are much higher heat rating.    Do you have any mods that might be messing with things? Have you set re-entry heating to higher than normal value in the game difficulty settings?

the second is engines cutting out at over 18km

RAPIER engines don't flame out till 29km,  though I can't climb above 23-24km on most designs without loosing speed.  I usually light up the NERV engines at this point but obviously you may as well keep the RAPIERS going till they turn themselves off because it all helps.

assuming no glitchy wierdness, the only advice i can give here is that inline cockpits are less vulnerable because the front of the plane always gets the worst of the heat, having something in front of the cockpit makes it a lot cooler.   Also , you are aware that heating is dependent on altitude as well as speed.  Namely, the higher you go , the faster you can go without getting too hot.    It's a tradeoff, you need to work out what speed profile is best for your design.  If you're getting too hot, raise the nose so your altitude goes up more quickly and your speed increases more slowly.

[mk1980]

if heat is a major problem, you could just replace the mk2 cockpit with an mk2 inline cockpit (or passenger cabin) and put an mk2 adapter tank in front with a shock cone intake or a shielded docking port as the nose. the docking port has insane heat resistance. i never managed to burn one off even at really insane speeds (like 1200m/s at 10 km or 1600 m/s at 22km) and it's useful for docking the craft once in orbit. the shock cone has better aerodynamics but worse heat resistance (still A LOT better than the cockpit), and it would provide enough air for your 2 rapiers, so you could replace the precoolers with something else.

[AeroGav]

Back to flight profiles again - 

the air breathing/jet powered/rapier powered (no oxidizer)  part of flight has only two things in it that really matter.

First, you have to be able to get over the sound barrier.  You have now learned how to do that, well done.

Second, you need to milk those jet engines for all the speed and altitude you can get before turning on the rocket/closed cycle engines.   

The RAPIER jets should keep running in air breathing mode till 29km ,  though by that point the flames coming out the back look like candles on a birthday cake, and tiny thrust output.   Above 20km thrust falls very quickly with increasing altitude. I think it halves with every additional 1.5km.  Most likely your ship cannot sustain flight  above 23km on RAPIER alone because thrust will have fallen below that needed to sustain level flight.

You can help yourself here by making sure you reach at least 1130 m/s at some point before passing through your airbreathing max altitude (23km).   That is because 1130m/s is the  speed at which  RAPIER produce max power.  Flying at the speed where they give max power means you can get as high as possible before you reach a point where the thrust is no longer sufficient to sustain level flight.

In fact, going slightly over this speed may be a worthwhile tradeoff.  Sure, your air breathing ceiling will be slightly lower, but at 1300m/s it only produces 20% less power than at the optimum 1130m/s, whereas the engine output halves every 1.5km above 20km.  Don't go over 1300 so long as the rapiers are burning though. because after 1300 their thrust falls off really quickly   and also of course going that fast below 30km is likely to cause overheating.

So...    after passing the sound barrier your next goal is to reach 1150-1300, but you don't have to do this at 15km, use your discretion based on how hot the ship is getting and also how fast it is accelerating.  Pull the nose up to reduce heat load, but since you are flying into thinner air this is also like throttling back on your engine.   If you're still gaining speed rapidly, that's no bad thing, excessive rates of acceleration can make things hard to control.

Lastly,  the rapid way RAPIER engines gain thrust after crossing the sound barrier means i sometimes overshoot my airbreathing cieling and intended speedrun altitude.  I can end up zooming up from 10km so fast i can't stop the climb in time, and end up zooming out past 23km while only doing 800 m/s,  then on past 29km when the engines stop and eventually coasting back down.   IF this happens, don't light the rockets and try to keep climbing,  instead coast back down to an altitude where the RAPIERs relight and do some more acceleration.  You don't want to turn on the rockets till you've gotten all the speed out of your jets you possibly can.

 

TL; DR

1. start lowering the nose around 15km when in the supersonic climb.  An easy but not quite the most efficient way of doing this is to set prograde on SAS

2. this should level you off at 19km, then go back into stability assist mode to hold level flight while accelerating.   You want 1130m/s before getting much over 20km.   Use your own judgement, it's ok to climb a bit faster if you're getting too hot and are accelerating well.

3. in a very slow climb, try to reach 1300m/s before 23km.   After that, raise the nose by however much it takes to stop getting any faster.

4. when you can no longer climb at a significant rate without loosing speed, activate the nukes

5. the added power from the nukes will give you a kick, but try to keep speed below 1300 until the RAPIER flame out completely at 29km, so raise the nose enough to do this.

6. after that, ignore any speed limits, just try to keep AoA between 5 and 10 degrees.

Edited June 9, 2016 by AeroGav

[jsisidore]

1 hour ago, Val said:

Closed Cycle is what the Rocket Mode on the RAPIERs are called. 

Nervs don't have a Jet mode.

I'll get back to you when I'm done testing my changes to the Triana.

If you can share your craft, I could also take a look at it.

 I meant rapier engines.

I would, but first I need to learn how to do it.

[AeroGav]

2 minutes ago, jsisidore said:

 I meant rapier engines.

I would, but first I need to learn how to do it.

Sign up to this site, KerbalX !

https://kerbalx.com/

Sorry about my long posts btw.   I appreciate some people may not like big walls of text - that's why i linked my 2 youtube videos, i've annotated them which speech bubbles to show what's happenning.    Have you tried watching them, how does my plane fly differently to yours?

Spoiler

 

Part 1 - climb to 17km, includes passing the sound barrier

 

part 2 - 18km to orbit, includes speedrun and transition to rocket power

 

 

[Val]

I updated the Triana with new landing gear that doesn't explode and made a boring video of the launch.

It shows that the craft has no issue going at 1000 m/s at 10 km, while climbing and accelerating.

(Video should be done uploading and processing 2 hours after posting this. I have to go to bed.)

[bewing]

7 hours ago, jsisidore said:

Just made a test, at 15km and 855m/s the cockpit has exploded, so more or less you can go higher than 800km/s

correction: 875km/s

now at 825km/s

I think you need to know physics in this game...

now 800km/s

I give up.

These numbers are just impossible. Either your game is corrupted, or you are using some bad mods, or you are doing something very very wrong. There is no cockpit in the game that can explode at less than 1000 m/s, period, even at sea level. At 15km, all cockpits can survive 1100 m/s indefinitely.

[jsisidore]

I've installed the engineer mod thingy and I think that I'm too heavy. Anyway, my design is impossible to achieve the orbit.

[jsisidore]

16 hours ago, AeroGav said:

Sign up to this site, KerbalX !

https://kerbalx.com/

Sorry about my long posts btw.   I appreciate some people may not like big walls of text - that's why i linked my 2 youtube videos, i've annotated them which speech bubbles to show what's happenning.    Have you tried watching them, how does my plane fly differently to yours?

  Reveal hidden contents

 

Part 1 - climb to 17km, includes passing the sound barrier

 

part 2 - 18km to orbit, includes speedrun and transition to rocket power

 

 

 

Yes I did, very informative. I prefer videos and pictures than long explanations when I'm learning something.

Edited June 10, 2016 by jsisidore