Mk3 SSTO w/ Nukes?

[NightshineRecorralis]

So I've been experimenting with nuclear powered Mk3 SSTOs, and I can't exactly say my attempts have been fruitful. Here is the craft I've come up with, and while it can get to orbit empty, it can only do just that. After loading an orange tank into the cargo bay, it won't even go higher than 15k meters. Tips would be greatly appreciated on how you deal with and fly large SSTOs.

https://www.dropbox.com/s/gt6hc4ffxufj55k/Wyvern KX.craft?dl=0

Questionable flying:

Orbiting but empty:

[bewing]

From the pix, it looks like your ascent is way too steep, and it looks like your rear stabilizer is in the exhaust path of some of your engines (which zeroes out their thrust).

[NightshineRecorralis]

My ascent profile? The one in orbit was a typical one with two separate accelerations and a powerdive, but it has an empty cargobay. The top pic is the one that wouldn't go out of the atmosphere at all

@bewing

Yeah, the rear wings were blocking a lot of thrust from the bottom engines, I should probably fix that.

[bewing]

Similarly to a gravity turn -- if you can keep prograde locked, you will get to orbit with a lot more fuel left.

[NightshineRecorralis]

1 hour ago, bewing said:

Similarly to a gravity turn -- if you can keep prograde locked, you will get to orbit with a lot more fuel left.

 

If it was that simple to make a long range SSTO, I wouldn't be posting here, as this was my first attempt at a large nuclear SSTO, anything helpful would be nice.

[Tidus Klein]

Ssto's work best when staying low for aslong as you can to build up speed, then once your at your absolute temperature limit pitch up to about 40-50 degrees and burn. Yours seems to have a fair amount of drag too, you dont need all the wing area, with rockets and Ssto's less is more.

[AeroGav]

In your first pic you are doing 300 m/s.   This is in the middle of the transonic high drag region, so i hope you don't hang around there too long. I try to stay slower than 240 and then, when doing so requires an angle of attack over 7 degrees to maintain lift, i drop the nose to go supersonic - get over 400 m/s before resuming climb.   I sometimes ask the nukes to help in this bit if i'm flying an underpowered machine, but you clearly had no trouble with that.

 

The second pic is concerning.   You are pitched up past the stall angle, making no lift and huge drag.

You are in an aeroplane - fly it is as one.    Most of the time you want to be at the angle of attack that gives you the best lift drag ratio, which is about 5 degrees at supersonic speed.  This is also basically your minimum drag speed for a given altitude and ship design, and also your best glide speed (lift drag ratio is king!).       The only exceptions to this are when you are transitioning through the sound barrier, as  i mentioned in previous paragraph, or when doing your 20km speedrun, trying to milk 1300 m/s out of the jet engines, when you are probably going to be holding the nose down.

One you transition to expensive rocket power,  pitch for optimum L/D.  Over time, you will get faster and higher, orbital freefall effect will support more and more of your weight, meaning you need less lift, which is just as well as lift and drag both decline as the air gets thin.      

 

Here is me flying a spaceplane to orbit on nuke power through the upper atmosphere.  It's low TWR design, weighs 16 tons at that point, 1 nuke.

Here is a rather overpowered version doing the same, 2 nukes and less weight ....  that thing pretty much jumps out of the atmosphere once i end the speedrun and pitch up to 5 alpha.

  I terms of design,  I normally aim for one jet engine per 30 tons of t/o weight.    

If it's a completely oxidizer free design, then you want one nuke per 15 tons.  

However, since you have some rapiers as they are the best ssto jet engine anyway, and because it's hard to avoid oxidizer tankage in mk2 and mk3 fuselages even if you want to , you can cut down on the nuke requirement (they are heavy) by running some rapiers in closed cycle for a short time.

A RAPIER in closed cycle makes 180kn, three times what a nuke does.   The first part of the closed cycle climb requires the most power, because once you get above mach 5.5 orbital effect is supporting most of your weight, and since the lift drag ratio wont have changed much, that means less drag and therefore less thrust needed to make progress.

So for example, on a 90 ton ship,  you could have 3 rapiers and just 3 nukes instead of 6.    When you've gone as fast as you can airbreathing, switch on the nukes and pitch to 5 alpha.   If you are loosing speed, switch over ONE of the rapiers to closed cycle, and let the other two continue in jet mode until they flame out, milking every last drop out of the atmosphere.

I did build a cargo mk3 recently with 6 nukes/6 rapiers.  

https://kerbalx.com/AeroGav/Storm-Badger

I was planning on 180 tons MTOW but it finished up lighter than expected and feels a bit overpowered,  but you can compare it with a pure rapier design here.

https://kerbalx.com/AeroGav/Kendrel2

This is a more "efficient" design, with 3 engines of each type at about 100 tons gross.  Brings 48 kerbals interplanetary.

https://kerbalx.com/AeroGav/Star-Empress

 

I don't know if oxidizer free ssto would be able to match the payload fraction of a pure rapier ship if only going to 72km orbit.  If you are flying to Minmus, obviously the nervs will win.

[NightshineRecorralis]

2 hours ago, AeroGav said:

Snip~

The first pic is the problematic aircraft, the second was was practically out of atmo, so I pitched up to go more AP time, due to lower TW ratios

[AeroGav]

7 minutes ago, NightshineRecorralis said:

The first pic is the problematic aircraft, the second was was practically out of atmo, so I pitched up to go more AP time, due to lower TW ratios

Ah ok .. so if the second aircraft never got above 15km it probably never even got supersonic then?

I'm not sure how many nukes you have vs gross weight of craft. 

Looks like your strategy was a zoom climb in atmosphere with the jets followed by an attempt to stabilize out of atmosphere with nukes.   Its kind of the opposite of my approach which is to fly constant 5 degree AoA at best lift/drag. 

[NightshineRecorralis]

1 minute ago, AeroGav said:

Ah ok .. so if the second aircraft never got above 15km it probably never even got supersonic then?

I'm not sure how many nukes you have vs gross weight of craft. 

Looks like your strategy was a zoom climb in atmosphere with the jets followed by an attempt to stabilize out of atmosphere with nukes.   Its kind of the opposite of my approach which is to fly constant 5 degree AoA at best lift/drag. 

*first*

but yeah, It was hovering at around 400m/s even during powerdive. For the second one, my ascent profile was to get to 500m/s before 5000m, hold 2* above prograde until 9km, hold 3* above horizon until 15km, switch on nukes, wait until Rapiers drop below 179.5kN and switch those to closed cycle. Hold 15*above horizon until 25km, then pitch up to 45* to maximize time to AP. I circularized with ~20,000 units of LF.

 

The other one couldn't even get high enough for me to use the nukes

[AeroGav]

18 minutes ago, NightshineRecorralis said:

*first*

but yeah, It was hovering at around 400m/s even during powerdive. For the second one, my ascent profile was to get to 500m/s before 5000m, hold 2* above prograde until 9km, hold 3* above horizon until 15km, switch on nukes, wait until Rapiers drop below 179.5kN and switch those to closed cycle. Hold 15*above horizon until 25km, then pitch up to 45* to maximize time to AP. I circularized with ~20,000 units of LF.

 

The other one couldn't even get high enough for me to use the nukes

So, what's the gross weight and how many engines of each type?  All that changed between the two was cargo right?

I prefer to talk "angle of attack" ie. difference between nose angle and prograde rather than "pitch angles",   Pitch doesn't tell you whether you're going too fast for the airframe, too slow for it, or are stalling.  You can stall at 0 degree pitch if prograde is a descent steeper than 30 degrees :-)

Having got through the sound barrier, my goal is to accelerate to mach 4.5 (1350m/s) between 15/20km.   I usually try level off for that, when heat bars start appearing, i let it climb a couple of km so the heat is less, at some point you can no longer accelerate, then i start the nukes and pitch up to 5-7 degrees above prograde.   And like i said, you don't need to switch all the rapiers at once,  you can let some continue to milk the atmosphere for jet thrust.  On a well streamlined design you need about 60kn - 1 nuke's worth - of thrust per 15 tons. A rapier in closed cycle has three times the power of the nuke, so you can calculate  how many rapiers to switch based around that.   Obviously once you pass the flameout altitude you can switch all the rest over if you want.

Looks like your profile is based off a zoom climb,  i think it works ok for high  TWR designs  and for rockets,   but i've moved away from it to a more "cruising airplane" type climb.

Edited November 12, 2016 by AeroGav

[NightshineRecorralis]

 

My aoa was really never over 10* while flying, only increased it when after punching through 20km.

Gross weight is something like 350t w/ cargo. Can't check right now since I'm not at my computer.

@AeroGav

Edited November 12, 2016 by NightshineRecorralis

[AeroGav]

25 minutes ago, NightshineRecorralis said:

 

My aoa was really never over 10* while flying, only increased it when after punching through 20km.

 

@AeroGav

Ok.   You're still making significant lift after 20km though, at least you will be if going faster than 1400 as you should be.   If you haven't yet got enough orbital effect to counteract gravity and your climb rate is decreasing,  it is better to use lift than engine thrust directly - since you should be getting lift drag ratio of at least 2:1 if not more (the star empress passenger liner does over 3:1 hypersonic)

[AeroGav]

20 hours ago, NightshineRecorralis said:

So I've been experimenting with nuclear powered Mk3 SSTOs, and I can't exactly say my attempts have been fruitful. Here is the craft I've come up with, and while it can get to orbit empty, it can only do just that. After loading an orange tank into the cargo bay, it won't even go higher than 15k meters. Tips would be greatly appreciated on how you deal with and fly large SSTOs.

https://www.dropbox.com/s/gt6hc4ffxufj55k/Wyvern KX.craft?dl=0

 

Sorry chap, i didn't realise the craft was on dropbox.

Analysing now..

OK.   The short version appears to be that the craft has various issues causing excess drag which you attempted to get around by fitting more and more jet engines.   These extra engines are more mass the nukes have to carry from 20km to orbit.   Also,  for a nuke powered SSTO there was rather a lot (75 tons)  of oxidizer onboard.

Details of some of the drag probs below if you're interested

  

Spoiler

Cones on the rapiers - good.   No cones on the nukes though - drag.

Rear of ship - you have a mk3 to 2.5m adapter, the 2.5m adapter has a 2.5m node and you attached a 1.25m engine directly to it.   You need an adapter or this will produce excess drag.  There are four mk1 engine nacelles attached radially to the 2.5m adapter and they are partially clipped inside, but the nacelles have no nose cones on them - flat plate drag !

You also have long tailcones attached radially and partially clipped inside in a visually appealing manner - makes a nice feathering effect, but the game just sees another fuselage stack . These appear to have NCS adapters on the front, but no small nose cones on the front of these, so a small drag penalty .

Radiators in the belly.  Draggy waste of time.   They dont work in the upper atmosphere, so wont help with launch or re-entry overheating.  They can be useful to get rid of the heat from launch if you are re-entering immediately after launch - in that case use deployable radiators when in space - less drag.

 

 

On my version of your ship i stripped it down to 5 Rapiers and 6 Nukes.  I took away all the oxidizer tanks I could, but it's still got oxidizer in the 2.5m adapters front and back as well as the 2.5m to 1.25m adapter at the nose.

https://kerbalx.com/AeroGav/Wyvern-Gav-b
I'm quite happy to take this off KerbalX when you've downloaded it as it's still your ship after all.   Anyway the question is can it even break the sound barrier with half the number of engines?

Spoiler

I tabbed out of the game and when i came back to it we were on an escape trajectory from Kerbin with over 2900 and over 5300 oxidizer remaining , not counting the cargo which was of course locked.   Still too much oxidizier capacity on this design !

Launch profile,  after takeoff let it find it's own pitch with SAS off.  It will settle at about 2deg AoA which with 5 deg incidence on the wings gives a lot of lift.   After a while it'll be approaching 6km and encroaching on the transonic region.  At this point you can use prograde on sas to lock it at near zero aoa and stop it trying to climb till properly through the sound barrier.   It more or less takes care of itself after that,  it'll overshoot past 29km a few times and flame out - wait for it to come back down.  I got up to mach 5 before i ran out of patience and made a run for orbit.  If it threatens to overheat then turning SAS off causes the nose to rise a little which helps to get you out of the clag.

 

 

Edited November 12, 2016 by AeroGav

[Jarin]

21 hours ago, NightshineRecorralis said:

[regarding prograde lock] If it was that simple to make a long range SSTO, I wouldn't be posting here, as this was my first attempt at a large nuclear SSTO, anything helpful would be nice.

 

7 hours ago, NightshineRecorralis said:

My aoa was really never over 10* while flying, only increased it when after punching through 20km.

Haven't played with the design yet (I'll do that next) but I wanted to address your comments here. Your ascent profile is almost certainly causing you trouble. Do you have the thrust to go supersonic at or below 10km? Because if you can do your acceleration there, it greatly helps overall. On heavy craft like this, I'll be kissing 1000m/s at sea level before ascending at all. My AoA is never over 15 degrees at *all* clear to orbit. You should build to be capable of accelerating at a 5-10 degree incline up to 20km altitude. Diving for speed is something I file under "advanced technique, do not bother unless already very experienced with other designs". 

Really, it sounds like a large part of your problem is just straight-up not enough atmospheric thrust. 

[NightshineRecorralis]

3 minutes ago, Jarin said:

 

Haven't played with the design yet (I'll do that next) but I wanted to address your comments here. Your ascent profile is almost certainly causing you trouble. Do you have the thrust to go supersonic at or below 10km? Because if you can do your acceleration there, it greatly helps overall. On heavy craft like this, I'll be kissing 1000m/s at sea level before ascending at all. My AoA is never over 15 degrees at *all* clear to orbit. You should build to be capable of accelerating at a 5-10 degree incline up to 20km altitude. Diving for speed is something I file under "advanced technique, do not bother unless already very experienced with other designs". 

Really, it sounds like a large part of your problem is just straight-up not enough atmospheric thrust. 

Kinda sorta fixed the problem with this one: https://kerbalx.com/NightshineRecorralis/Wyvern-KX (Ignore the KX)

Full mission here: 

 

Edited November 12, 2016 by NightshineRecorralis

[Jarin]

Glad you got a working design. I've got an album with my flight-profile suggestions for the old design here. Advice should hold for future designs as well.

Turns out I was wrong about not enough thrust. It just needed a bit of patience.