Spaceplane with nuke: How do you do it?

[ShadowZone]

I've been trying for most of yesterday to create a spaceplane that can carry 6 Kerbals and 3,5t of cargo to orbit. I usually manage to get the thing up to 58km or something but the nuke engine is not powerful enough to get the thing in orbit. The plane weighs ~35t all combined and uses 4 turbojets and 1 nuke. Stock aerodynamics.

What am I doing wrong? Did Squad tweak the turbojets in 0.90? Because I imagine I did not have that many problems getting spaceplanes to orbit in 0.25

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Edited February 1, 2015 by ShadowZone
Added info about aero-model.

[Janos1986]

The nuke's thrust is abysmal, the idea is that your plane should be able to slingshot out of the atmosphere with turbojets alone, then you turn nukes on when you're already in vacuum so there's no air to slow you down.

It's hard but not impossible. I have trouble doing that myself.

[FlyingPete]

You can get into orbit with just jets and RCS thrusters in some cases. The trick is to keep your climb slow so you can build up speed in the atmosphere. You want to have a suborbital trajectory of something like 90km x 40km when the jets die. Then you coast to apoapsis and circularize with your rockets/monoprop engines/ions.

[ShadowZone]

You can get into orbit with just jets and RCS thrusters in some cases. The trick is to keep your climb slow so you can build up speed in the atmosphere. You want to have a suborbital trajectory of something like 90km x 40km when the jets die. Then you coast to apoapsis and circularize with your rockets/monoprop engines/ions.

Okay in the past I did it this way: get up to about 35km, then fire rockets until circularized. What's an optimal flight path to do it your way? Because either I can't get decent speeds or my vertical speed keeps dropping and I lose altitude.

Edited February 1, 2015 by ShadowZone

[Wanderfound]

As always, the answer varies a lot depending upon what aero model you're flying in. Here's how to do it in FAR:

[ShadowZone]

Forgot to mention: completely stock (craft parts and aerodynamics - if you can call it that)

[Pecan]

Forgot to mention: completely stock (craft parts and aerodynamics - if you can call it that)

Jet ascent falls into three phases in any aerodynamics model, although the speeds and altitudes will change. In stock:

1. Get above the thick, draggy lower atmosphere: climb-out to 15km-20km at 45-degrees or so, caring more about rate of climb than horizontal speed. Make coffee.
   
2. Build horizontal speed: pitch-down to provide more forward thrust but keep climbing. Between 20km-30km try to keep your horizontal speed around 1km/s plus 100m/s per 1km altitude above 20km (1,000m/s at 20km, 1,100m/s at 21km, etc). Adjust pitch/climb-rate to maintain something like this so you break 2km/s and get orbital speed around 30km-33km.
   
3. Circularise: nurse the throttle to maintain as much horizontal speed as possible while drifting to Ap, building Pe while you do it. If you intake-spam like me you can burn the jet all the way to space and get a periapsis over 50km, leaving only about 30m/s for the rockets to circularise a 75km orbit.
   

Different designs fly differently so use this only as a rough guide.

[Starhawk]

If you're interested, I have an imgur album showing detailed progress using an ascent just like Pecan described. The spaceplane in the album uses two turbojets and one nuke and masses about 27 t. It's 100% stock.

Good luck and happy landings!

[GoSlash27]

Shadowzone,

You've got a lot more spaceplane there than you need for that job.

The single engine design I outline here is capable of putting 6 kerbals and 3.5 tonnes of payload into orbit.

I notice you've got a whole lot of wings on that thing for being 35 tonnes.

You've also got what appears to be insufficient intake area for your engines.

Finally, you've got more engines than you need. Remember that moar engines doesn't add anything but weight and drag when starvation sets in.

My guess is your drag is overpowering your thrust and killing your acceleration while you're still below 32km altitude. This leaves you with insufficient speed to establish periapsis and your nuke unable to make up the difference in such a short time frame.

Follow Pecan's lead on the launch profile.

Best,

-Slashy

Edited February 1, 2015 by GoSlash27

[wasmic]

You can get into orbit with just jets and RCS thrusters in some cases. The trick is to keep your climb slow so you can build up speed in the atmosphere. You want to have a suborbital trajectory of something like 90km x 40km when the jets die. Then you coast to apoapsis and circularize with your rockets/monoprop engines/ions.

Doing it this way will require insane amounts of airhogging, though. It's completely possible to do an SSTO where the jet's flameout at 30 km, you'll just need more thrust and delta-v for the rocket engine.

[GoSlash27]

Doing it this way will require insane amounts of airhogging, though. It's completely possible to do an SSTO where the jet's flameout at 30 km, you'll just need more thrust and delta-v for the rocket engine.

Not true. This can be done with as little as .018 m^2/engine, which is about half of what most people use for intakes. It's just a matter of being patient around 30km and letting the speed build.

Observe:

http://s52.photobucket.com/user/GoSlash27/slideshow/KSP/intake%20tests

Any of these intake combos are sufficient to achieve orbital periapsis:

Intakes per engine

3x XM-G50

2x circular intake

1x circular intake + 2x XM-G50

1x Ram intake + 1x XM-G50

1x Shock cone + 1x XM-G50

Once you've established the periapsis, it takes very little thrust and DV to circularize.

Best,

-Slashy

Edited February 1, 2015 by GoSlash27

[Red Iron Crown]

Slashy, surely you mean apoapsis rather than periapsis, yes?

[Pecan]

Not true. This can be done with as little as .018 m^2/engine, which is about half of what most people use for intakes. It's just a matter of being patient around 30km and letting the speed build...

Indeed.

Although I DO intake-spam (up to 8 RAM intakes per jet) that's mostly because I'm too impatient to raise the periapsis that 'low', preferring to keep the jet ticking-over all the way up.

Besides, intake-spam makes designs more forgiving on ascent for newbies, and I don't think it looks too bad:

(provided you accept that that particular one is deliberately trying to avoid looking like everyone else's Mk2 spaceplanes)

[MarvinKitFox]

Slashy, surely you mean apoapsis rather than periapsis, yes?

No he means periapsis.

The idea is to be at the *lowest* alt of an almost-orbit.

Typically something like a 75km x 35km, with you at the 35 side

The reason most people fail is because, like the OP, they do not understand this simple rule, and try to burn to space as soon as they get an apoapsis above the atmosphere.

Getting to orbit from ,say, a 75km x -120km (with you at 35km) requires stiff rocket thrust.

Getting to orbit from a 75km x 35km, with you at 35km, requires you to just coast up, and burn under a 100m/s to circularise.

[GoSlash27]

Slashy, surely you mean apoapsis rather than periapsis, yes?

It's kinda confusing, but I mean what Marvin said. Since the flight has an apoapsis the entire time, I am differentiating by referring to the current apoapsis as what will become the final periapsis upon circularization.

If you have enough velocity, you will drift right up to orbital altitude with hardly any drag and have almost all of your orbital velocity when you get there.

You can generate a whole lot of velocity from turbojets without very much intake area, it's just a matter of patience.

And of course, achieving orbit is all about velocity, not altitude.

tl;dr

Establishing orbit in a turbojet spaceplane requires a lot less rocket thrust and a lot less intake area than many people assume.

Best,

-Slashy

Edited February 1, 2015 by GoSlash27

[CrisK]

ShadowZone, You only need two of those turbojets. The additional turbojets, along with the fuel, are just extra weight. I highly recommend downloading a couple of Wanderfound's Kerbodyne designs to study.

[Pecan]

[Orbital periapsis is] kinda confusing...

Just as a side-note, the navball will switch from 'surface' to 'orbital' mode once the current periapsis is above 24km, which will probably be quite a bit after you've got an apoapsis at the required orbital altitude. It should be easy to achieve that with minimal intakes or, in my case, patience ^^.

[Laie]

The reason most people fail is because, like the OP, they do not understand this simple rule, and try to burn to space as soon as they get an apoapsis above the atmosphere.

Getting to orbit from ,say, a 75km x -120km (with you at 35km) requires stiff rocket thrust.

Getting to orbit from a 75km x 35km, with you at 35km, requires you to just coast up, and burn under a 100m/s to circularise.

The numbers you've chosen aren't too suitable to illustrate the point -- the difference between those two scenarios is approx 150m/s worth of rocket thrust, and you don't need very high-powered engines either way. That 150m/s would also take you out of the atmosphere within 5 minutes or so, while milking the jets even further can easily take another 20 minutes -- most of that not for the milking per se, but because even after you're done, the shallow ascent takes about half an orbit until you finally leave the atmosphere and can timewarp. A case could be made that in this example, the extra rocket thrust would be fuel well spent.

But yes, you can and should start working on circularization while you're still at an altitude where the jets work well -- that what all this "getting speed" stuff really is about, after all. Then again, it's a diminishing returns thing -- Compared to the first 100km of periapsis altitude, the last 100km require a lot more patience and save a lot less fuel.

So unless you're doing a challenge, the question of when to ignite the rockets ("let's get this over with") can to a large part be decided by taste. But if you're using the jets for the sole purpose of getting to space, you're doing it wrong: Just compare the delta-V requirements of a rocket that can take you "to space" with that of a rocket that will put you in a proper orbit. Also, this: https://what-if.xkcd.com/58/

Edited February 1, 2015 by Laie

[GoSlash27]

Laie,

The difference is a lot more than it would appear. Using the recommended intake area, a turbojet will flame out at 32km altitude while going 1.2km/sec, but is capable of exceeding 2.2 km/sec (essentially orbital velocity when you throw in rotation) at the same altitude if you build speed. More speed means more air, which means more thrust and thus more speed...

That's a 1 km/sec DV difference that must be made up in a very short time if you elect to not hang around at 32km altitude building speed which requires a whole lot more fuel and much more powerful rockets to complete the burn.

Turbojet and RAPIER spaceplanes work best if not flown like rockets, which is why the profile is so important.

Best,

-Slashy

[ShadowZone]

Thanks for the advice. Seems like I have to get back to the drawing board since my plane refuses to pitch up once above 12k if I use only 2 Turbojets. Any idea why that happens? Because I remember that being one of my main problems with designing spaceplanes in KSP... those bloody things keep losing vertical speed once a certain altitude is reached.

[Starhawk]

Lack of lift sounds most likely. Could be lack of control surfaces (not enough pitch authority), but I would try more wings first.

Good luck, and happy landings!

[GoSlash27]

Thanks for the advice. Seems like I have to get back to the drawing board since my plane refuses to pitch up once above 12k if I use only 2 Turbojets. Any idea why that happens? Because I remember that being one of my main problems with designing spaceplanes in KSP... those bloody things keep losing vertical speed once a certain altitude is reached.

ShadowZone,

It depends. It sounds like a lack of pitch authority as you approach terminal velocity. This is usually due to a drag misalignment more often than static imbalance in my experience.

Make sure your intakes are as far back as possible and add control surfaces (elevators *and* canards) in a balanced fashion to give you more pitch authority.

Also, you shouldn't be anywhere near terminal velocity at 12 km. You want to be climbing ballistically to get out of the soup. You don't want to approach Vt until you're at 25 km or so.

Best,

-Slashy

*edit*

If you've been using high- mounted wings, that'd do it. Wings exhibit less drag than other parts at low angles of attack and high speed. This can create a pitch moment that pushes your nose down.

Edited February 1, 2015 by GoSlash27

[Laie]

Laie,

The difference is a lot more than it would appear.

[...]Turbojet and RAPIER spaceplanes work best if not flown like rockets, which is why the profile is so important.

I never wanted to dispute the latter -- if it came out like this, I've done something wrong. I tried to saddle the horse from behind: all the advice about building speed is actually about raising periapsis. As I put it, one "start working on circularization while [..] still at an altitude where the jets work".

However, there's diminishing returns: In terms of surface speed at 35km, we're talking about 1100, 2000 and 2200m/s, respectively, for an periapsis of -500, -300 and -100km. Now, one probably has to go faster than 1100@35 for the jets to function at all; and most planes couldn't accelerate further without climbing as well. But that's besides the point. What I'm trying to get at is that the first several hundred km of periapsis altitude are low-hanging fruit, while the last meters are a lot of patient fiddling for little gain.

A periapsis of -100km can be improved upon, but that's neither as easy nor as worthwhile as having it brought to -100km in the first place.

- - - Updated - - -

Thanks for the advice. Seems like I have to get back to the drawing board since my plane refuses to pitch up once above 12k if I use only 2 Turbojets. Any idea why that happens? Because I remember that being one of my main problems with designing spaceplanes in KSP... those bloody things keep losing vertical speed once a certain altitude is reached.

Maybe you're going ballistic -- happens easily if you have a drag (read: intakes) behind the CoM, and go too fast. That usually only happens / becomes noticable somewhere around 20km+, though, so lack of lift or uneven drag seems more likely. In case of ballistics, more pitch authority helps, and/or climbing steeper. My personal rule of thumb is that I try to climb quickly enough so that I'm always just a little short of terminal velocity.