SSTO are all small?

[juanml82]

16 hours ago, AeroGav said:

I have a stock 5 crew minmus SSTO - could do with updating for 1.13 (could use a service bay, some radiators to deploy from within and moving the solar array inside the bay)  , but it still works.

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

The problem with building large pure stock craft

1. there is a shortage of wing parts of sufficient size.  

2. spaceplane parts generally are just smaller than the largest available rocket stuff. This means you need more parts to make something the same size,  so your creation becomes too wobbly.   Wobble is even less desirable in wing/control surface parts than it is in rocket body parts,   but you can't use struts because they cause huge drag.     Spaceplanes need to have low drag above all else, it's worth sacrificing more weight for less drag.  And unlike a rocket, which might shed some of its struts as boosters fall off, a plane wears its struts full time.

3. fuel transfer problems.  If you use only Rapier engines, this is a non issue because they drain evenly from every tank even in rocket mode.

However, Rapier have the worst vacuum ISP in rocket mode of any engine,  they are no good for an interplanetary spaceplane.   For that you need NERV, but they drain fuel from the front tanks first, unbalancing your plane, and won't pull fuel from the wing tanks etc.   So, you either need a lot of fuel ducts (massive drag, due to game bug).   Or transfer manually, which gets unmanageable with too many engines or tanks.

Very easily solved with mods ( GPOspeedfuelpump, or tac fuel balancer)   but pure stock it's a major headache.

 

Re - engines

I build a lot of stuff in the 30-40 ton class.

With 2 NERV and 1 Rapier,   I just run the Rapier in air breathe mode all the way to cutoff at 29km, even though it is unable to lift me beyond 23km on its own.  After 23km, I start the 2 nukes up,  which have 800ISP,  but might as well keep the Rapier going at 3200ISP till it quits , no matter how little power it's making.

I did try taking some oxidizer, and running a bit of closed cycle mode from say 29km to 34km.   However,  i actually had slightly less delta V once i reached orbit than if just left the oxidizer tanks empty.   Yes, you launch with more total fuel , yes, you have better TWR when the Rapier is going in closed cycle mode,  but you're also squandering liquid fuel at 300ISP when it would have given you 800 run through the nukes,  and you also use more LF getting all that oxidizer up to 29-34km so you can burn it in the first place.

It's also harder to fly - you're heavier, so you climb slower for a given speed and get hotter, closer to blowing up.   Getting through the sound barrier is harder.   It's harder to hold a nice steady flight profile when your TWR is fluctuating all over the place like that - underpowered - rapiers in closed cycle - waaahhh! massively overpowered ! splutter ! out of ox 5 seconds later ! now underpowered again !

 

Now there's other combos

 

2 Rapier, 1 Nuke

This is what my very first spaceplane(s) used

Because you have more RApier, you can get higher on pure airbreathing, but not much higher because thrust halves with every 1.5km up there.  So at 24km you can go no higher and must switch the rapier over to closed cycle to keep going up - one nuke on its own is not enough.   This means you miss out on  milking air breathing mode on your rapier all the way to 29km.     In closed cycle, you are masssively overpowered,  ideally you'd only switch one RAPIER to rocket mode and keep the other air breathing, that'd give plenty enough power along with the single nuke, but obviously you can't on a 3 engine ship due to asymmetric thrust .

 

2 Whiplash, 2 Terrier, 1 Nuke

I'll soon be in a position to try this in my latest career game.  I think it could be the best yet.

2 nukes gives plenty of power to push a 30 ton ship the rest of the way to orbit once the air breathers quit, but that's 6 tons of reactor weight you're carrying everywhere you go.    If I only had one nuke, that'd save 3 tons.  Once in orbit , that'd be plenty.    In the climb to orbit,  above 35km drag is under 20kn, so the 60kn from a single nuke would be fine.   But when the jet engines first conk out, you're seeing drag numbers of 50-70kn.     

So my idea is

0. Start the Nuke when the Whiplashes start to struggle.

1. Shortly afterward, start the Terriers to maintain climb rate as jet power declines further.

1.  Jettison the Whiplash when they flame out.  They weigh 1.8 tons each, that will boost my delta V a lot.  They only cost 2000 credits per engine, unlike the Rapier (6000) and nuke (13000?) so are cheap enough to throw away.

2. Jettison the Terrier pods when empty.  Only bring enough fuel to reach 35km or so.  Terrier are also very cheap engines (400 cost?)

3.  The nuke takes you the rest of the way to orbit then on to wherever you're going.

Before you start hollering whats the point of an spaceplane if it throws away 4400 Kredits worth of engines every flight,  this should give enough delta v to fly to duna and back, comfortably,  fly around a bit in its atmosphere and possibly land a few times or drive over the surface a bit.  Any rocket that can do the same will cost more than 4400 kredits.  A lot more.

Another option, useful before unlocking Rapiers in career, is two Whiplash, one Aerospike. This doesn't give you, at all, the kind of dV you get out of a Nuke, so it's either only useful for ferrying things to/from LKO, or they require refueling in LKO to get elsewhere. But you avoid the low TWR of the nukes. For LKO operations, like rescue contracts, they are good enough.

[Jestersage]

20 hours ago, AeroGav said:

Re - engines

I build a lot of stuff in the 30-40 ton class.

With 2 NERV and 1 Rapier,   I just run the Rapier in air breathe mode all the way to cutoff at 29km, even though it is unable to lift me beyond 23km on its own.  After 23km, I start the 2 nukes up,  which have 800ISP,  but might as well keep the Rapier going at 3200ISP till it quits , no matter how little power it's making.

I did try taking some oxidizer, and running a bit of closed cycle mode from say 29km to 34km.   However,  i actually had slightly less delta V once i reached orbit than if just left the oxidizer tanks empty.   Yes, you launch with more total fuel , yes, you have better TWR when the Rapier is going in closed cycle mode,  but you're also squandering liquid fuel at 300ISP when it would have given you 800 run through the nukes,  and you also use more LF getting all that oxidizer up to 29-34km so you can burn it in the first place.

It's also harder to fly - you're heavier, so you climb slower for a given speed and get hotter, closer to blowing up.   Getting through the sound barrier is harder.   It's harder to hold a nice steady flight profile when your TWR is fluctuating all over the place like that - underpowered - rapiers in closed cycle - waaahhh! massively overpowered ! splutter ! out of ox 5 seconds later ! now underpowered again !

 

Now there's other combos

 

2 Rapier, 1 Nuke

This is what my very first spaceplane(s) used

Because you have more RApier, you can get higher on pure airbreathing, but not much higher because thrust halves with every 1.5km up there.  So at 24km you can go no higher and must switch the rapier over to closed cycle to keep going up - one nuke on its own is not enough.   This means you miss out on  milking air breathing mode on your rapier all the way to 29km.     In closed cycle, you are masssively overpowered,  ideally you'd only switch one RAPIER to rocket mode and keep the other air breathing, that'd give plenty enough power along with the single nuke, but obviously you can't on a 3 engine ship due to asymmetric thrust .

 

2 Whiplash, 2 Terrier, 1 Nuke

I'll soon be in a position to try this in my latest career game.  I think it could be the best yet.

2 nukes gives plenty of power to push a 30 ton ship the rest of the way to orbit once the air breathers quit, but that's 6 tons of reactor weight you're carrying everywhere you go.    If I only had one nuke, that'd save 3 tons.  Once in orbit , that'd be plenty.    In the climb to orbit,  above 35km drag is under 20kn, so the 60kn from a single nuke would be fine.   But when the jet engines first conk out, you're seeing drag numbers of 50-70kn.     

So my idea is

0. Start the Nuke when the Whiplashes start to struggle.

1. Shortly afterward, start the Terriers to maintain climb rate as jet power declines further.

1.  Jettison the Whiplash when they flame out.  They weigh 1.8 tons each, that will boost my delta V a lot.  They only cost 2000 credits per engine, unlike the Rapier (6000) and nuke (13000?) so are cheap enough to throw away.

2. Jettison the Terrier pods when empty.  Only bring enough fuel to reach 35km or so.  Terrier are also very cheap engines (400 cost?)

3.  The nuke takes you the rest of the way to orbit then on to wherever you're going.

Before you start hollering whats the point of an spaceplane if it throws away 4400 Kredits worth of engines every flight,  this should give enough delta v to fly to duna and back, comfortably,  fly around a bit in its atmosphere and possibly land a few times or drive over the surface a bit.  Any rocket that can do the same will cost more than 4400 kredits.  A lot more.

Are these for Mk-1, Mk2, or Mk3 configurations?

Also, Your Point 2 "Jettison the Terrier pods" makes it no longer SSTO. Just a reusable spaceplane ala Space Shuttle.

[AeroGav]

On 13/07/2016 at 11:49 PM, Jestersage said:

Another question I want to confirm: VSSTO is just not as efficent as the Standard Spaceplane SSTO, right? My understanding is that SSTO also used the lift from the lifting surface to achieve orbit.

Just to clear up some terminology -

SSTO allows the entire vessel to be recovered which may lower costs, but the downside is reduced payload fraction.  

Wings and Jet engines increase the payload fraction to orbit,  making SSTO somewhat more practical.  However,  not every spaceplane is SSTO.   The winners of all payload mass ratio challenges are spaceplanes that jettison stuff on the way up, combining the best aspects of staged rockets along with wing lift and air breathing.

However, you can still put a larger payload in orbit with a rocket, because you can simply build a much bigger rocket with stock parts.

How do Jets and Wings help?

Jet engines do this by having an ISP 10x greater than any rocket engine.   However, they are also heavy.  Wingless launchers (ie. rockets) require a thrust weight ratio over 1, so that means a lot of jet engines which become dead weight over 25km/Mach 4.5.    

Wings allow you to get away with a lower TWR.   Lift : Drag ratio can exceed 8 to 1 or more at low speeds,  and generally stays above 2.5 : 1 even at hypersonic velocities.  So , you don't need as much engine.   On a pure rocket propelled vehicle, this is of marginal benefit, but bearing in mind the aforementioned issue of jet engine mass you can see why minimising engine mass is a good thing.   More so if the rocket engine of choice in the upper atmosphere is the NERV, which is very efficient but also very heavy.