Delta-V for SSTOs?

[Cairol]

For rockets, the current charts say, you need about 3400 d/v to get to orbit.

How much do you need to do the same orbit with an SSTO?
Because I often find myself thinking I have plenty of fuel in my plane, but as soon as I switch to closed cycle, my fuel drains much faster than expected, often causing me to fail to orbit.
And then usually I tend to enter a loop of MOAR FUEL -> TOO HEAVY -> MOAR ENGINES -> TOO FEW FUEL -> MOAR FUEL -> TOO HEAVY -> MOAR... .

Are there tipps for determining needed d/v, maybe also in relation to TWR for SSTOs?

[Foxster]

There's some tricky stuff in there like how much the payload weighs, how much dV you want left by orbit, how draggy the craft is because of the extras you have bolted on (e.g. RCS), etc. 

If there's an easy way to figure this out I don't know what it is. So, for me, it's still trial and error every time. 

But one big tip is to start small and keep it that way. Use the smallest possible capsule. Keep the number of stacks to the minimum. Don't over-wing or over-control. Use the slimmest parts (i.e. mk1).  Start with the minimum of engines to make the thing move and only very reluctantly add more. 

 

[mk1980]

i don't think there's an easy way to calculate that at all. far too many factors are in play.

assuming you build your SSTO with the top tier RAPIER engines, you can realistically get about 1600 m/s out of them in the super efficient air breathing mode, and they'll get yout to about 22+ km altitude in that mode. from there, you'll need something like ~1000 m/s in closed cycle mode to get to orbit (maybe a bit less if you're really good at that stuff).

it's hard to tell how much LF you'll consume during the air breathing phase, since that depends heavily on the drag of your plane and also to some extent on the thrust/weight ratio (you'll spend more if you packed too little thrust, but you'll also spend more fuel in closed cycle mode later if you packed *too many* engines) - so there's always a question of balancing 

you can probably make an informed decision by draining some of the LF out of your tanks and look at the closed cycle deltaV (ie. a rough simulation how much "rocket" dV you'll have at the end of the airbreathing part). if that number is too low ( < 1000 m/s) chances are that you won't make it up (or - even more embarassing - you won't have enough dV for the reetry burn )

a few things i've learned regarding planes (that may not be immediately obvious)

- it's not always helpful to add more fuel to get to orbit. there are cases where additional fuel will not improve the situation but actually make it *worse*. there is such a thing as too much fuel.

- there are also situations where too many (or too few) engines will make a design unsuccessful (as mentioned earlier). forget everything about thrust to weight ratio you learned from rockets. planes are different. if your plane has a TWR of > 1 on the runway. you have too many engines. 

- the ramjet engines (whiplash/rapier) and to a lesser extent also the panther afterburner engine have relatively low thrust on the runway ("static thrust"), but gain a lot of extra thrust once you accelerate a bit. a rapier can go from ~180 kN (?) static thrust all the way up to ~460 kN thrust at mach 3.7. it will start losing thrust beyond that point.

- aerodynamcis are extremely important. most of the energy built up to get to orbit comes from the airbreathing phase, and the jet mode only works in atmosphere. so unlike rockets, you can't just get out of the thick atmo ASAP to avoid most of the aero drag. you have to stay in the atmo long enough to build succicient speed to make it to orbit.

- wings provide lift, but also create drag. there's a fine balance between enough wings to take off and not fall down immediately and too many wings that will make your plane less efficient when you try to get it to mach 5+ during the ascent 

 

[Cairol]

Running out of oxidizer while carrying too much LF and/or too much engines is my most frequent problem.
I then start to switch LF tanks to O2 tanks and when I finally made it to orbit, I check my fuel and sometimes I can get rid of one or two tanks alltogether
for the next trips and subsequently even leave one engine at home.
But until I reach that optimizing loop, it's a lot of try and error for me to get that thing to orbit. And that's not what I'm used to in RL for solving my problems.
Sadly, it's much more complex than with a rocket. And that's why I only do space planes / SSTOs for short periods and then get frustrated and leave them alone for a while.
But I guess that's what makes space planes and especially SSTOs challenging - and also rewarding, if you nailed it at last.

Funny side-note: I really got crazy with my new SSTO today, because it was suddenly losing all engine thrust and all controls shortly after lift-off without any apparent reason.
I checked my fuel, I checked the intake air, I checked the prop. fulfillment meter, I checked if I had a flame-out. But all parameters were completely fine. But I was still losing power suddenly on every flight.
Turned out that I simply lost my RemoteTech signal, because I forgot to bring an antenna with me. The internal antenna of the probe core only has a 3 km range.
Normally I only forget my parachutes...

[Signo]

Well, I have some "numbers" I can share - they are all based on my experience and style, so they are not absolute at all - I use a very slow ascent profile: I pitch up 5° at take off and I keep it like that; pretty boring but quite effective to me.

I usually add at least 400U of LF per rapier, it is more than enough to manage my slow ascents and to save a few for a safe landing. 

Then I add a minimum 900/1100 LF+O supply. It is barely enough to get to LKO, if you want to improve the range you need to sacrifice the payload.

I managed to lift planes a few kilograms in excess of 25t (6 to 7t payload to LKO included) with a single rapier, 60t with a twin rapier (12t payload) and more than 130t (32t payload) with a quad engine craft. I do not have anything bigger but I suppose it is possible to exceed 275t if you make it a "okto".

Ram/nuke propelled SSTOs of the same weight have better range but worse payload, they are usually my choice to ferry kerbals around while I'd rather have a rapier propelled cargo.

Drag is the most important thing to take into account: a couple of exposed solars or RCS thrusters can really set back the overall performance - I try to hide any possible drag source inside a bay. RCS thrusters can work in very narrow and apparently closed environments. Control surfaces are a drag source too. Not to mention struts and fuel lines. Less is more.

I try to have a lifting surface of 10 per 30t of craft weight.

All my crafts share a few design tweaks regarding AoA - wings have got a +5° AoA while intakes get a -5°: the craft is supposed to be climbing with a 5° pitch, so the wings will have a relative AoA of +10 improving the lift and the intakes will have a perfect 0°: improving the engine feed will raise a little bit the effective ceiling in open cycle.

Do not forget that your craft is supposed to get back to KSC - there are some components that can handle heat better than others, in my opinion the best is the shielded docking port so it should be your preferred nosecone. Same for wings - FAT wings are pretty good at "goin' up" but they suck at "goin' down" due to their low temperature ceiling. Big-S wings can withstand a "worst case scenario" way better.

There are, of course, several alternative choices. I hope this can help.

 

 

Cheers and good luck.

Edited June 16, 2016 by Signo

[cantab]

It's not so clear cut. But generally I'd say you need about 1000 m/s of rocket delta-V, and then sufficient additional liquid fuel to reach 1400 m/s and 20 km on the jets. You also want enough rocket TWR that it greatly exceeds your drag. And ascent profiles matter a lot.

[AeroGav]

If you have the Big S shuttle wings unlocked then there is no need for any jet fuel tanks in the fuselage - the wings, air intakes, big s strakes, and even ncs adapters hold more than enough liquid fuel for the airbreathing side of things.

My own rule of thumb is one airbreather for every 30 tons of mass and one NERV for every 15 tons.   I actually find it hard to control excessively overpowered jets, when the engines get their surge in power as you reach supersonic speeds .  It all depends on your control method - if you've locked a specific pitch angle with SAS then i suppose the  worst that'll happen is you might overheat (unlikely ) or waste a little more jet fuel (big deal).  If you have the wings angled at 5 degrees incidence and are flying on prograde marker, or are flying with pitch trim however, so your AoA is constant and not able to change control input easily or quickly, then excessive acceleration after passing sound barrier , whilst still at fairly low altitude, creates a huge amount of lift and it can be difficult to keep the climb rate under control.   I can find myself broaching flame out altitude on the jets in a zoom climb, without having made a speedrun, or even gotten much over mach 2.

As for the closed cycle part of the climb,  remember it does not need a TWR > 1.     Rockets need TWR to avoid falling back to earth, but you have wings for that.   Hypersonic flight sees them at their worst, but even in this regime you  should be seeing a lift:drag ratio of at least 2.5 to 1.   So, for a spaceplane, a thrust ratio of 0.4 to 1 is equivalent to a TWR of 1 to 1 in a rocket - it's the minmum you need to keep from falling back down/loosing airspeed and staling.

Most people say 1.5 to 1 is a good TWR to aim for in a launch rocket, so it's equivalent would be 0.6 to 1 in a spaceplane  - means you have 50% more thrust than you need to merely counteract the drag that comes from generating the lift that stops you falling back to earth.

 

[Corona688]

In a sense it doesn't matter if your craft is a SSTO or not.  If it can hit 3000 m/s straight up from a standing start, it's got at least the oomph it needs to make orbit.  The spaceplane part makes it trickier, in that you can't just aim it straight up to wait and see.

Delta-V calculations for air-breathing engines may not be meaningful.  You're liable to spend minutes at a time all but coasting and your jets will mostly be operating at their worst possible ISP (either too thick or too thin air).  These uncertainties aren't going to make the numbers better, they'll make it a whole lot worse.

So I'd just calculate the delta v of the liquid fuelled part.  If your spaceplane can reach 1500m/s at 25,000m, how much more delta-V do you need to get to orbit from there?

[AeroGav]

32 minutes ago, Corona688 said:

 

Delta-V calculations for air-breathing engines may not be meaningful.  You're liable to spend minutes at a time all but coasting and your jets will mostly be operating at their worst possible ISP (either too thick or too thin air).  These uncertainties aren't going to make the numbers better, they'll make it a whole lot worse.

So I'd just calculate the delta v of the liquid fuelled part.  If your spaceplane can reach 1500m/s at 25,000m, how much more delta-V do you need to get to orbit from there?

Actually the ISP of all engines is the same in the stock game regardless of throttle setting or altitude/airspeed regeime in which they are being run.    Operate a RAPIER out of its comfort zone , too slow,  or too high (halves every 1.5km above 20km) , or even too fast (takes some doing but thrust falls off a cliff past mach 4.5) then it's thrust drops but fuel consumption falls proportionately.   Of course,   operating at low thrust levels like this does increase what i like to call "treading water" losses - the spaceplane equivalent of gravity losses, the proportion of your engine power that is gobbled up just hanging on to the speed and altitude of that you already have.  However, it is very unlikely that you'll run out of jet fuel before reaching flameout altitude however and if you do it's easy enough to bring more.  

There's very much a glass cieling on what you can achieve with the air breathing engines alone due to the altitude and velocity modifiers.  With one rapier and 25 tons of ship i can get to maybe 1300 m/s and 22km before excess power drops to zero.   Double the number of rapiers and you can reach 1350, 24km - the power is falling off so fast at this point of the envelope, you have to ask if it's worth the extra 2 ton mass of the engine that's being dragged up to orbit. 

The closed cycle part of the ascent is something you could theoretically calculate delta V s so yes you could go 

orbital velocity 2200 - air breathing limit 1300 = 900 dv

but you are probably going to suffer larger and more variable drag losses being deeper in the atmosphere.    Lift / drag ratio makes a significant impact.  Thrust weight ratio does too and a high TWR makes you less vulnerable to poor L/D, but then more engine mass is needed.

 

[Corona688]

3 minutes ago, AeroGav said:

...then it's thrust drops but fuel consumption falls proportionately.

Did that change?  When I was first playing with spaceplanes, I remember turning on a jet engine in vaccuum for the hell of it, and while it produced no thrust, it still gulped fuel.

Edited June 16, 2016 by Corona688

[AeroGav]

43 minutes ago, Corona688 said:

Did that change?  When I was first playing with spaceplanes, I remember turning on a jet engine in vaccuum for the hell of it, and while it produced no thrust, it still gulped fuel.

I don't know when exactly but sometime just before the game became payware.    There's some obsolete info on the wiki still - says that NERV engine has higher fuel consumption at sea level where it's ISP is only 130.  Nowadays the fuel  flow is the same vacuum and in atmo, but it only makes 13kn thrust at ground level  vs 60 kn in space.     When i first tried a plane with a NERV,  I was discouraged by how weak it seemed because it would barely move on the runway.  Of course, get it up to 10km and it's already giving 55kn, which is a whole other matter.

[Corona688]

14 minutes ago, AeroGav said:

Nowadays the fuel  flow is the same vacuum and in atmo, but it only makes 13kn thrust at ground level  vs 60 kn in space.

Specific impulse is thrust over flow rate.  If you get less thrust with the same flow, that's a lower ISP.

Edited June 16, 2016 by Corona688

[AeroGav]

26 minutes ago, Corona688 said:

Specific impulse is thrust over flow rate.  If you get less thrust with the same flow, that's a lower ISP.

yes, it used to  be constant thrust and flow varied, now it's more realistic constant FF but thrust varies.

However jet engines always give same isp, but max thrust is subject to a multiplier.

[Corona688]

6 minutes ago, AeroGav said:

However jet engines always give same isp, but max thrust is subject to a multiplier.

Oh, I see -- for jet engines specifically.  Thanks.

[Stoney3K]

Your flight profile also matters a LOT. If you spend too much time building horizontal speed and lose vertical speed in the process, it means you need to get that vertical speed (which kicks your apoapsis up) back using the rocket engines, or your aircraft will incinerate because of atmospheric heating. Spend too little time building horizontal speed and you will not have enough Delta-V to circularize because your orbit is too eccentric.

There is a "goldy locks" zone which has the optimum flight path angle but I have not been able to find it yet. I can usually get up to Mach 4 on my air breathing RAPIERs reaching 22km, but when I switch to rocket mode, my flight path is too shallow so I need to waste a lot of Delta-V to get my apoapsis out of the atmosphere.

Edited June 17, 2016 by Stoney3K

[AeroGav]

9 hours ago, Stoney3K said:

Your flight profile also matters a LOT. If you spend too much time building horizontal speed and lose vertical speed in the process, it means you need to get that vertical speed (which kicks your apoapsis up) back using the rocket engines, or your aircraft will incinerate because of atmospheric heating. Spend too little time building horizontal speed and you will not have enough Delta-V to circularize because your orbit is too eccentric.

There is a "goldy locks" zone which has the optimum flight path angle but I have not been able to find it yet. I can usually get up to Mach 4 on my air breathing RAPIERs reaching 22km, but when I switch to rocket mode, my flight path is too shallow so I need to waste a lot of Delta-V to get my apoapsis out of the atmosphere.

My craft have quite large wings and can generate enough lift rise naturally as speed goes up.   Eventually orbital effect starts taking over from lift.   If you have the right amount of wing there is no need to ever raise the nose more than 5 degrees above prograde to rise fast enough not to incinerate.   During the airbreathing speedrun i'm often holding the nose down

 

[Stoney3K]

11 hours ago, AeroGav said:

My craft have quite large wings and can generate enough lift rise naturally as speed goes up.   Eventually orbital effect starts taking over from lift.   If you have the right amount of wing there is no need to ever raise the nose more than 5 degrees above prograde to rise fast enough not to incinerate.   During the airbreathing speedrun i'm often holding the nose down.

I was talking about flight path angle, which means the direction your craft is going (the angle of the prograde vector). So I'm talking about the ratio between horizontal and vertical speed, if that is too low (surface prograde marker too close to the horizon), you won't raise your apoapsis, if it's too steep, you don't have enough horizontal velocity and you need to squander a lot of rocket fuel to circularize.

What you're posting about is the angle of attack, which is the difference between the prograde vector and your aircraft's nose attitude. I usually don't go far off prograde because I design my aircraft to have lift when flying nose forward.

[Signo]

You can go "orbital" with a 0° degree pitch - horizontal speed rules for planes.

[Cairol]

A lot of good information here.
I'll try to extract and transfer this knowledge to my space plane designs and ascent profile.
It's still tricky though, as it's seemingly impossible to give as exact numbers as it's possible with regular rocket ascents. Too many factors to cope with.
So I guess it's still a good chunk of try and error. Aka, "the kerbal way"...