Heavy Lift SSTO Delta-v

[HeliosPh0enix]

I wanted to design a heavy SSTO that carries a large amount of crew and has an ISRU with drills in the back for refueling.  Without any fuel or engines, it weighs around 70 tons.  I recently watched a video by Mark Thrimm that gave multipliers that you could use to determine how much liquid fuel, rocket fuel, and RAPIERS you need based on your dry mass.  I would use those multipliers for this purpose, but they only give enough fuel to get to LKO and then be able to de-orbit.  I want this SSTO to be able to, at the very least, be able to get anywhere in Kerbin's SOI, and potentially to Duna, which requires significantly more delta-v than LKO and de-orbiting.  Does anyone have different multipliers to use, or at the very least equations that I can plug stuff into in order to find how much fuel and engines I need?  On the other hand, does anyone have any better methods or comprehensive tutorials they could point me to?  Just getting started into SSTOs and am still trying to figure it out.  Any help is appreciated!
Note:  I have KER if there is anything there that can help with calculations, just point me to the right submenus and clicky things.  Thanks!

[ElWanderer]

Once you're in space, you don't need to do any spaceplane-specific calculations (unless visiting Laythe!) Find out how much delta-v it will take to get where you want to go from LKO from a delta-v map, then use the rocket equation to work out how much fuel you'll need for your dry mass of 70t. Add some for luck. Then add the mass of that to the dry mass you're plugging into your SSTO equations.

e.g. if you need 50t of fuel to get to Duna's surface (or to get back after refueling), you effectively have a spaceplane with a dry mass of 120t to get into orbit of Kerbin.

This changes if you can refuel in LKO, as then some of the fuel capacity used to get into orbit can be reused to go places in space.

Edit: ah, wait. Adding engines and fuel tanks will increase your dry mass, reducing the delta-v you have once you reach orbit, so you would need to iterate adding extra fuel to account for the extra mass, then check if you need extra engines/fuel to get into orbit, and so on until the difference is negligible.

Edited December 7, 2017 by ElWanderer

[FleshJeb]

You can use Mark's numbers as a starting point.

This will be an iterative process:

Find out how much delta-V you want to have remaining in orbit.

Use the Rocket Equation to figure out how much more fuel it would take to add that dV.

Add this amount of fuel to your "dry mass".

Run it through Mark's rules of thumb again.

Repeat process.

The craft will keep getting bigger, but it should eventually converge to a solution.

[AeroGav]

 I have many interplanetary SSTO on my KerbalX page, though none are an exact match for your spec.   There's a small self refuelling SSTO (mk1, five seat) called Starsailor and a big liner called Lusitania.   The Stretch Kerboliner is a large mining passenger craft but it was built some time ago , i don't know if it still works.

My rule of thumb is one RAPIER and one Panther per 50 tons or so of launch mass,  and one nuke per 20 tons.     If you follow low drag design principles and fly the right profile, that is sufficient engine to get to orbit with low drag/gravity losses.    Once in orbit,  I check the remaining delta V readout on Kerbal engineer.  

You want 2k delta V leftover in low kerbin orbit to be able to comfortably land on Minmus and refuel.   If you got 3k or more, you can straight shot Duna and brim the tanks there.

If it is able to get to orbit comfortably on the relatively low number of engines i am suggesting, but does not have enough delta v left,  then there is just too much payload for a plane of that size, either make it larger or reduce the number of passenger cabins.   Since it is able to get to space easily on that number of engines, efficiency is probably already close to optimum.

 

I wrote a tutorial covering drag reduction and flight profiles amongst other things - 

 

 

[bewing]

Making a flyable spaceplane is hard. Designing it to SSTO to LKO is very hard. Making it get to Minmus on its first tank of fuel requires heroic levels of SSTO design skillz. 

You speak very casually of making a spaceplane that can get from the runway to Duna and refine its own fuel and then go anywhere it pleases afterward. It's not just a question of scaling up.

 

 

 

Edited December 9, 2017 by bewing

[Wanderfound]

See here for an interplanetary Mk3 demonstrator:

https://kerbalx.com/Wanderfound/Kerbotruck-Interplanetary

 

 

It'll easily do KSC to Minmus direct, and can hop from there to almost anywhere.

If you wanted more passengers on that, you'd just dump the lab, shorten the cargo bay and add a passenger cabin. The seaplane floats and hydrofoils are optional, but useful on Laythe.

 

Basic Mk3 interplanetary tricks:

* Streamline. Wing incidence to keep the nose on prograde, and put all of the clutter into the cargo bay.

* Engine balance. Enough RAPIERs to get in the air and up to speed, but no more. Enough nukes to get the transfers done, but no more.

* Fuel balance. Enough oxidiser to lift your apoapsis; everything else devoted to LF apart from a minimal ore capacity. Circularise on your nukes.

Edited December 12, 2017 by Wanderfound

[AeroGav]

The thing is , if you can build an interplanetary SSTO, why does it need mk3 cargo bays to carry stations and rovers?    Why not just have your Kerbals live on the plane?   It is more mobile than a rover.

[AeroGav]

Got some more data points for you - my liquid fuel only payload to orbit fraction entry had a fuel fraction of 22%.      Thus, this is close to the minimum fuel fraction you can have and get to orbit.

Voodoo Ray,  a Panther-Nerv liquid only SSTO, has a 45% fuel fraction and makes orbit with 2k delta V left.   Because the airbreathing engine only gets it up to 14km and 750m/s, the flight to orbit uses more fuel than on a RAPIER powered craft - one of mine has similar delta V in orbit, but only has a 39% fuel fraction on account of reaching 1350 at 22km air breathing.

[Gordon Fecyk]

On ‎12‎/‎9‎/‎2017 at 11:55 AM, bewing said:

Making a flyable spaceplane is hard. Designing it to SSTO to LKO is very hard. Making it get to Minmus on its first tank of fuel requires heroic levels of SSTO design skillz.

Introducing some Skillz:

I modified this design to turn the short cargo bay upside down, move its orange tank payload back to the long cargo bay, then add a small ISRU, one radial tank and one large drill to the small cargo bay. Finally added some radiators and solar panels to the inside for power and cooling. Can barely, but safely, land on Minmus with a little bit of a safety margin.

I'll have to dig up that particular version and post a link to it when I get home today. Don't know how well it would fly in stock aero, but it doesn't absolutely need Ferram Aerospace.

Here's a Zip with two versions of this craft:

• The FAR version uses the elevons as air brakes (uses the Brakes action group) and seems to pick up speed much easier in FAR aero. It seems to need less liquid fuel so it can haul up more oxidizer and get more dV in LKO as a result. It uses the nose canards as flaps for ascent assistance.
• The Stock version doesn't need air brakes so much, but it needs a lot more liquid fuel and considerably less oxidizer because it loiters a lot longer in the atmosphere. At times I had to toggle the rockets to exceed Mach 1, but once I reached 12 km up it was easier to pick up speed. Maybe a slight wing incidence would help.
• Both versions can get to 100 km LKO with just under 1300 m/s left, which is enough to get to Minmus and land if you are careful. Once landed you can mine and refuel. 

It's possible to save considerable amounts of mass by removing the hitchhiker can and that relay satellite that's loaded in the bay.

This craft's original mission was to take 40 tonnes to 100 km LKO and return safely, but it's proven rather versatile with different payloads. You could fit docking ports, EVA struts, landers, and other stuff for retrieving things from LKO, launching landers to the Mun and Minmus, launching relays, and I even took a bunch of tourists to the periphery of Kerbin's SOI on it.

Edited December 13, 2017 by Gordon Fecyk

[AeroGav]

I would love to install Ferram again, but someone comes along with a stock challenge/stock craft needing fix before i finish building any craft and i got to uninstall once more

[Gordon Fecyk]

21 minutes ago, AeroGav said:

I would love to install Ferram again, but someone comes along with a stock challenge/stock craft needing fix before i finish building any craft and i got to uninstall once more.

This is why I keep more than one KSP installation around. As long as they're on the same computer I think it's OK, EULA-wise.

The Fat-Star should fly in stock aero, and probably have an easier time climbing at lower speed, what with all of the wing span compared to your Steven-Tylo craft. Now the Steven-Tylo? There is some skillz.

Edited December 12, 2017 by Gordon Fecyk

[AeroGav]

1 minute ago, Gordon Fecyk said:

This is why I keep more than one KSP installation around. As long as they're on the same computer I think it's OK, EULA-wise.

The Fat-Star should fly in stock aero, and probably have an easier time climbing at lower speed, what with all of the wing span compared to your Steven-Tylo craft. Now there is some skillz.

Did you see Steven Tylo land on the Island airbase at the end of its mission to Jool (stock aero).    I'm guessing it comes in a little hotter on FAR

 

[Spricigo]

3 minutes ago, Gordon Fecyk said:

This is why I keep more than one KSP installation around. As long as they're on the same computer I think it's OK, EULA-wise.

Like any sane people, I just scrolled down and hit accept.

But if I'm Steam let me install KSP on how many machines I want. So, maybe Squad don't mind multiple installs as long as we don't start to give away or resell KSP copies.

Notice, I'm not suggesting that people use it as a 'creative way' to play KSP without paying for it*, but if I'm not mistaken steam even allow sharing KSP: I can let my steam freinds play KSP while I'm not playing.

 

*And for anyone trying to be 'creative': You are a selfish idiot.

[Gordon Fecyk]

3 hours ago, Spricigo said:

But if I'm Steam let me install KSP on how many machines I want. So, maybe Squad don't mind multiple installs as long as we don't start to give away or resell KSP copies.

I'm digressing from the OP's topic, but I won't stray for long, promise.

If you're a Steam user, you can copy the folder %programfiles%\Steam\steamapps\common\Kerbal Space Program to other locations and launch ksp.exe or ksp_x64.exe from there. You can even add command line options by making shortcuts to these and editing the shortcuts. The user launching those needs read/write access to those folders; Modify permissions in Windows is sufficient and you don't need local Administrator access or privilege elevation. Then you can have a stock install, an install with one set of add-ons, another with a different set of add-ons, and so on.

Please use this responsibly. I believe multiple copies on one computer is reasonable, but copies on multiple computers is not without buying extra licenses.

[HeliosPh0enix]

23 hours ago, Wanderfound said:

See here for an interplanetary Mk3 demonstrator:

https://kerbalx.com/Wanderfound/Kerbotruck-Interplanetary

 

 

It'll easily do KSC to Minmus direct, and can hop from there to almost anywhere.

If you wanted more passengers on that, you'd just dump the lab, shorten the cargo bay and add a passenger cabin. The seaplane floats and hydrofoils are optional, but useful on Laythe.

 

Basic Mk3 interplanetary tricks:

* Streamline. Wing incidence to keep the nose on prograde, and put all of the clutter into the cargo bay.

* Engine balance. Enough RAPIERs to get in the air and up to speed, but no more. Enough nukes to get the transfers done, but no more.

* Fuel balance. Enough oxidiser to lift your apoapsis; everything else devoted to LF apart from a minimal ore capacity. Circularise on your nukes.

If you wanted to lift a large cargo load (taking up the whole cargo bay) into LKO, then you could just take away the refinery gear and lab, correct?  It wouldn't have the ability to land and refuel itself, but orbital refueling stations work too.

If those little stubby wings provide enough lift for that to fly, then I have been seriously overestimating the amount of lift that functioning spaceplanes need.  My wings are always massive compared to those.  Is there a (relatively) simple way to find out how big your wings need to be with stock KSP atmosphere?

[Wanderfound]

1 hour ago, HeliosPh0enix said:

If you wanted to lift a large cargo load (taking up the whole cargo bay) into LKO, then you could just take away the refinery gear and lab, correct?  It wouldn't have the ability to land and refuel itself, but orbital refueling stations work too.

Yup.

https://kerbalx.com/Wanderfound/Kerbotruck-Compact-Lander

That design can easily handle 40 ton payloads. Add a couple more engines and it'll do 60+.

 

1 hour ago, HeliosPh0enix said:

If those little stubby wings provide enough lift for that to fly, then I have been seriously overestimating the amount of lift that functioning spaceplanes need.  My wings are always massive compared to those.  Is there a (relatively) simple way to find out how big your wings need to be with stock KSP atmosphere?

Note that the canards, tailplane and strakes add substantial lift as well. And the wing incidence is important to avoid fuselage drag.

My designs tend towards minimalist wings, but that isn't the only way to go. See this thread:

 

Edited December 13, 2017 by Wanderfound

[AeroGav]

12 hours ago, HeliosPh0enix said:

 

If those little stubby wings provide enough lift for that to fly, then I have been seriously overestimating the amount of lift that functioning spaceplanes need.  My wings are always massive compared to those.  Is there a (relatively) simple way to find out how big your wings need to be with stock KSP atmosphere?

You can certainly takeoff with even smaller wings than Wanderfound is using, with the correct design.

A vast majority of the drag comes from the fuselage - as he says, angling the wings up where they attach to the fuselage, enables you to keep the fuselage closer to prograde and thus reduce its drag.     However,  more wings still benefits, because you can reduce angle of attack even more (all the way to prograde hold) and at that point, adding more wings allows you to fly higher for any given airspeed, where drag is less.      But, beyond a point, this can take you above the altitude where your air breathing engines can make power.

Once you switch to rocket power,  you're back into a situation where the higher you can be without increasing your Angle of Attack, the better.   So more wings = less drag.  But,  on a chemical powered lifter like Wanderfound's, this is not very important.    Each Rapier engine makes 180kn,   he has four of them,  so whether the plane has 100 of 50kn drag is not going to hurt things.

If all your rocket mode thrust is coming from NERVs,  it changes.    Each nerv weighs 3 tons and only makes 60kn,  and you still need jet engines for the air breathing phase.

It is easier to add wings to reduce drag , than extra NERVs.

For example, four big S wing strakes hold the same fuel as one mk1 liquid fuel fuselage (400 units).       The four strakes have a dry mass of 400kg instead of 250kg, so that's 150kg additional mass to use the wing parts.

But,  you'd have to swap twenty mk1 liquid fuel fuselages over to eighty big S wing strakes before you'd add 3000kg dry mass to your craft - equivalent to the mass of one extra NERV.

Taken to the logical conclusion, you end up with monstrosities like this (43.7% payload fraction , later i added one more nerv , even more strakes, and got it to 50%)

or this SSTOliner

Every godamned square inch of fuselage for revenue passengers.  No room for oxidizer, galleys or lavatories (wear nappies or do it out the airlock) on my airline!

Again with an oxidizer SSTO,  math changes.    Wings don't hold rocket fuel,  they are just dead weight.   

Fuselage tanks have lower mass per unit of fuel carried which is what matters when you're guzzling gas at 305 ISP  and drag is less than 10% of your thrust on that final blaze of glory.

Wing tanks have more mass but lower drag per unit of fuel capacity, and they enable you to fly a more lofted, lower drag profile, which matters when your dry mass is going to suck no matter what thanks to the nerv engines,  and a 50% drag reduction is the difference between not accelerating at all and making orbit.