Looking for specifics for a Laythe and back SSTO/spaceplane.

[panzer1b]

Ok, well after ive just about maxed out optimizing my Tri-Fighter lineup, im starting a newer build (that doesnt have any specific appearance requirements in dead set on), and would like to be able to get it to laythe, and back to kerbin, and land somewhere on kerbin (after landing on laythe) without any droptanks and being able to bring a 1 ton scout vehicle to laythe, without taking advantage of stuff like gravity slingshots ect (just a standard transfer at optimal angle). Id like to be able to pick where i land on laythe/kerbin as well, but it doesnt need to be on some odd orbit, just near equator is fine (so ill need a little bit more dV then what is the bare minimum to get to laythe and return).

Id like to know the following:

How much LF/O i should have total?

How much lift coefficient i should have in total?

How much intake area i should have total (or how many and what intakes)?

How many and what engines i should have?

How heavy should the total thing be about (including 1 ton of payload to laythe max)?

How much dV do i need for such a craft?

While designs that people have made already that are capable of what id like to do are appreciated, i enjoy building my own craft, so while i may borrow someone else's concept, i rarely actually use full craft made by someone else. That means, generic specs like how much fuel, what engines, how to best ascent profile, ect are what id prefer to use. Anyways, one hurdle ive come up with myself, is that for a true SSTO (no ditching anything), it is extremely difficult to get into orbit without bringing enough engine mass to just kill the vaccum dV down to below 5-6K (too low for a laythe and return without ditching parts or . While nukes are the best for interplanetary, they are heavy, and then having 1-3 extra jets is just killing the dV in space (and i actually need these engines once at laythe so even if i was willing to ditch parts, theyd have to stay). The vehicle (modified GAZ-49P) can fit inside a standard mk2 docking bay (the long version), so that also needs to be dragged into orbit.

Finally, i run STOCK game parts and stock aerodynamics, so ill obviously be using jets to achieve orbit, then nuke for the entire rest of the trip (im rather patient so a single nuke should be ok provided the whole thing isnt above 30 tons), unless this isnt actually the best method to get to laythe and back.

Edited December 13, 2014 by panzer1b

[bitslizer]

1) I think that's not the right question, should be how much DV, you need enough DV for LKO to Laythe transfer, aerocapture and chute landing for free, enough DV to carry the LF for the Laythe ascend and DV for the Laythe to Kerbin transfer (again aero capture at Kerbin for free). How much LF/Ox being a funciton of the mass of the craft vs DV requirement

2) any to none.... you can build a SSTO without wings to goto Laythe and back

3) my general rule of thumb for Kerbin SSTO is about 8 intakes per turbojet.

4) that depends on the size of your craft, if you build a craft that can SSTO in kerbin it should be able to SSTO in laythe without problems. I think cupcakes have build with as little as 1 turbojet and 1 rocket engine

5) again depending on your design preference it can be pretty small (relatively) to fairly big.

Edited December 12, 2014 by bitslizer

[numerobis]

(1) The round-trip requires a 3300 m/s deltaV from rockets. I pack about 4 km/s from turbojets for the landing and takeoff (and I assume a ballpark 10,000s Isp for turbojets).

(2) Ballpark, whatever lift coefficient has you lift off from the runway at X m/s will let you land on Laythe at 1,000m altitude at X m/s. That's because the air is thinner, but you burned a large fraction of your mass in fuel, and the two roughly cancel out.

(3) Your intake requirements at Laythe are dwarfed by those at Kerbin. For the same mass, you need half as many intakes on Laythe (because gravity is less) -- but you'll be much lighter because of the fuel you used. So just make sure you can reach orbit on Kerbin.

(4) Your thrust requirements at Laythe are dwarfed by those at Kerbin. For the same mass, you need 80% the jet thrust (because gravity is less) -- but etc etc.

Basically, build a plane that can lift off, and has 3300 m/s deltaV in orbit, and has a bit of jet fuel left. Then you can go to Laythe and back.

[panzer1b]

(1) The round-trip requires a 3300 m/s deltaV from rockets. I pack about 4 km/s from turbojets for the landing and takeoff (and I assume a ballpark 10,000s Isp for turbojets).

(2) Ballpark, whatever lift coefficient has you lift off from the runway at X m/s will let you land on Laythe at 1,000m altitude at X m/s. That's because the air is thinner, but you burned a large fraction of your mass in fuel, and the two roughly cancel out.

(3) Your intake requirements at Laythe are dwarfed by those at Kerbin. For the same mass, you need half as many intakes on Laythe (because gravity is less) -- but you'll be much lighter because of the fuel you used. So just make sure you can reach orbit on Kerbin.

(4) Your thrust requirements at Laythe are dwarfed by those at Kerbin. For the same mass, you need 80% the jet thrust (because gravity is less) -- but etc etc.

Basically, build a plane that can lift off, and has 3300 m/s deltaV in orbit, and has a bit of jet fuel left. Then you can go to Laythe and back.

i have a plane that has 4.5k dV in orbit after getting into LKO(not counting a little extra LF, like 50 or so of it).

so what am i doing wrong when it comes to approaches (i must be wasting fuel on inefficient burns ect or whatnot)?

Any tips on how to get there in the most efficient method possible?

I could also use some tips on setting up a proper aerobraking at laythe, as while i have done it many times, i seem to be using ALOT (200-400 dV) to even set up a trajectory thatll get close enough for aerobraking. Not to mention it takes me a lot of dV to get a laythe encounter in the 1st place once im in jool's soi

[Empiro]

How much delta-V are you using to get to Jool? It shouldn't take more than 2000 m/s, and if you're using more than that it means that you're probably not launching at the right transfer window. I'd recommend getting the Kerbal Alarm Clock mod which tells you the proper times to launch. If you're still having trouble, come back and we can give more specific advice.

The key to getting a good close encounter is to start early. Before you even enter the body's SOI, you can make small adjustments. Double click on the body to see your path within that SOI, and make small adjustments. Normal/Anti-Normal burns have a huge effect on the inclination, while pro/retrograde burns have a huge effect on the PE. The earlier the better, though if the adjustments are too sensitive, you can always wait until a bit later. I usually make a mid-course correction to get a good close encounter with Jool (at close to 0 inclination, and PE just outside the atmosphere), and then make a second correction as soon as I enter Jool's SOI to get a good close encounter with Laythe.

I like the precise node mod because it lets me tweak maneuver nodes really carefully.

Edited December 12, 2014 by Empiro

[panzer1b]

How much delta-V are you using to get to Jool? It shouldn't take more than 2000 m/s, and if you're using more than that it means that you're probably not launching at the right transfer window. I'd recommend getting the Kerbal Alarm Clock mod which tells you the proper times to launch. If you're still having trouble, come back and we can give more specific advice.

The key to getting a good close encounter is to start early. Before you even enter the body's SOI, you can make small adjustments. Double click on the body to see your path within that SOI, and make small adjustments. Normal/Anti-Normal burns have a huge effect on the inclination, while pro/retrograde burns have a huge effect on the PE. The earlier the better, though if the adjustments are too sensitive, you can always wait until a bit later. I usually make a mid-course correction to get a good close encounter with Jool (at close to 0 inclination, and PE just outside the atmosphere), and then make a second correction as soon as I enter Jool's SOI to get a good close encounter with Laythe.

I like the precise node mod because it lets me tweak maneuver nodes really carefully.

Most approaches use around 1900-2100 dV, depending on how many correction burns i do mid flight (if i get it perfect it takes 1900ish to get to jool SOI). This doesnt count anyhing when i get to jool system. So ofc intercepting laythe adds anywhere from 200-500 dV ontop of this (provided i can even get a nice intercept with laythe in the 1st place that doesnt require bouncing off another moon or something).

Also, when i get to jool, is it best to try to go straight for laythe? or is it better to aerobrake in jool atmo, and then try to get to laythe?

What can i do to minimize the dV needs once im at the jool system/returning from jool to kerbin?

[Empiro]

What is your PE at Jool? If you put a maneuver node down as soon as you enter Jool, it shouldn't be too costly to get the Laythe encounter. If you're finding it too expensive, you can even try to get the encounter before you enter Jool's SOI (just make sure not to warp right when you switch SOIs).

Another option is as you said, to try to Aerobrake and put your AP just past Laythe's orbit. You're guaranteed to eventually get an encounter with Laythe. Once you do, if you make the adjustments as early as possible before entering Laythe's SOI, it shouldn't take much delta-V to put yourself in Laythe's atmosphere.

I'm not sure what the best approach is to get back to Kerbin from Laythe. I would suspect that a Tylo gravity assist at just the right time would be ideal, but I think it would be hard to get everything set up just right. I think a straight burn from low Laythe orbit on a trajectory that puts you out of Jool's SOI and straight back to Kerbin would be both straightforward and fairly efficient because of Laythe's relatively high orbital velocity and gravity. However, my math still works out to about 2000 m/s to get back from Laythe. This means that I'd actually recommend 5000 m/s after taking off Kerbin to allow for getting to Laythe (~2000 m/s), taking off from Laythe (~700 m/s), getting back from Laythe (~2000 m/s), and also giving you some extra breathing room (~300 m/s) to make small mistakes and adjustments.

Edited December 12, 2014 by Empiro

[numerobis]

Most approaches use around 1900-2100 dV, depending on how many correction burns i do mid flight (if i get it perfect it takes 1900ish to get to jool SOI). This doesnt count anyhing when i get to jool system. So ofc intercepting laythe adds anywhere from 200-500 dV ontop of this (provided i can even get a nice intercept with laythe in the 1st place that doesnt require bouncing off another moon or something).

Your "of course" is odd. Are you waiting until you're in Jool's SOI to get an intercept? You should do that while in Kerbin's SOI, and then it'll just require a couple corrections (once when you exit Kerbin's SOI, another when you enter Jool's), with less than 10 m/s each time.

[numerobis]

Empiro: it's about 2100 m/s Kerbin-Laythe with plenty of margin for corrections, and a bit less than 1100 m/s for the return.

A couple hundred of those m/s you can do using jets, since orbital velocity at Laythe is less than 1850 m/s surface.

[panzer1b]

What is your PE at Jool? If you put a maneuver node down as soon as you enter Jool, it shouldn't be too costly to get the Laythe encounter. If you're finding it too expensive, you can even try to get the encounter before you enter Jool's SOI (just make sure not to warp right when you switch SOIs).

Another option is as you said, to try to Aerobrake and put your AP just past Laythe's orbit. You're guaranteed to eventually get an encounter with Laythe. Once you do, if you make the adjustments as early as possible before entering Laythe's SOI, it shouldn't take much delta-V to put yourself in Laythe's atmosphere.

I'm not sure what the best approach is to get back to Kerbin from Laythe. I would suspect that a Tylo gravity assist at just the right time would be ideal, but I think it would be hard to get everything set up just right. I think a straight burn from low Laythe orbit on a trajectory that puts you out of Jool's SOI and straight back to Kerbin would be both straightforward and fairly efficient because of Laythe's relatively high orbital velocity and gravity. However, my math still works out to about 2000 m/s to get back from Laythe. This means that I'd actually recommend 5000 m/s after taking off Kerbin to allow for getting to Laythe (~2000 m/s), taking off from Laythe (~700 m/s), getting back from Laythe (~2000 m/s), and also giving you some extra breathing room (~300 m/s) to make small mistakes and adjustments.

My PE at jool is very close to near laythe's (i usually try to grab it on the way there straight up), but i can easily go lower/higher if needed. If its way off, i usually try to get a gravity assist off of a moon (if im gonna have to do a braking burn, might as well do it in a gravity well).

So if its 5K dV, i could easily get that much with droptanks on my current ssto build, hopefully being enough for a proper laythe expedition.

Empiro: it's about 2100 m/s Kerbin-Laythe with plenty of margin for corrections, and a bit less than 1100 m/s for the return.

A couple hundred of those m/s you can do using jets, since orbital velocity at Laythe is less than 1850 m/s surface.

Ok, can i get an explatation of why it takes about 1/2 the dV to get to kerbin vs leaving kerbin?

by these calcs, counting an extra 300dV for corrections, i could THEORETICALLY make a round trip under 3500dV, not counting laythe landing/takeoff (landing is next to none as i can choose to use parachutes or even glide down on wings). Is this true, or does it take above 4K dV to do both ways (not counting landing) like the previous post?

Finally, are there any ways to optimize the laythe expedition in terms of gravity assists ect? Is it extremely hard to set up a gravity assist? or is it an actually viable method of travel for someone with moderate interplanetary skills?

Also, i just checked with MJ, my craft gets to kerbin LKO of 80ish km circle with 4982 dV left on the rockets. Im probably going to add jet droptanks, as well it would help ALOT if my tansk had max oxy capacity

Edited December 13, 2014 by panzer1b

[numerobis]

I did an exhaustive calculation of two paths to do the return here. Cantab corroborated it by looking at delta-V tables.

There's basically two reasons that the paths aren't symmetric:

(a) when doing a Hohmann transfer, it's normal for the two burns to have different requirements. For example, LKO to the altitude of Mun is 850 up, then 375 to circularize up there -- and the return is 375 up there, and 850 to circularize at LKO. The return, however, you can use the atmosphere for aerobraking. That means it's 1200 m/s to get to Mun's altitude and circularize there, and less than 400 m/s to return! For the Kerbin-Jool trip (or Kerbin-Laythe, same thing), you can aerobrake at both ends.

( The Oberth effect makes a big difference. To leave Jool's orbit you'd need 1500 m/s. But since you're leaving from an orbit around Jool at 27000km, you already have some speed, which you can use. And in fact, since you're in an orbit about Laythe at 55km, you've got that much more: at the appointed time you're going around the sun at a few km/s faster than Jool is. You are also deep in their gravity wells, which you have to fight your way out of, but overall, instead of 1500 m/s you have under 1100 m/s to burn.

Gravity assists can save you about 1km/s on the way up. The path is Mun, Eve, Kerbin, Kerbin again, and that last one bounces you out to Jool. It's about 1km/s to get to Eve, the rest is small corrections. It's pretty much exactly the same as how the Galileo probe got to Jupiter IRL. PLAD has pulled it off for just 1011 m/s from LKO to Jool.

You could save a few hundred on the way back from Laythe, too, by bouncing around the Joolian satellites. I think the lower bound on that is a 650 m/s return -- that's how much it costs just to get to Tylo -- but maybe you could do even better somehow with an assist off Laythe itself? Still, we're really splitting hairs at that point.

With jets boosting you 100 m/s beyond orbital speed at Kerbin, PLAD's trajectory, aerobrake directly into Laythe, and jets providing an extra 300 m/s beyond orbital speed at Laythe for the return, you could, theoretically, make the return trip with just over 1700 m/s. Good luck

Edited December 13, 2014 by numerobis

[Empiro]

Empiro: it's about 2100 m/s Kerbin-Laythe with plenty of margin for corrections, and a bit less than 1100 m/s for the return.

A couple hundred of those m/s you can do using jets, since orbital velocity at Laythe is less than 1850 m/s surface.

Yeah I made a mistake in my calculations (by assuming that you burn out of Laythe and then burn out of Jool, which is extremely inefficient), but my numbers still come out to around 1300 m/s. I'll probably just run a test tonight though.

I guess how much you can get on jets depends on which mods you're using -- I always play with FAR, so you can't get as fast on Turbojets / Rapiers as you can in stock aerodynamics.

Still, it does mean that 4500 m/s is plenty to get back, and I think that you'd gain very little from using any sort of gravity assists (versus the amount of work you have to put in).

Panzer, the reason why it takes less delta-V to get back is because of the Oberth effect. Laythe is moving very fast around Jool (3220 m/s), in addition you're moving very fast around Laythe (1800 m/s, though you had to pay that cost using your jet and rocket engines to get into orbit around Laythe). Combined, it means that even though you have to pay to first escape Laythe, the cost is much lower overall due to the savings from the Oberth effect. Edit: mumerobis' explanation is more complete than mine since he's correct that the two halves of the transfer orbit are asymmetric.

Edited December 13, 2014 by Empiro

[GoSlash27]

The original question never specified how much payload we are talking about here, and that's the single most important thing.

Also a disclaimer, I have zero experience at making "legit" Kerbin to Laythe SSTO spaceplanes. Just not something I ever personally had a need for. So whatever I propose should be evaluated with that fact in mind...

I do have a good amount of experience at making efficient SSTO spaceplanes that are multi- hop capable and LV-N mass movers, which is really what we're talking about here.

The requirement is for an SSTO spaceplane that can make 2 atmospheric burns to orbit from fuel alone with reserve and can carry a single LV-N with enough fuel/ oxidizer to make 5KM/sec DV in space.

This is, IMO, easily do-able.

A twin engine dual hop SSTO would have 2 turbojets, 1200 units of fuel, 28 "strake" wing panels, and would have the capacity to lift 9 tonnes of payload to orbit with it.

That 9 tonnes would be the LV-N and 6.75 tonnes of fuel/ oxidizer/ tanks, which is way more than you'd need to move 25 tonnes of mass that much at a vacuum Isp of 800.

So my preliminary design would be:

1 Mk 2 cockpit

3 short LF+O Mk2 sections

1 LV-N

4 Mk-1 jet fuel fuselage

2 turbojet

4 XM-G50 radial scoop intakes

2 shock cone intakes

28 "strake" wing panels

Other assorted/ massless parts as necessary.

Best,

-Slashy

[numerobis]

A few minor improvements on Slashy's proposed plane:

• Instead of 3 short LF+O tanks, use 3 Mk2 adapters, or 1 adapter up front, and two Mk2 bicouplers on the back (and stick the turbojets on those). Same amount of fuel, same drag and lift, but reduces the dry mass of those tanks from 750kg to 600kg. You can reduce it even more by using a T800 and a T100, but then you get a bit more drag and a bit less lift, and it's only 37.5kg less.
  
• Use 2 Mk2 short liquidfuel tanks rather than 4 Mk1 tanks. Same amount of fuel, but less dry mass (500 kg rather than 600 kg), less drag, more lift.
  
• Use 5 ram air intakes rather than 4 radial scoops and 2 shock cones. Slightly more intake area, 40 kg less.
  

Now you have 290kg to spend on more fun endeavours. That's enough for 4 rover wheels, with mass left over! Or you could just take the fuel savings to the bank.

[panzer1b]

My older "HK-303 Tri-Fighter V" on its 1st successful to laythe and back expedition

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This is what made laythe and back, ofc with barely any fuel left at the end, but it is ratrher impressive something that began as a short range intereptor could do. Its such a good craft, long range, very good TWR (provided ur willing to gun rapiers in rocket mode and cut down fuel economy), ok aerodynamics (a little unstable when empty, but nothing the reaction wheels/wings cant deal with), can carry missiles (i didnt bring for laythe trip as its at least .5 tons extra for 2 of em), and is in my opinuion rather good looking.

Ofc it cant carry cargo, and attempts at making cargo variants of the HK-303 didnt go so well, so well, making a 100% new cargo SSTO which can at teh bare minimum carry 2 tons to laythe. Hopefully itll be a success.

The original question never specified how much payload we are talking about here, and that's the single most important thing.

Also a disclaimer, I have zero experience at making "legit" Kerbin to Laythe SSTO spaceplanes. Just not something I ever personally had a need for. So whatever I propose should be evaluated with that fact in mind...

I do have a good amount of experience at making efficient SSTO spaceplanes that are multi- hop capable and LV-N mass movers, which is really what we're talking about here.

The requirement is for an SSTO spaceplane that can make 2 atmospheric burns to orbit from fuel alone with reserve and can carry a single LV-N with enough fuel/ oxidizer to make 5KM/sec DV in space.

This is, IMO, easily do-able.

A twin engine dual hop SSTO would have 2 turbojets, 1200 units of fuel, 28 "strake" wing panels, and would have the capacity to lift 9 tonnes of payload to orbit with it.

That 9 tonnes would be the LV-N and 6.75 tonnes of fuel/ oxidizer/ tanks, which is way more than you'd need to move 25 tonnes of mass that much at a vacuum Isp of 800.

So my preliminary design would be:

1 Mk 2 cockpit

3 short LF+O Mk2 sections

1 LV-N

4 Mk-1 jet fuel fuselage

2 turbojet

4 XM-G50 radial scoop intakes

2 shock cone intakes

28 "strake" wing panels

Other assorted/ massless parts as necessary.

Best,

-Slashy

Ill try it, might make a good way to lift to LKO, then ofc move to another interplanetary SSTO like the one im working on.

Also, why the strake wings, is there a particular reason they are better (other then looks which is rather opinionated)?

ANyways, thanks for all the great tips guys, i finally achieved what i felt was impossible with a craft below 6K dV!

Edited December 13, 2014 by panzer1b

[numerobis]

The strakes have the best lift:mass ratio of all the standard wings, and they also have anomalously low drag.

You can do better with the delta-deluxe, but that one acts a bit oddly.

[numerobis]

BTW, my personal view is that each Kerbal on an interplanetary trip should have their own crew cabin, if not two of them. Gotta give them room to stretch out their legs, have a warm meal, play cards...