Tylo lander help !

[AeroGav]

Well, this is no good.       I've just spent the past few hours making Kerbal marmalade.

https://kerbalx.com/AeroGav/Tylo-impactor

 

I'm supposed to be planning a Jool 5 mission.      The giant spaceplane, with two CRG-100 bays is looking good, it will go direct from Kerbin and splash down on Laythe.    The IRSU gear can then refuel us, and the lander deployed to see the other moons.

I wanted to bring a hab, cupola and lander can to every body to keep a "deluxe" feel to this mission.  I decided to move the IRSU gear off the plane to the lander since the return trip costs to Tylo are so frightening.   I decided we'd visit this place last and decoupe the converter here, to make the ascent a little easier.   There's docking ports between the crewed bit and the rockomax tank, after meeting up with the plane, we can undock the tank/engine bit and this frees up a port on the lander to dock with the port inside the plane's cargo bay.    That way, Kerbals got some hab space for the way back as well as on the way out.

According to the wiki, need 2k dV to land on Tylo and this has 2800.   TWR is over unity as well, but it can't do it.  Crashed into surface at   400m/s with fuel about to run out.

Any fixes need to not make it wider, since we got to fit in a mk3 bay.   We can add length easily enough, but  I'd prefer not to topple over on landing.   I'm thinking maybe a couple of "boosters" to attach to the top of the stack, sparks or something?

Poodle    Vac TWR  14    ISP 350

Dart         Vac TWR  18    ISP 340

Spark      Vac TWR  18   ISP 320

 

Edited January 10, 2017 by AeroGav

[GoSlash27]

AeroGav,

You can shave a couple tonnes off of that by replacing the Poodle with an Aerospike. It'll have plenty of thrust, and you'll be able to add more fuel for additional DV without overloading it.

Conversely, you can use what you have and perform a zero descent rate landing. You have plenty of DV for that approach.

Best,
-Slashy

*edit* wait... that looks fishy. No way that thing is only 2 tonnes dry. I think MJ is misleading you on those DV numbers.

 

[Foxster]

Can you post a craft file?

Some early thoughts though...

You generally need more than 2000dV for Tylo. I'd budget 2500+.

A TWR of just >1 isn't really going to cut it. You need to be able to decelerate, not just maintain speed. Try for 1.5+.

I think you should be able to get the three capsules down and back without needing an ISRU. Let me have a play...  

Edited January 10, 2017 by Foxster

[AeroGav]

11 minutes ago, Foxster said:

Can you post a craft file?

Some early thoughts though...

You generally need more than 2000dV for Tylo. I'd budget 2500+.

A TWR of just >1 isn't really going to cut it. You need to be able to decelerate, not just maintain speed. Try for 1.5+.

I think you should be able to get the three capsules down and back without needing an ISRU. Let me have a play...  

https://kerbalx.com/AeroGav/Tylo-impactor

Added link to this in OP ! 

34 minutes ago, GoSlash27 said:

No way that thing is only 2 tonnes dry. I think MJ is misleading you on those DV numbers.

 

Has to be - I made a new version with a drop tank and 4 sparks bound to an action group.    This managed to arrest the vertical velocity but we were still going 200m/s sideways when the fuel ran out.

40 tons gross wt.   of which 24 are fuel.

17 minutes ago, Foxster said:

 

I think you should be able to get the three capsules down and back without needing an ISRU. Let me have a play...  

The mission architecture requires one anyway, so i figured put it on the lander and ditch it before takeoff, since we won't need it again.

[Rhomphaia]

1 hour ago, GoSlash27 said:

AeroGav,

You can shave a couple tonnes off of that by replacing the Poodle with an Aerospike. It'll have plenty of thrust, and you'll be able to add more fuel for additional DV without overloading it.

Conversely, you can use what you have and perform a zero descent rate landing. You have plenty of DV for that approach.

Best,
-Slashy

*edit* wait... that looks fishy. No way that thing is only 2 tonnes dry. I think MJ is misleading you on those DV numbers.

 

The 2 tonnes looks to be what is north of the decoupler, It is indeed throwing off KERs readouts, since it is calculating the DV without it, should be 2550ish

TWR is also off by a bit.

 

MechJeb seems to handle the staging better than KER on this boilerplate (the Ore and monoprop tanks are partially filled for ballast) and matches manual calculation

Spoiler

 

Edited January 10, 2017 by Rhomphaia

[Spricigo]

2 hours ago, GoSlash27 said:

zero descent rate landing.

sorry for the ignorance, but what exactly it means?

[GoSlash27]

Aerogav,

 According to my numbers, you need a heck of a lot more than that to make 2km/ sec DV on Tylo.

Assuming a payload of 14t and DV budget of 2km/ sec, My spreadsheet recommends a Skipper with an X200-32 plus an x200-8. If you fill up that last tank, it gives 2200 m/sec with a total mass of 39.3 tonnes.

 There are lighter solutions for higher efficiency engines, but they'd require multiple engines to do it.

HTHs,
-Slashy

15 minutes ago, Spricigo said:

sorry for the ignorance, but what exactly it means?

 Spricigo,

 It's basically a reverse Hohmann transfer. You set Pe to just above the surface, then brake at Pe to arrest your horizontal velocity without climbing or descending. It's the most fuel efficient method.

Best,
-Slashy

Edited January 10, 2017 by GoSlash27

[AeroGav]

1 hour ago, GoSlash27 said:

 

 Spricigo,

 It's basically a reverse Hohmann transfer. You set Pe to just above the surface, then brake at Pe to arrest your horizontal velocity without climbing or descending. It's the most fuel efficient method.

Best,
-Slashy

What should the AP be, or does this not matter?

 

1 hour ago, GoSlash27 said:

Aerogav,

 According to my numbers, you need a heck of a lot more than that to make 2km/ sec DV on Tylo.

Assuming a payload of 14t and DV budget of 2km/ sec, My spreadsheet recommends a Skipper with an X200-32 plus an x200-8. If you fill up that last tank, it gives 2200 m/sec with a total mass of 39.3 tonnes.

 There are lighter solutions for higher efficiency engines, but they'd require multiple engines to do it.

HTHs,
-Slashy

So, how much thrust do i need, what's the optimum TWR? 

If the lander weighs 16T empty, and Tylo has a grav of 7.85 , that's 125kn of gravity.   At 250kn the Poodle ought to be enough, and it's nice and efficient (350isp), which is why i chose it.  In practice, we could clearly do with more power and more fuel - twin aerospikes? triple aerospikes?

[GoSlash27]

AeroGav, the lower the better for DV, but the quoted figure is for a 10km altitude. Incidentally, it's 2,270 m/sec from there, so you'd best pack some extra.

Best,
-Slashy

[AeroGav]

50 minutes ago, GoSlash27 said:

AeroGav, the lower the better for DV, but the quoted figure is for a 10km altitude. Incidentally, it's 2,270 m/sec from there, so you'd best pack some extra.

Best,
-Slashy

OK, i made it with the zero descent profile you described - 40 units of fuel left !

That was with an extra x200-8 attached to the top of the vehicle via decoupler,  which had four chains of oscar b's hanging below it.  Made it look like the thing was wearing a Swagman's hat, but i digress.  For extra power i had four twitches clustered round the bottom set on an action group.

Just about managed to not topple over.  Discovered that the ladders don't work - when the kerbal moves from one set to the next, he looses his grip and falls.    Fortunately he survived, but when trying to climb up the bottom set, i found out that one of those twitch engines blocks him from getting above the fourth rung.     Good to find these things out in a simulation eh?

The IRSU worked as advertised and it was fun to watch the separatrons lift it clear and sideways then smash it into the ground.  Getting back to orbit was tight as well - we were free of the irsu, but we'd also lost our drop tank.    V. low orbit !

 

 

[GoSlash27]

59 minutes ago, AeroGav said:

So, how much thrust do i need, what's the optimum TWR? 

If the lander weighs 16T empty, and Tylo has a grav of 7.85 , that's 125kn of gravity.   At 250kn the Poodle ought to be enough, and it's nice and efficient (350isp), which is why i chose it.  In practice, we could clearly do with more power and more fuel - twin aerospikes? triple aerospikes?

I assumed that your payload was 14 tonnes. Based on that, Tylo's gravity, and the DV budget, my spreadsheet solves the reverse rocket equation for each engine.

So for 2,270 m/sec, a Tylo t/w of at least 1.2, here's what I get:

Terrier: 6 engines, 37 FL-T100 tanks, Stage mass 37.8 t
Poodle: 2 engines, 5 x200-8 tanks, 38.4 t
Skipper: 1 engine, 6 x200-8 tanks, 40.4 tonnes.

 I recommended the Skipper because it's the lightest and cheapest single engine solution. I'd actually recommend packing a lot more DV than that, and the Skipper has room for growth.

 Best,
-Slashy

[Firemetal]

Do you need that many Kerbals to land? My advice is to minimize dry mass and maximize propellant mass. This will give you the ideal amount of DV for a Tylo landing. Also try and pick the highest thrust, highest ISP engine you can. I'd suggest an aerospike with RCS or reaction wheels to rotate the craft.

Fire

Edit: Actually read the OP. DV required for a Tylo landing is about 2100 m/s but as @Foxster said, you probably want a few hundred m/s wiggle room.

Edited January 10, 2017 by Firemetal

[Foxster]

It is a tad quick/dirty and could do with some tarting up but this did the job down and back up with 1000dV to spare at 50km orbit...

That's an Aerospike and a Skipper as engines. Enable crossfeed on the decoupler coming down so both tanks can be used by the Skipper. Refuel on the surface, disable the crossfeed. Then stage when the first stage is empty on the way up.  

Update: Doh! What's the point of the Aerospike? Just remove that and the de-coupler and it works fine.

Update 2: I like this one better. There's a bit less margin but its lighter and less tippy. Uses two Aerospikes and one Terrier...

Edited January 10, 2017 by Foxster

[Aegolius13]

Does the mothership drop you off in low Taylor orbit or are you burning some fuel to get there?  If the latter, you could consider using a higher ISP engine for the stuff that requires less TWR (either out radially,  or on top pointing backwards). Then switch to the main engine for landing and ascent.  

Not sure it would work that well here, though.   A nuke is going to be pretty big and heavy,  and does not fit the best with a 2.5m design.   Ions could be an option,  but your ship is pretty big so thrust would be tiny. 

My latest Jool 5 vacuum lander is a three stager, that starts with Tyler and sheds stages as the moons get smaller.  The top stage is an ion, and it helps my second stage Sparks with getting to and from Vall.  But the lander payload is much smaller than yours.