dV to get into Laythe orbit?

[1of6Billion]

There he is, poor Gusford. The brainiacs back on Kerbin where sure 3100m/s dv would be enough to get back into orbit from the surface.

However whatever he tries simulation he runs. 3300m/s seems to be an absolute minimum. Always falling 200m/s short.

So, anyone got an idea what's going wrong here? Or did management mess up by going for the lowest bidder on the dV charts?

[MarvinKitFox]

Umm, 2900 should do the job.

You are'nt maybe lookign at the *vacuum* delta-v for your ship, are you?

Layhte is not in vacuum!

Care to post the ship .craft file, or even your whole savegame file? (info on mods in there if you do so, plz)

Edited November 9, 2014 by MarvinKitFox

[Starman4308]

First off, those delta-V maps are approximations; exact delta-V requirements depend on TWR profile and how you manage your gravity turn.

Double check that you're accounting for atmospheric Isp: if you've got a vacuum engine, you might be losing a good amount of delta-V during the initial ascent*. There could also be issues with an imperfect gravity turn or a bad TWR: if your engine is struggling to get off the ground, you'll lose a lot of delta-V during the first part of the ascent, and if you're going horizontal too quickly or too slowly, that also burns up delta-V. I unfortunately can't give you much guidance on how to do an optimal gravity turn: I've never been to Laythe. I imagine it would be more horizontal than Kerbin ascents, courtesy of the thinner atmosphere, but the optimal turn requires simulation, either mathematical or just by trying until you get it right.

*One thing which helps is that air gets thin very quickly, so if you're not dawdling in sea-level soup, you reach near-vacuum efficiency very quickly. One rather spectacular example is the LV-N, which reaches 390 Isp before it reaches 2 km altitude at Kerbin.

Otherwise: time for a rescue misson, or alt-F12, depending on your patience level.

EDIT: And if you've got any monopropellant, you're going to have to figure out if you want to use it or if you want to dump it. In theory, it's rocket fuel, and since it has low specific impulse, you'd want to use up the monopropellant at launch or shortly thereafter (the issue is that, since RCS such a large vacuum/sea-level Isp ratio, it might be worthwhile to hold onto it for a few kilometers). In practice, that approach would require some fancy footwork, as you'd need to rapidly switch between staging mode, to turn the craft and fire stages, and docking mode so you can hold down W to translate forwards.

If you don't want to deal with switching so often, the question becomes simple: do you get more delta-V by hanging onto that monopropellant until the LF/O rockets are exhausted, or do you get more by burning all of it before launch?

Edited November 9, 2014 by Starman4308

[Claw]

Or, if it's not working out and you have another vessel in orbit, you can jump ship at the last minute and use the jetpack to circularize. Then it becomes a "Rescue Gusford from orbit" contract. Just remember to hand carry all that precious science with you.

Cheers,

~Claw

[numerobis]

If you've got a pair of 48-7S and 4.1t of mass on top (which is my guess from the picture), then there's a risk you exceed terminal velocity fairly quickly; are you keeping that under control when you launch?

3.1 km/s seems tight anyway. You need about 1.9 km/s orbit speed, so that leaves you only 1.2 km/s to lose in gravity and atmospheric losses.

[1of6Billion]

Umm, 2900 should do the job.

You are'nt maybe lookign at the *vacuum* delta-v for your ship, are you?

Layhte is not in vacuum!

Care to post the ship .craft file, or even your whole savegame file? (info on mods in there if you do so, plz)

Well yes I am... Heads will roll back at the VAB.

Ascend-profile is done by Mechjeb. That takes care of terminal velocity issues. The ship in orbit does not have a pod or even a chair.

I could try to start the gravity turn a bit earlier.

@Starman, I can use RCS in stage mode by pressing 'H'

Edited November 9, 2014 by 1of6Billion
pic!

[numerobis]

You'll definitely want to start the turn much earlier than on Kerbin. Air pressure starts lower and falls faster on Laythe. If you start to turn on Kerbin at 8km, the equivalent air density on Laythe is 5.5km. If you end your turn on Kerbin at 40km, the equivalent air density on Laythe is 31km.

By 2km, you're halfway to vacuum Isp.

Looks like you have RCS installed; you could use some at low altitude to jump-start you to terminal velocity (reducing your gravity losses at the start) -- and to reduce the mass you need to lift to orbit.

[magnemoe]

Well yes I am... Heads will roll back at the VAB.

http://i.imgur.com/XPsrvQi.png

Ascend-profile is done by Mechjeb. That takes care of terminal velocity issues. The ship in orbit does not have a pod or even a chair.

I could try to start the gravity turn a bit earlier.

@Starman, I can use RCS in stage mode by pressing 'H'

dV for upper stage is not listed.

You also have an weird fuel line.

Check the dV in wab, just for the upper stage, note that the 48-7S is far better than the radial ants so it might help to trasfer some fuel to lower stage.

Remember that the atmosphere end lower on Laythe so you can orbit around 60 km I think.

If you run out of fuel don't panic, make sure the ship point forward in orbit, eva, grab science from pod, let go and do an eva burn in the direction the ship is facing. 1/4 of the eva fuel should work.

[1of6Billion]

It's clear, this thing is not going to get back into space without cheating. Too bad, I was doing well with my Grand Tour of the Jool system.

When in the VAB, there are 2966m/s (in Atmo!!!) with RCS reduced to 1.5 (Need that for docking). That should be enough... So I just burned too much fuel landing.

[1of6Billion]

dV for upper stage is not listed.

You also have an weird fuel line.

Check the dV in wab, just for the upper stage, note that the 48-7S is far better than the radial ants so it might help to trasfer some fuel to lower stage.

Remember that the atmosphere end lower on Laythe so you can orbit around 60 km I think.

If you run out of fuel don't panic, make sure the ship point forward in orbit, eva, grab science from pod, let go and do an eva burn in the direction the ship is facing. 1/4 of the eva fuel should work.

Weird fuel line only looks weird. It works pretty well. That upper stage drains first. Ants are not used. After visiting Vall (next target) the lower stage would stay there. Those 2 ants are enough for Bop en Pol.

[Starman4308]

Weird fuel line only looks weird. It works pretty well. That upper stage drains first. Ants are not used. After visiting Vall (next target) the lower stage would stay there. Those 2 ants are enough for Bop en Pol.

You may be able to increase your available delta-V, then, by transferring fuel into the upper stage, disabling the fuel tank, staging, re-activating the fuel tank, and firing the Ants. While the Ants are less efficient, you may increase overall available delta-V because you ditch the weight of the lower stage. You have not exhausted your options. Sure, it might scrub other missions, but if you would prefer to rescue without cheating, this is the way to go.

And seriously: burn some of that RCS. If you do it right, you need just tiny touches of RCS to rendezvous and dock, and every gram of RCS used during ascent will increase your delta-V. Plus, if you have RCS aboard your orbital vehicle, you can use that to rendezvous. This is often the more efficient way to do it: the lander has to carry that RCS fuel back up to orbit, while if you rendezvous with your orbiter, you don't need to bother carrying the RCS back up. The exception is if you have a very lightweight lander and a heavyweight orbital vehicle, because it costs more RCS to maneuver a heavy vehicle.

[MarvinKitFox]

ummm.

My best tests show that thing can get to orbit, if you are anywhere near the equator.

You only have about 18-20m/s margin though.

How capable (and how pilotable) is your obital ship?

Consider *not* going for an orbit but rather a high suborbital hop. You definitely have the juice to make a suborbital with 10 minutes++ of vacuum time.

Just hop up, intercept with your orbital craft, dock and run for it.

Equally, how about pushing a bit?

Pop up to a nice suborbital lob, and push it!

Your Eva pack is good for about 50m/s on that ship's mass, and refills itself(unless your mods remove the refill, many do)

[1of6Billion]

You may be able to increase your available delta-V, then, by transferring fuel into the upper stage, disabling the fuel tank, staging, re-activating the fuel tank, and firing the Ants. While the Ants are less efficient, you may increase overall available delta-V because you ditch the weight of the lower stage. You have not exhausted your options. Sure, it might scrub other missions, but if you would prefer to rescue without cheating, this is the way to go.

And seriously: burn some of that RCS. If you do it right, you need just tiny touches of RCS to rendezvous and dock, and every gram of RCS used during ascent will increase your delta-V. Plus, if you have RCS aboard your orbital vehicle, you can use that to rendezvous. This is often the more efficient way to do it: the lander has to carry that RCS fuel back up to orbit, while if you rendezvous with your orbiter, you don't need to bother carrying the RCS back up. The exception is if you have a very lightweight lander and a heavyweight orbital vehicle, because it costs more RCS to maneuver a heavy vehicle.

I tried burning off most RCS during lift off. (0.5 left) It helped a bit, but not enough. Still 160m/s short.

The upper stage with 2 ants has a TWR of 0.66 It will not get me into orbit.

edit: nope. Got to 50km. No chance of circularising.

Edited November 9, 2014 by 1of6Billion

[1of6Billion]

ummm.

My best tests show that thing can get to orbit, if you are anywhere near the equator.

You only have about 18-20m/s margin though.

How capable (and how pilotable) is your obital ship?

Consider *not* going for an orbit but rather a high suborbital hop. You definitely have the juice to make a suborbital with 10 minutes++ of vacuum time.

Just hop up, intercept with your orbital craft, dock and run for it.

Equally, how about pushing a bit?

Pop up to a nice suborbital lob, and push it!

Your Eva pack is good for about 50m/s on that ship's mass, and refills itself(unless your mods remove the refill, many do)

This is crazy. Will try that tomorrow

[Starman4308]

The upper stage with 2 ants has a TWR of 0.66 It will not get me into orbit.

You don't need high TWR to circularize. You only need high TWR for launch; otherwise, you can coast on a combination of the upwards kick from the lower stages plus lower effective gravity*. You'll spend a bit more delta-V due to having less TWR, and correspondingly higher gravity losses, but if you get enough delta-V out of staging, that'll get you closer to low Laythe orbit.

*Mostly coming out of already being partway to orbit, with a tiny bit coming from having some additional distance from the planet's center.

[cantab]

Figures for a Laythe ascent do seem to vary widely. I can only put it down to Laythe ascents not getting anything like the attention to optimising that Kerbin ascents do.

[KerikBalm]

Why didn't you use an airbreather? A turbojet engine can basically get you into orbit on Laythe, and then just a tiny push with an ant or and ion engine will get your PE up (you could even try storing some intake air)

I know its too late now.

BTW, if you install NEAR or FAR, that design should get to orbit (atmosphere becomes less of a soup)

[1of6Billion]

Why didn't you use an airbreather? A turbojet engine can basically get you into orbit on Laythe, and then just a tiny push with an ant or and ion engine will get your PE up (you could even try storing some intake air)

I know its too late now.

BTW, if you install NEAR or FAR, that design should get to orbit (atmosphere becomes less of a soup)

It's a mission to put boots on every Joolian moon. Not strictly Laythe.

[cantab]

The reason I didn't opt with jets for Laythe myself was lack of familiarity and concerns about predictability. Fuel usage on jet-engine craft is more sensitive to ascent profiles, and on an unfamiliar world that brings additional risks. (And I try to minimise quickloading). A simple rocket packing enough delta-V felt more dependable.

[KerikBalm]

It's a mission to put boots on every Joolian moon. Not strictly Laythe.

Yea, but some moons should get their own lander.

Its pretty hard to make a reusable lander that works on tylo and laythe, and then its inefficient to take it to val/bop/pol.

A jet engine lander can be tiny: 1 turbotjet, 1 FL-T200 tank with half the oxidizer emptied, 3 fins as landing struts, 3 radial intakes, 1 parachute (or pair of radials), 1 docking port, 2x24-77s radial rockets for orbital insertion, docking port + rcs ports.

That will have plenty of dV, you don't need wings, just get up to near orbital velocity, point prograde, and then full throttle (and then throttle back as neccessary when it starts to flame out). You should easily get your Apoapsis well out of the atmosphere, with a perapsis high in the atmosphere.

With such small landers, you can easily take multiple to visit the moons.

If you want, you can even reuse the lander can.

* For the other moons, I use one lander, I jetison parts on tylo, but its single stage for the other moons

[1of6Billion]

Well, despite all great advise from you guys, I gave up. I launched a Refuel mission. Less than 30 units of oxidiser and fuel where needed.

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