Stranded on Tylo with 2970dV?

[ChiefJohnson]

Hello fellow Spacemen,

my Kerbonaut finally landed savely on Tylo. Now I hope to get him back home to his family and friends. MechJeb says I have 2970dV left. But I can't get to a 10km orbit where my return ship will pick me up, while the dV-maps claims I just need 2270dV.

Am I a bad little spaceguy or has MechJeb made some mistakes? I am afraid it did some wrong calculations and put the dV in the wrong stage. I will add a picuture to make it more clear.

Stages viewed by MechJeb (numers as seen in Mechjeb, not the staging sequence at the bottom left side) :

4: Will be left on the ground, just there for the legs (2 of the radial tanks).

3: Two left radial boosters with a little fuel left (612dV), will be dropped soon

2: This is where the most dV should be, but its 0... dV is on the next stage, which is wrong

1. This is where all the dV is located, but this stage already dropped the engine, so no dV at all (only science, command pod, MonoProp and some utility stuff)

So is the dV is calculated in stage 1 instead of stage 2, therefore I have less dV left because stage 1 is 3 tons heavier than stage 2?

Would be nice if someone could explain me my mistake. Also I would love to know if there is any possibility to save my Kerbonaut:/ Haven't lost one for a long time, except once to the allmighty kraken.

Thank you.

Edit:

Thanks for the answers so far, somehow I failed uploading the picture, here it is now:

Edited October 6, 2015 by ChiefJohnson

[Van Disaster]

There's a few rescue options:

* Move the return ship to a lower orbit

* get as far as you can to orbit, jump out, circularize on space suit RCS

* drop some more fuel next to your lander & refill it

* send an entire new lander

I usually crosscheck MJ and KER, at times one has been wrong. Adding a real picture might help

[FancyMouse]

What about a calculation by hand? dV is not a magic number and you should be able to calculate yourself. It may not match exactly what tools say because the mass game shows may not be precise, but still should be within 1% error range.

And yes, what you guessed is actually happening in a lot of cases, but without case by case analysis it's hard to say for sure

[Empiro]

2970 should be enough, and unless your craft is really oddly designed and triggering some sort of mechjeb bug, the readout should be accurate. A picture would be helpful.

Assuming you do have that much delta-V, it's going to depend on your TWR and ascent trajectory. How close can you get to orbit? If you're just a 100-200 m/s difference, then a better path will make all the difference.

[Mastikator]

Tylo escape velocity is 3069m/s according to the wiki, so any rocket with less can't leave.

Unless you do multiple runs of getting out with your kerbal and push on the back of it (and at that point you might as well enable infinite fuel)

[Empiro]

He just needs orbit, and not escape, so 2900+ ought to be enough, assuming the TWR is sufficient.

[JedTech]

Send us a screenshot please. We will band together to save your Kerbal!

- - - Updated - - -

It looks like you have asparagus staging setup? Are there fuel lines which pump fuel from the radial tanks to the center tanks?

Are the center tanks full?

[Padishar]

I suspect this is caused by the root part of the vessel not being in the "payload" (e.g. the decoupler) so MJ is staging it off during the simulation because it thinks the capsule and other stuff is just dead weight. If this is the case then there isn't a lot you can do about it now, your vessel doesn't have enough dV to get to orbit. A rough calculation shows that stage 2 should only have about 1090m/s so you will be quite a way short of the 2270 required. You might just manage to EVA and circularize your kerbal with the jetpack (it has about 600m/s)...

[Tatonf]

I agree with Padishar. Your first stage doesn't have that much Delta-V.

[Starhawk]

The math checks out, as far as I can see.

Try this: Leave the surface, ditching the empty sections. Pitch over almost horizontal (eastwards, of course), with just enough vertical to clear the terrain. Keep burning until apo is achieved, then shut down. Circularize at apo.

This should be the most efficient ascent profile.

Good luck.

Happy landings!

[FancyMouse]

3.8t start mass just sounds the mass of fuel tanks plus the engine with upper stage already ditched

[Padishar]

The math checks out, as far as I can see.

It really doesn't. The only way the masses make sense is if the final stage consists of the decoupler, two empty tanks and the engine because one of those parts is the root. This is also the only way that the deltaV can be showing in stage 1. The reported deltaV is much higher than it should be because MJ thinks the lander can and other stuff has been dropped when stage 1 was fired.

Edited October 5, 2015 by Padishar

[GoSlash27]

^ What Padishar said.

There's no way 1.78t accounts for the engine, empty tanks,decoupler, crew module, and additional stuff. It would account only for the engine and empty tanks, so MJ must assume that the decoupler is the payload.

I can't come up with an accurate figure without an idea of how much all that stuff above the crew module weighs, but it looks like at least another tonne.

If that's the case, it's 2.8 tonnes of not- fuel and 2 tonnes of fuel. 4.8/2.8= 1.71 Rwd. That would yield 1,700 m/sec (assuming this is an LV-T45), which won't make orbit. And honestly it's probably heavier than that.

The moral of the story: Never assume that MJ figures are correct. It's fine to use it as a sanity check, but you should compute the DV yourself (or with a spreadsheet).

How are you set for monoprop? If you have enough, you might be able to use it to circularize.

Best,

-Slashy

[Starhawk]

It really doesn't. The only way the masses make sense is if the final stage consists of the decoupler, two empty tanks and the engine because one of those parts is the root. This is also the only way that the deltaV can be showing in stage 1. The reported deltaV is much higher than it should be because MJ thinks the lander can and other stuff has been dropped when stage 1 was fired.

Yeah, you're right. As is Slashy. I wasn't paying attention. My bad.

Happy landings!

[Foxster]

The profile of your lift from Tylo is also very important. It makes a big difference on a craft with marginal dV to spare.

Aim to turn immediately after lift-off and then do the shallowest climb that just allows the vertical speed to increase. In other words, skim the surface in close to a straight line until you build enough horizontal speed to achieve orbit. In MJ's ascent profile this may be between 5 and 20 degrees or so depending on the craft's TWR.

[Padishar]

The profile of your lift from Tylo is also very important. It makes a big difference on a craft with marginal dV to spare.

Alas, the vessel actually only has about 1700 m/s so it is a long way from being a "craft with marginal dV to spare".

[ChiefJohnson]

"2.8 tonnes of not- fuel and 2 tonnes of fuel. 4.8/2.8= 1.71 Rwd. That would yield 1,700 m/sec"

Is the calculation of dV that simple? If so, I will not rely on MechJeb so much for the future. Thanks for the answers, I will try to get off with the MonoProp left, tho I don't think that will be enough. I will probably try to design a better lander and try to rescue my Kerbonaut! Thanks you very much!

[Streetwind]

"2.8 tonnes of not- fuel and 2 tonnes of fuel. 4.8/2.8= 1.71 Rwd. That would yield 1,700 m/sec"

Is the calculation of dV that simple?

It's not... this is just a coincidence where the numbers align. If you took the same engine but twice the fuel in the same dry mass, you'd get 6.8/2.8 = 2.42 but 2785 m/s dV. You can make arbitrary exampels where the numbers run apart further still, and an engine with a different Isp will return a different dV figure even for the 1.7 mass ratio.

That said, the rocket equation really isn't hard. The logarithm is the only part I can't readily do in my head.

[Jovus]

If that's the case, it's 2.8 tonnes of not- fuel and 2 tonnes of fuel. 4.8/2.8= 1.71 Rwd. That would yield 1,700 m/sec (assuming this is an LV-T45), which won't make orbit. And honestly it's probably heavier than that.

That said, if this calculation is correct, you should still be able to save the Kerbal without too many shenanigans, if only just. An EVA pack has ~550 m/s, and low Tylo orbit costs ~2300m/s.

If not, and you both don't need to worry about the mothership's delta-v to within a km/s and are feeling daring, you could always try a suborbital rescue. Burn the return stage as much as you can at the right time, then slow the mothership down to catch your kerbal. After he's caught, reattain orbit.

[Padishar]

Is the calculation of dV that simple?

While there is a bit left out of that calculation, the full one is still pretty simple. Basically, for any particular burn (strictly whenever the engines being used are changed or non-propellant mass is decoupled) the formula boils down to:

deltaV = Isp * g * ln(m0 / m1)

where m0 is the total mass at the start of the burn, m1 is the mass at the end of the burn, Isp is the effective Isp of the engine(s) in seconds and g is 9.80665 m/s^2.

So, to calculate the deltaV that stage 2 should have we just plug in the numbers (assuming the engine is an LV-T45 and there is 2t of fuel to be burned):

deltaV = 320 * 9.80665 * ln(6.833 / 4.833)

deltaV = 1086.72 m/s

With the 612 m/s from stage 3 this gives a total of 1698 m/s. As I said on the previous page, adding in the kerbal's EVA backpack of approx. 600 m/s gives 2298 m/s which should be just enough if you can do the EVA burn accurately (and quickly) enough.

As for doing it in your head, yes, natural log is harder to estimate than log in base 10 but you can still get a ballpark figure using these rough values.

[table=width: 500]

[tr]

[td]Mass ratio (m0 / m1)[/td]

[td]Approx. deltaV[/td]

[/tr]

[tr]

[td]1.65[/td]

[td]5 x Isp[/td]

[/tr]

[tr]

[td]2.7[/td]

[td]10 x Isp[/td]

[/tr]

[tr]

[td]4.5[/td]

[td]15 x Isp[/td]

[/tr]

[tr]

[td]7.4[/td]

[td]20 x Isp[/td]

[/tr]

[/table]

You might find it easier to generate your own list based on percentage of mass of fuel, e.g. 10% would mean the burn is using 10% of the whole mass, so m0 / m1 = 1/0.9 = 1.111, 20% => m0/m1 = 1 / 0.8 = 1.25, etc. You can stop at 90% as the wet/dry tank mass ratios in KSP never get much better than that (except when using RF/RO).

Edited October 6, 2015 by Padishar

[cantab]

Yeah, I've been caught out by KER doing that, assuming I'll decouple parts of my ship before I actually will.

At this point your best solution is probably to fly off Tylo then bail out and jetpack to orbit. That's what I had to do on my Tylo mission, because I forgot one of the steps when adding up the delta-V requirements.

[Captain H@dock]

Is the calculation of dV that simple? If so, I will not rely on MechJeb so much for the future. Thanks for the answers, I will try to get off with the MonoProp left, tho I don't think that will be enough. I will probably try to design a better lander and try to rescue my Kerbonaut! Thanks you very much!

Well, the equation is always that simple.

Also, nobody should have to guesstimate natural log in the 21th century. Just prepare a spreadsheet with dry weight, fuel weight (or wet weight), and Isp as inputs, and voila, you get dV for single stage ships.

It's then up to you to enrich it with spreadsheet wizardry (using id and vlookup for quick tanks and engines referencing, options for extra tanks or payload) for what your use cases are.

Also, this thread is making me slightly nervous regarding my Tylo re-usable lander (no ISRU). Luckily, I won't know if I've made a terrible mistake until 2 kerbal years from now...

[Warzouz]

Also, this thread is making me slightly nervous regarding my Tylo re-usable lander (no ISRU). Luckily, I won't know if I've made a terrible mistake until 2 kerbal years from now...

I had a one manned SSTO lander in beta 0.9* with quite low TWR (1.02 when starting descent) which used 2 LVN. It weighted around 15tons and had 6000m/s of fuel. I barely got back to orbit (50m/s left). Hopefully, the tug was there to bring me back to my Laythe station. for refuel.

In 1.0, LVN are heavier, so this ship wouldn't work any more. Further more, I had quite a lot of physic-less parts. I didn't tried to redo an equivalent ship for 1.0.

* As I wasn't sure that lander could successfully land at Tylo, I brought along another staged lander (thus not reusable). This one was lighter.

[Kelvin Kerbin]

My math shows that it should take ~18dV to change orbits from 20km to 10km. I checked it in KSP too and that seems about right. This dV map (and several others):

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

shows 3070dV to 20km. That would imply around 3050 to 10km. Quick math shows it takes a dV of about 200m/s to reach 10km in Tylo's gravity and it takes around 2030 dV to orbit Tylo at 10km so an orbital dV of 2070 is really close to an idealized engine (infinite TWR). I think the dV map you used is ideal and from a practical perspective it takes a lot more than 2070 dV to reach a 10km orbit?

I see that it will take your ship around 60s to expend it's 3 km/s dV. That is not ideal so it takes your ship 60s to fight gravity over the ideal scenario and with a gravity of 7.8m/s^2 that would be 468 m/s additional dV (at least). Since the first stage has the worst TWR the losses fighting gravity are probably a lot worse and then you probably didn't do everything perfectly.

Just use the Hyperedit mod and then you won't loose your Kerbal.

[Captain H@dock]

I had a one manned SSTO lander in beta 0.9* with quite low TWR (1.02 when starting descent) which used 2 LVN. It weighted around 15tons and had 6000m/s of fuel. I barely got back to orbit (50m/s left). Hopefully, the tug was there to bring me back to my Laythe station. for refuel.

In 1.0, LVN are heavier, so this ship wouldn't work any more. Further more, I had quite a lot of physic-less parts. I didn't tried to redo an equivalent ship for 1.0.

* As I wasn't sure that lander could successfully land at Tylo, I brought along another staged lander (thus not reusable). This one was lighter.

LV-n isn't possible with a reasonable size according to my planning. The only engine in 1.0.4 (apart from the super big ones) with enough TWR and ISP is the aerospike. Payload fraction is about ridiculous and dV is marginal (5.2K dV), but TWR is higher than 1.0 from orbit, and above 2.0 from take-off. Except if my math/data are wrong.