dV for local shuttle?

[KatzOhki]

I have a small shuttle design that currently has around 140-220dV. Just wondering, is this enough for local travel? e.g. from Mun to Kerbin and Minmus or around the Jool system. Not needed to land only move from one orbit to the next.

If not, do you have a recommendation for the dV need for such task?

Edited May 12, 2015 by KatzOhki

[Tex_NL]

Please note that with the new aerodynamics the launch costs on atmospheric worlds has changed. But non-atmospheric data has not changed.

[KatzOhki]

Yeah OK, I've seen that, but it's not clear to me how much dV is needed to move from Bop to Pol for example.

[capran]

There's various tools for calculating transfer windows. You should try them out. I've been using Transfer Window Planner.

[Randazzo]

140-220 is not enough to go anywhere. It's barely (and perhaps not) enough to set up a rendezvous and deorbit again. If you want to go to the mun or minmus and back, you'll need at minimum ~1300, and that's assuming you aerobrake over Kerbin.

I get the impression this is perhaps a one-way trip vehicle, but it still needs a little more range.

I would need to see the craft to offer any useful ideas for improving it.

Edited May 12, 2015 by Randazzo

[KatzOhki]

I forgot that transfer window planner works for moon to moon.

OK I can describe it pretty easily. It's a Mk2 cockpit, Mk2 to 1.25 adapter, docking port, 2x24-77 engines and that's pretty much it. It looks good as heck, but yeah as I feared it's pretty underwhelming performance. I'll look into using Ion instead. It has to be rather slim to fit in my cargo bay.

Edit: Originally I had the Mk55 engines, which look nice, but changed to the 24-77s to improve performance.

Edited May 12, 2015 by KatzOhki

[Alias72]

Yeah OK, I've seen that, but it's not clear to me how much dV is needed to move from Bop to Pol for example.

proceed along the DV chart adding values as you go. From bop low to Pol low.

900 + 220 +160 + 820 = 2100 m/s

This is the minimum DV to transfer. It uses Hohmann transfers. Hohmann transfers require very specific windows that may occur only once in a year or two. If you need to leave now then you can spend DV for time with a less efficient transfer.

A shuttle for Munar travel would need 2(310+860) = 2340 m/s so I would go for 2500m/s for a shuttle

[Randazzo]

I forgot that transfer window planner works for moon to moon.

OK I can describe it pretty easily. It's a Mk2 cockpit, Mk2 to 1.25 adapter, docking port, 2x24-77 engines and that's pretty much it. It looks good as heck, but yeah as I feared it's pretty underwhelming performance. I'll look into using Ion instead. It has to be rather slim to fit in my cargo bay.

I imagine you're somewhat restricted on length as well then? It's going to need more fuel to increase it's budget. A short mk2 rocket fuel section might do it without compromising the overall silhouette too much.

[KatzOhki]

Adding a Mk2 LF+O Short tank only bumps it up to about 350dV, but yeah I want it to be short (partly for looks, but also to fit yeah). I think I might keep it as is and just use it for a short range ship to station transport (sort of like a dinghy) and then build something up for a little bit longer range in-system, probably using Ion. I'm working on building up my support fleet for my capital ships and I was hoping to get something with a little more range that would fit in the cargo bay. Thanks for all the tips guys

[Randazzo]

There's something wrong with these calculations.

I'm willing to bet KER/Mechjeb is calculating the dV with the shuttle still attached to the mothership, thus carrying it's weight. Two 24x77's should get WAY more out of a short tank than 130 dV.

Either that or you're hauling around a shipment of ACME Anvils for orbital deployment. I always wondered where they came from.

Edit2: Based on your description of the original design, assuming you used the short adapter (and I even added an extra .5 tons for good measure), it should have ~1000 dV.

Edited May 12, 2015 by Randazzo

[KatzOhki]

I might have missed a couply items, most notably the Mk2 Docking port. Here's my parts list:

Part	Full	Dry	#
Mk2 Cockpit	2	2	1
Mk2 Docking Port	0.6	0.6	1
Mk2 to 1.25 adapter	1.7	0.21	1
Mk. 55 	0.9	0.9	0
24-77	0.09	0.09	2
Clampotron	0.05	0.05	1
Thruster Block	0.05	0.05	2
RCS port	0.05	0.05	2
Mk2 LF+O Short	1.75	0.26	0

Not using MehcJeb, just my own calculations.

Edit: If I replace the docking port with the fuel instead of just adding it I only get another 50 dV.

[Randazzo]

Here's my best approximation of your initial description:

And with the extra fuel tank:

[KatzOhki]

Yep. Pretty well spot-on. WTH is wrong with my calculator then? I have ISP*LN(FullMass/DryMass) = dV

[Starwhip]

I might have missed a couply items, most notably the Mk2 Docking port. Here's my parts list:

Part	Full	Dry	#
Mk2 Cockpit	2	2	1
Mk2 Docking Port	0.6	0.6	1
Mk2 to 1.25 adapter	1.7	0.21	1
Mk. 55 	0.9	0.9	0
24-77	0.09	0.09	2
Clampotron	0.05	0.05	1
Thruster Block	0.05	0.05	2
RCS port	0.05	0.05	2
Mk2 LF+O Short	1.75	0.26	0

Not using MehcJeb, just my own calculations.

Edit: If I replace the docking port with the fuel instead of just adding it I only get another 50 dV.

Can you show us this equation? It doesn't seem right.

Edit: no, it's ISP x 9.82 x ln(m/m1)

The one you have is for another version of Isp. The in-game engines have their Isp's in seconds, and you must multiply by the gravitational constant. In ksp this is 9.82

Edited May 12, 2015 by Starwhip

[Randazzo]

The "rocket equation" uses exhaust velocity where you're using Isp. You can get the exhaust velocity by multiplying the Isp by standard gravity, hence the step Starwhip added to the equation.

[KatzOhki]

SMH... Well I feel pretty dumb because I feel like I read that and must have just forgotten to multiply by the 9.82. Hey, that's actually good news because it means all the stuff on my calculator is MASSIVELY overpowered! Cool. 2154 dV then. That's not bad.

Edit: Actually that's a bit worrying because it's telling me my multi-purpose lander should have something like 37000 dV, which seems crazy high.

Edited May 12, 2015 by KatzOhki

[Randazzo]

Edit: Actually that's a bit worrying because it's telling me my multi-purpose lander should have something like 37000 dV, which seems crazy high.

I should like to see this lander. lol

On the upside, you're ready for the Jool-5 tour.

[Rickenbacker]

As a rule of thumb, my Kerbin workhorse shuttles have around 1500m/s Delta-V. That lets them get to the Mun or Minmus, enter orbit, rendezvous, return and reenter with some margin.

[KatzOhki]

I should like to see this lander. lol

On the upside, you're ready for the Jool-5 tour.

Mk1-2 Pod			1
X200-8			2
LV-909			4
FL-T400			4
Docking Sr.			1
Sepratron I			0
Mk16-XL Chute			4
24-77			12
Sratus-V Cyl Mono			2
RV-105			8
Mk25 Chute			1
LT-1			4
RCL01			1
OX-STAT			4
Telus-LV			1
Communotron 16			1
Tiny Science			4

I designed it to be a landing support craft way back in pre-0.25. It was set up to be able to land and return to orbit from 90% of moons/planets. With the *9.82 correction applied to its numbers it seems like it has WAY more dV than it should, so there is probably something else wrong with my calculations.

I originally set up this spreadsheet so that I could play with numbers during a lull at work, but it looks like I'm probably going to have to get KER to verify designs.

Edit: I'm still going off the numbers on the wiki, so maybe that's the problem?

Edited May 13, 2015 by KatzOhki

[Randazzo]

Edit: I'm still going off the numbers on the wiki, so maybe that's the problem?

That's possible.

I've tried to work out what numbers you're using to do the math, but I'm a touch confused by the numbers in that spreadsheet data, such as why you have a 12 in wet mass column for the 24-77's. Is it set up opposite of the first batch of numbers you put up? IE, 12 engines.

[KatzOhki]

Oh yeah sorry I just copied the quantity numbers that time. Here's the full data I have.

Part	Full	Dry	#
Mk1-2 Pod	4	4	1
X200-8	4.5	0.5	2
LV-909	0.5	0.5	4
FL-T400	2.25	0.25	4
Docking Sr.	0.2	0.2	1
Sepratron I	0.0725	0.0125	0
Mk16-XL Chute	0.3	0.3	4
24-77	0.09	0.09	12
Sratus-V Cyl Mono	0.75	0.75	2
RV-105	0.05	0.05	8
Mk25 Chute	0.2	0.2	1
LT-1	0.05	0.05	4
RCL01	0.5	0.5	1
OX-STAT	0.005	0.005	4
Telus-LV	0.005	0.005	1
Communotron 16	0.005	0.005	1
Tiny Science	0.005	0.005	4

The spreadsheet:

https://docs.google.com/spreadsheets/d/1WVAbl1HUctPlSr0fT5ijVUB9LgEvQ6qmZNBCwWY9qzs/edit?usp=sharing

Edited May 13, 2015 by KatzOhki
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[Randazzo]

Well, here's what I've got.

Wet Mass - 29.33

Dry mass - 13.33

Isp - 303.75 - I averaged the Isp of all the engines on that list (12x290)+(4x345) / 16 = 303.75 and that could be the wrong way to do it.

So, plugging that all in: (303.75 * 9.81) * Ln (29.33/13.33) = 2354 dV

Edit: This assumes a single stage with fuel fed evenly to all engines.

Edited May 13, 2015 by Randazzo

[KatzOhki]

That definitely could be the error in my calculations. My ISP adds all the ISPs, which I think is not the right way to do it. The KSP Wiki page on ISP has another different way of doing it. Using that I get an ISP of 421s. That formula calculates dV directly as 4131, using that ISP in the standard way gives me dV=3252. Those numbers seem more "realistic" and here's the weird thing, getting rid of the *9.82 going back to the original values I had gives me dV=3832. Somehow all these numbers all seem really close.

[Randazzo]

That definitely could be the error in my calculations. My ISP adds all the ISPs, which I think is not the right way to do it. The KSP Wiki page on ISP has another different way of doing it. Using that I get an ISP of 421s. That formula calculates dV directly as 4131, using that ISP in the standard way gives me dV=3252. Those numbers seem more "realistic" and here's the weird thing, getting rid of the *9.82 going back to the original values I had gives me dV=3832. Somehow all these numbers all seem really close.

The Isp is not a cumulative thing though. No matter how many 24-77's you add, it's never going to raise the ISP beyond the 909's 345. In fact, it will never raise it at all in relation to the 909.

[KatzOhki]

Yeah I think the two mistakes I had were: Cumulating the ISP and omitting the 9.82. Somehow, in at least the one case, that made the numbers come out almost correct.