real dV needed for dunar orbit

[KerikBalm]

So I was playing around with the rocket equations and a graphing program, so I could optimise eninges for various purposes (mainly least fuel used for a given lander + engine, for designing reusable landers that dock with orbiting fuel depots).

And I realized, according to the rocket equations, the Aerospike engine can't get much more than 800 m/s of dV (assuming a lander can, engine, and tanks... legs and chutes drops it even more for practical fuel amounts) without shedding fuel tanks.

I know I've made duna landers before using just aerospikes.... but the dV map shows that I need 1380 m/s to get to orbit.

Given the thin atmosphere and relatively low gravity, I doubt there is such a difference if one lands at 0 meters or 4,000 meters elevation.

I'm also guessing that this figure doesn't take into account the planet's rotation... ie 1380 is what you need at the poles, but not the equator?

What is a realistic dV design goal for an equatorial duna lander?

Using the 1380 figure, it seems that nukes are the only way to a duna SSTO, but I know that is not the case

(although at any rate, it seems I'll be using LV-Ns for minimum fuel use per mission, unless the real dV needed is really low)

*edit* oops, was using ISP rather than exhaust velocity (it, off by a factor of 9.8) *edit*

The question still remains though

Edited June 7, 2014 by KerikBalm

[Pecan]

Personally, I use 48-7Ss on my Duna lander, but it's vastly over-engineered. One thing that makes your question more complicated is tht you are landing and then returning - you can use parachutes to land, or at least assist with landing, so your deltaV requirement is greatly reduced. Just EVA and repack them on the ground. Tavert's mass-optimal engine charts are a great resource for this sort of design question (if not the deltaV calcs themselves): forum.kerbalspaceprogram.com/threads/45155-Mass-optimal-engine-type-vs-delta-V-payload-and-min-TWR

[KerikBalm]

Yes, I'm aware of all that....

I've seen those engine charts before, and I'm making my own graphs (so that I can see how much dV adding x mass in parachutes costs my lander, so I can optimize how many chutes to use, since using engines to brake also costs dV).

The "problem" with those charts, is they show mass optimal, not fuel optimal.

If I have a 48-7s, weighing .1 tons, vs a LV-N, weighing 2.25 tons, for the same mass craft, I get to carry 2.15 tons of extra fuel&tanks if I use the 48-72.

There is a certain desin dV, where both a 48-7s and a LV-N powered craft would weigh the same to acheive that dV.

Below that dV requirement, the 48-7S is mass optimal, above that, the LV-N is mass optimal.

However - at that dV, the 48-7s is burning 2.15 more tons of fuel than the LV-N.

Go slightly below that dV, and the 48-7s craft can weigh less but it will still burn more fuel than the LV-N because it can start with 2.15 tons more fuel.

When looking at the mass optimal charts, its fine if I don't want to re-use a vessel.

I want to look at fuel optimal, the craft that can get me to and from the surface of duna using the least fuel per trip. This is not neccessarily the lightest craft that can make the trip. My graphs can show me which lander uses more fuel to acheive a given dV.

I just need to know.... do I really need 1380 m/s to get to orbit from Duna, or if I make all my landings equatorial, can I use a craft capable of less dV (thus less fuel use, thus less resupply missions to the Duna orbital fuel depot)

[Xavven]

I think you might need to double-check your formulas. My 2-kerbal lander has a 3.46 t payload (lander can, scientific instruments, legs, lights, batteries, etc.). Plugging it into my delta-v spreadsheet, I get 2105 m/s of delta-v with just a single Rockomax X200-8 Fuel Tank (360 units of liquid fuel, like a FL-T800), and it has 18.5 m/s² of acceleration in microgravity(that's a TWR of 6 on Duna). This lander weighs 9.46 t total when including the tank, fuel, and engine.

Payload 3.46

Engine 1 Aerospike

Engine 2

Tank 1 Large 360 Tank

Tank 2

Initial Mass (m0) 9.46

Final Mass (m1) 5.46

Mass Ratio (m0/m1) 1.73

Total Thrust 175

Thrust to Weight Full 18.5

Thrust to Weight Dry 32.1

Delta-V in Atmosphere (ÃŽâ€v) 2094

Delta-V in Vacuum (ÃŽâ€v) 2105

*edit* and to answer your question, yes it does take 1380 ÃŽâ€v to ascend from Duna. In fact, I usually end up using a little more than that because my ascent profile isn't mechjeb perfect.

Edited June 7, 2014 by Xavven

[KerikBalm]

Yea, check my edit. By using the value for ISP in seconds (800), rather than exhaust velocity (roughly 8000), I got a figure about 10x too low - 8,000 m/s is about the upper limit, not 800.

I guess I can ask another question... how much dV do you need to get into a polar orbit?

How much does dV vary between the lowest point and the highest point (I hear people talk about this often for Eve, where the difference is very large... but I'm curious about Duna)

[Pecan]

Sorry, I have no better information than I've already given. Totally understand your intentions though - who really cares what the lander masses; you just want to know how much extra fuel you'll need to give it each trip. Excellent! Please give me a nudge when you have the graphs, I'd love to see them :-)

The deltaV chart I'm using shows 1,300m/s for Duna orbit though, not 1,380. Meanwhile, don't forget you'll need some fuel for rendezvous. You'll also need to top-up with monopropellant every now and again, of course.

[Yasmy]

Duna's surface speed is about 31 m/s. So it'll cost about that much more to launch into polar orbit, and twice that to launch into a retrograde orbit.

[Xavven]

Yea, check my edit. By using the value for ISP in seconds (800), rather than exhaust velocity (roughly 8000), I got a figure about 10x too low - 8,000 m/s is about the upper limit, not 800.

Ah, now I understand your edit. Approximately 8400 m/s of delta v for a single stage at 390 ISP is an upper limit because the mass ratio of fuel tanks themselves is 9, and you therefore cannot get a better mass ratio than 9 in a single stage (i.e. no dropping tanks or engines).

I guess I can ask another question... how much dV do you need to get into a polar orbit?

How much does dV vary between the lowest point and the highest point (I hear people talk about this often for Eve, where the difference is very large... but I'm curious about Duna)

The delta V required to lift off of Eve varies tremendously by starting altitude because its atmosphere is so thick. Since the most delta V efficient ascent through an atmosphere is terminal velocity, and terminal velocity at Eve is very low (we're talking about 58-118 m/s all the way up to 10,000 m altitude!), you spend a lot of time fighting its tremendous gravity.

With such a thin atmosphere at Duna, terminal velocity is already 278 m/s by the time you hit 5000 m altitude. Lifting off from sea level, this means that keeping your accelerometer at about 1.3 g gives you an optimal ascent through the first 3000 m, at which point you can already start your "gravity turn". In other words, the atmosphere is much less of an issue. Plus, the delta V requirements are so low to begin with, that it's easy to just over-engineer your lander and be done with it.

[cantab]

I believe that for Duna the difference between high and low points matters more for landing. If you're coming in to land on high ground, your chutes will give you a lot less drag. Then again even targeting low ground a chute-only Duna landing isn't easy, so using a little engine thrust makes more sense anyway. The thinner air at higher altitudes affects planes more though, some might only be able to safely land at a suitable altitude.

[KerikBalm]

Hmm, I hadn't realized the rotation of Duna was so slow... 31 m/s is nothing....

-Just checked, it takes 3x longer to rotate than kerbin does... combined with a smaller radius = meh... about as irrelevant as landing on the Mun

IIRC drogues pop at 10k on Duna, and the mains pop at 9k. That should mean that any landing site under 8km should be accessible without using much dV to cushion the landing.

I know that Eve's problem is mainly the atmosphere, but the high gravity also matters...

For duna, I suppose I'll just use m*g*h = 1/2 M * v^2 -> g*h = v^2 g= 3 m/s (roughly), so a difference of 5km in landing site should be a difference of

2*3*5,000 = v^2 -> v^2 = 30,000 v= 173 m/s

So a rough estimate is 170 m/s less dV needed to ascend from a 5km higher starting location.

I guess I could have calculated the surface velocity too using the wiki... (I didn't feel like throwing together a mission in a new snadbox, and going there to observe it).

I guess I'll just set the design goal for 1,400 m/s to have a nice error margin, then decide about the crew and composition of the rest of the lander(s) -

I used to always try and put enough chutes on for an unpowered touchdown, but now I'm seeing that they are easily costing me more fuel/dV on the ascent than a quick thrust before touchdown would cost - although thats not a problem on non-reusable landers where I can jetison the chutes along with the first empty tanks on ascent.

More tricky will be seeing if I can save any fuel using modded electric fans on an duna plane... they're dead weight, and don't get it going very fast, nor can it fy very high in that atmosphere... I'm not sure how much my Kerbin testing will translate to duna (sure, I can correlate an air pressure on Kerbin to an altitude at duna... but Duna's got less gravity, so I could fly at a lower AoA, and thus a higher speed).

[Xavven]

Hmm, I hadn't realized the rotation of Duna was so slow... 31 m/s is nothing....

-Just checked, it takes 3x longer to rotate than kerbin does... combined with a smaller radius = meh... about as irrelevant as landing on the Mun

IIRC drogues pop at 10k on Duna, and the mains pop at 9k. That should mean that any landing site under 8km should be accessible without using much dV to cushion the landing.

I know that Eve's problem is mainly the atmosphere, but the high gravity also matters...

For duna, I suppose I'll just use m*g*h = 1/2 M * v^2 -> g*h = v^2 g= 3 m/s (roughly), so a difference of 5km in landing site should be a difference of

2*3*5,000 = v^2 -> v^2 = 30,000 v= 173 m/s

So a rough estimate is 170 m/s less dV needed to ascend from a 5km higher starting location.

I guess I could have calculated the surface velocity too using the wiki... (I didn't feel like throwing together a mission in a new snadbox, and going there to observe it).

I guess I'll just set the design goal for 1,400 m/s to have a nice error margin, then decide about the crew and composition of the rest of the lander(s) -

I used to always try and put enough chutes on for an unpowered touchdown, but now I'm seeing that they are easily costing me more fuel/dV on the ascent than a quick thrust before touchdown would cost - although thats not a problem on non-reusable landers where I can jetison the chutes along with the first empty tanks on ascent.

More tricky will be seeing if I can save any fuel using modded electric fans on an duna plane... they're dead weight, and don't get it going very fast, nor can it fy very high in that atmosphere... I'm not sure how much my Kerbin testing will translate to duna (sure, I can correlate an air pressure on Kerbin to an altitude at duna... but Duna's got less gravity, so I could fly at a lower AoA, and thus a higher speed).

I highly recommend adding 500-1000 m/s of delta-v to the 1400 figure. It won't add that much weight and it's worth the safety margin. 1380 assumes a perfect ascent, and assumes you aren't cushioning your landing at all. It also doesn't account for the delta-v you use to deorbit, and then to rendezvous after ascent.

[KerikBalm]

An extra 1,000 dv lets you get most of the way to orbit, then about and land again, and then go back to orbit... sure its a nice capability, but its also going to mean I'll need to arrange for a lot more fuel shipments for my lander to visit the various places I want to go (I don't use hyper edit).

I think I'll just set my design dV to be an even 1,500 m/s, not including the RCS fuel for docking