Need help with rocket design

[Laie]

2: Nukes won't do in atmosphere. They are great in a vacuum, but not where there is some air.

Baloney. Some air doesn't hurt.

At an altitude of 2500-3000m, Nukes already have a better ISP than any other engine. And that's on Kerbin. On Duna, the atmosphere are so thin that it hardly matters. Just watch your TWR, that's all.

[Zylark]

Well, ok, nukes may be an option on Duna as it got a pretty thin atmosphere and low gravity. But still, those things weigh a ton, 2.25 tons to be precise. With poor thrust to weight. They might be frugal with fuel, but are also more suited for long low TWR burns. Something like a LV909 isn't all that thirsty, comes in at 0.5 tons and have comparable thrust to the nuke. Which makes them perfect for a small lander where every little kilogram matters. The tiny little Rockomax 48-7S are also pretty impressive. 100kg each. Four of those give better thrust than two LV909 _and_ shaves 600kg off the mass of the vehicle. Compared to two nukes, they give the same thrust and saves a whopping 4.1 tons.

With its third gravity of Kerbin, Duna is just there on the edge on where nukes are able to lift any meaningful payload to orbit. You get very little bang for your buck using nukes under those gravity conditions.

Oh, and Unknow: That little lander of mine have very slim margins of error. Both descent and ascent got to be done perfectly to save fuel. If first trip to Duna, try and aim for at least 2500dV (in atmosphere).

Edited October 29, 2014 by Zylark

[capi3101]

OP, I went ahead and threw together those lifter designs I mentioned in my previous post for your consideration:

This is the asparagus-staged lifter design. 3 pairs of boosters and a Skipper engine the core, with the Skipper set to about 92% thrust and the LV-T30 engines set to about 87% or so. You could use the engines at full thrust but then you'd lose some delta-V to atmospheric drag. The design was still capable of making orbit when I increased the NREP testing payload to 30 tonnes (though naturally I had to turn the engines back up to full).

This is the SSR design. My original specifications called for five stacks of triple-orange tanks, a short tank and Mainsails; KER assured me that I could make orbit with just three instead. And I did - went ahead and flew it on up...

That design handled 25 tonnes okay, though as you can see in the original screenie, the TWR is right there at 1.2, which in my experience is the minimum acceptable TWR for a rocket launch. If you go with a heavier payload there, you might want to add those fourth and fifth stacks. You can also see the price difference between the two boosters - the asparagus is actually cheaper. The steering on the SSR also blows once the engines are off; you'd probably want to add a Roundified tank or two and RCS blocks.

As far as using nukes go, you can use them as a main ascent engine if what you're building is small, like a long-range science probe; even then the crummy TWR is generally a factor. They're still probably best reserved for interplanetary flight, or maybe for orbital insertion - when thrust isn't as important as efficiency.

Edited October 29, 2014 by capi3101

[Jouni]

Well, ok, nukes may be an option on Duna as it got a pretty thin atmosphere and low gravity. But still, those things weigh a ton, 2.25 tons to be precise. With poor thrust to weight. They might be frugal with fuel, but are also more suited for long low TWR burns. Something like a LV909 isn't all that thirsty, comes in at 0.5 tons and have comparable thrust to the nuke. Which makes them perfect for a small lander where every little kilogram matters. The tiny little Rockomax 48-7S are also pretty impressive. 100kg each. Four of those give better thrust than two LV909 _and_ shaves 600kg off the mass of the vehicle. Compared to two nukes, they give the same thrust and saves a whopping 4.1 tons.

With its third gravity of Kerbin, Duna is just there on the edge on where nukes are able to lift any meaningful payload to orbit. You get very little bang for your buck using nukes under those gravity conditions.

Nuclear engines work perfectly well as lander engines on everywhere except Kerbin, Laythe, and Eve (and possibly even on Kerbin and Laythe). They're especially good on Tylo, if you want to build a single-stage lander. The key to using them efficiently is not wasting fuel for carrying a separate lander, but landing with the main ship instead.

Two nuclear engines and an X200-16 fuel tank will provide over 4400 m/s of delta-v for 5 tonnes of payload, with Duna TWR ranging from 2.20 to 3.88. That's more than enough to reach Duna from LKO, land there, and return to Kerbin. If the TWR sounds low, remember that the effective TWR is higher when using a couple of parachutes to slow down. The whole mission stage weights just 18.5 tonnes, so you can launch it with a reasonably small rocket.

[Zylark]

Some more rocket designs for the OP to have a look at. Again, designed to get a lander to Duna - in style. This from my current career, which is now just about to get ready for branching out to the other planets.

First order of business - a lander. Need be lightweight, packing a ton of science gear quite literally, and have ample life support should something, let us say, unfortunate happen.

Slightly bigger than the little one from my previous career, but also comes with a smidge more dV and more beefy parachutes. One drogue, three XL and two radial. That should save me plenty of dV in braking for touchdown. Staged in three phases. Drogue deploy at 2500 meters, Radials at 2000 and XLs at 1500. This to save stress on the vessel. If all chutes deploy at once it might just rip the entire thing apart as it comes screaming down the Duna atmosphere - thin as it is.

Ready for launch atop my 15 Ton lifter. Since I'm running with Stage Recovery, my lifters got more chutes than you can shake a reasonably sized stick at. The Radial engines of the main-stage don't kick in until the SRBs are spent.

A good 150km orbit reached. The final stage of my lifter have plenty of fuel left to de-orbit itself. More cash in the bank for me.

Next order of business, the mothership that'll haul the lander from LKO to Duna. As it happens, I already have one of these in use around Kerbin and its' moons. More of a multi-purpose Crew Shuttle that doubles as a heavy tug and lander. Perfect for landing on Ike for instance. To which I might add a Karbonite driller and refinery is already underway. So sent up a sistership to the CS Anton, the CS Bernard...

The CS Bernard hitched a ride to orbit on top of my 45 Ton lifter. Mind you, the Crew Shuttle do weigh in at 51 Tons fully fueled, but since it got its' own engines to do the circularization, that's not a problem. To not fight against the Kerbin souposphere, the central main-stage don't ignite until the SRBs are spent.

A little Hohman transfer after, and time to dock the two vessels together.

Apply liberal amounts of Space-Tape...

...and good to go. 4500dV is enough to get to Duna and back. To make things better, I don't need to haul the lander back, so that should buy me another 1000dV for the return trip. And even should I waste a ton of fuel getting to Duna, as said, I already got some fuel generating gear en-route to Ike.

---

Using a bigger lifter, I could have launched both vessels in one go. But I've yet not had any need to go bigger than the 45 Ton Payload lifter I already got as a sub-assembly - and designing a new and bigger one would require the usual rounds of trial and error. All I really had to make from scratch for this mission was the lander.

Oh, and the nukes on my Crew Shuttle, are the bi-modal ones from the Constellation mod. They give a bit better ISP than the stock nukes at 950 and give a lot more thrust at 110 - and generate 18U/min electricity when not in use.

Hope it gives the OP some inspiration

Edited October 31, 2014 by Zylark

[Unknow0059]

Thanks very much for the help guys! I think we are done, but what is dV and aV?

[Redshift OTF]

Are you still sliding a lot in your lander or is that solved? Maybe it is because you are trying to hover too much when landing and any slight angle will cause your rockets to give you side-ways movement at low speeds.

DeltaV is one of the most important things in the game. It is how much your craft can change speed and is dependant on your engines and how much fuel you have. OK, that's not a great description. When you perform a manoeuvre node and the curved bar goes down, that number is DeltaV.

[Unknow0059]

Are you still sliding a lot in your lander or is that solved? Maybe it is because you are trying to hover too much when landing and any slight angle will cause your rockets to give you side-ways movement at low speeds.

DeltaV is one of the most important things in the game. It is how much your craft can change speed and is dependant on your engines and how much fuel you have. OK, that's not a great description. When you perform a manoeuvre node and the curved bar goes down, that number is DeltaV.

Yeah thats solved. Is there a best/good engine to change my speed during burns?

[mhoram]

Yeah thats solved. Is there a best/good engine to change my speed during burns?

Best depends on what you want to achieve.

If you go for efficiency, then taverts "Mass-optimal engine type vs delta-V, payload, and min TWR" thread can help you select the best engines.

But other factors like thrust or mass might be taken into account.

And as a general rule, the higher the ISP of an engine, the better is its efficiency.

[Redshift OTF]

Yeah thats solved. Is there a best/good engine to change my speed during burns?

Simple answer, you want the most efficient engine in space, such as the LV-N nuclear rocket, that gives you lots of DeltaV without using too much fuel. If you are trying to take off from a planet you would need something more powerful that is less efficient, (you have to use more fuel for the same DeltaV), otherwise you may not have a rocket powerful enough to escape the planets gravity. Obviously in space you don't have to worry about that. I'm sure others can explain better.

[Zylark]

Thanks very much for the help guys! I think we are done, but what is dV and aV?

dV as others have said is the amount of force you need to change direction and velocity. The precise measurement of which is not for the faint-hearted, which is why we plebs got MechJeb and Kerbal Engineer to do it for us.

If ISP is how many kilometers per liter of fuel your cars' engine can do, then dV is how far your car can go with your current engine and amount of fuel.(*)

aV as you say, is not a thing, it's just reading the triangle in MechJeb wrong - it is a delta... you know, from the greek alphabet. Hence Delta Velocity - dV.

(*) Yes I know it is not that simple, but it is a nice analogy nonetheless.

Edited October 30, 2014 by Zylark