To Duna with Chem rocket only

[Jestersage]

So as a challenge to my self (and to honor the ITS), I decide to try building a Shuttle that use no NERV engine at all, but using only the "Rhino" for the Second stage, which will carry the shuttle in question (consist of a Mk3 cockpit, crew cab, 3x cargo bay with a remote control KLAW and a lab). However, I am only able to hit about 2.9k deltaV (whatever the KER use for figure).

So did I hitting the upper ceiling of chem rocket, or can I do something to boost the deltaV (without compromising the look)?

(Post screenshot/craft later if needed... I am tired)

 

Edit: Craft in pastebin: http://pastebin.com/ss1xZWkR

Edited February 9, 2017 by Jestersage

[George van Doorn]

13 minutes ago, Jestersage said:

So did I hitting the upper ceiling of chem rocket, or can I do something to boost the deltaV (without compromising the look)?

(Post screenshot/craft later if needed... I am tired)

Not at all, 2.9k is low as far as ceilings go.

Just remember the old KSP saying: more deltaV needed ->more fuel. More TWR needed -> more engines.

As far as where to put 'em, only by showing us the stuff can we comment on that.

[Streetwind]

Short answer: no, you are not hitting the limit of a chemical rocket. The Rhino's theoretical single-stage dV limit is about 7,325 m/s, with a more practical limit of severe diminishing returns once you go above 5,000 m/s.

Long answer: learn about mass fractions.

 

The only solution to your issue is to add more fuel or drop some dry mass, either of which may or may not compromise the looks. I don't know, I haven't seen your design.

[Blaarkies]

KR-2L has an Isp of 340s. Thats an upper ceiling of 7328.62m/s dv...at a TWR approaching 0 
 

You can reasonably gather about 3600m/s dv from such engine with adequate thrust for orbital maneuvers. Unless you are using the Rhino for aesthetics, i would recommend using the Poodle or Aero-Spike. 
Poodle gives you the best Isp that LFO-only has to offer, and the TWR is better than terriers.
AeroSpike has decent vacuum Isp, and amazing TWR. Depending on the type of burns you need to do, either of those could be more useful.

You shouldn't have more than 1 TWR for orbital maneuvers (I stick to 0.5 even), because that means you are dragging along massive engines which drains your dv...space doesn't care if you have a V8 engine, it only cares about the sum of your acceleration

TIP: Build the payload part, add some empty LFO tanks and engine. Check the weight. Now fill tanks, and check the weight. Go for 2/3 of craft mass being fuel...so empty craft weighs 1ton, filled craft weighs 3tons, then you get 3600m/s dv

[AeroGav]

What's the mission architecture btw?    

2900dV is way more than you need to get from low kerbin orbit to duna orbit.    

The ITS was intending to use ISRU to fill up for the return journey.

The landing itself will consume a little more delta V,  but you can save a lot of that with aerobraking and either wings or parachutes to control most of the descent, only reverting to propulsive lift for the final few metres.

Or, just make the lander separate.

The Rhino is an awesome vacuum engine,   it is only 3% less efficient than a Poodle but the only engines which can beat it's TWR have an ISP in the 310-315 range.   It's just too big for most projects.

[Reusables]

No. if it is second stage engine, you can get 5.8km/s of dv with 20t payload and TWR of 0.5(400t craft). Use at most 2k to circularize with it, and at least 3.8k is still left. Massive, but definitely better than nukes with the TWR.

If you are using it on the ground, just don't. It has not enough TWR and Isp in the atmosphere. Mammoth/Vector with drop-tanks will definitely work better.

Just to note..

2 hours ago, Blaarkies said:

AeroSpike has decent vacuum Isp, and amazing TWR. Depending on the type of burns you need to do, either of those could be more useful. 

The Rhino has more vacuum TWR(22.65) than the Aerospike(18.35), thanks to economy of scale (maybe). It's optimized for big payloads, I think.

[Blaarkies]

Just now, Abastro said:

No. if it is second stage engine, you can get 5.8km/s of dv with 20t payload and TWR of 0.5(400t craft). Use at most 2k to circularize with it, and at least 3.8k is still left. Massive, but definitely better than nukes with the TWR.

If you are using it on the ground, just don't. It has not enough TWR and Isp in the atmosphere. Mammoth/Vector with drop-tanks will definitely work better.

Just to note..

The Rhino has more vacuum TWR(22.65) than the Aerospike(18.35), thanks to economy of scale (maybe). It's optimized for big payloads, I think.

I don't think Duna Atmosphere will really affect Rhino's Isp, so landing on Duna is definately a better idea with the Rhino. My point is, that thing produces 200t of push...a 400t craft is not a shuttle anymore, it's almost a small mothership

[GoSlash27]

The Rhino is way too much engine for that ship. You only need to accelerate at .5G for an interplanetary burn.

Even so, you have enough DV to make the trip there and back in your current design. You just have to refuel in low Munar orbit and depart from there.

Best,
-Slashy

[Raptor9]

Don't forget about the mass of the Rhino as well. Having 340 ISP compared to the Poodle's 350 doesn't sound so bad, but the Rhino adds 9 tons to the overall mass of the ship.  That's 7.25 additional tons of engine compared to the Poodle.  On really large spacecraft, that probably won't make as big a difference, but if you want to move a payload less than 50 tons to Duna, I would go with a Poodle.

Of course, that little ol' Poodle ain't going to do anything for you if you're also trying to land the entire ship on Duna using propulsive landing. 

Edited February 9, 2017 by Raptor9

[Reusables]

1 hour ago, Blaarkies said:

I don't think Duna Atmosphere will really affect Rhino's Isp, so landing on Duna is definately a better idea with the Rhino. My point is, that thing produces 200t of push...a 400t craft is not a shuttle anymore, it's almost a small mothership

I see. 

Rhino is definitely overpowered for this purpose, if Kerbin TWR of 1 is not needed.

Then, I'd rather perform more researches for the 20t of payload OP needs.

With 3 Aerospikes, TWR 0.5 is gained for 110t rocket, with 4.0k dv to spare. 4 Aerospikes gives TWR of 0.5 for 146t rocket, with 4.5k dv to spare. (Used 'k' instead of km)

With 2 Poodles, TWR 0.5 can be gained for 101t rocket, with 3.9k dv to spare. 3 Poodle gives TWR of 0.5 for 152t rocket, with 4.6k dv to spare. Though, it's hard to cluster Poodles, so use it with smaller engines like Terriers and Aerospikes.

In contrast, A Rhino gives dv of 4.5k using 175t rocket. TWR is over 1, though.

(Besides; I've never thought about using Rhino to lift off from Duna. Thanks for the tip)

[GoSlash27]

2 hours ago, Blaarkies said:

a 400t craft is not a shuttle anymore, it's almost a small mothership

"Almost a small mothership" ?! O.O
I'm scared of whatever it is you're doing

-Slashy

[Jestersage]

Okay, just put the pastebin of the craft at the Opening Thread.

[Ohm is Futile]

3 hours ago, GoSlash27 said:

The Rhino is way too much engine for that ship. You only need to accelerate at .5G for an interplanetary burn.

I think this does a better job of telling me what the issue is with my own project in a single line than the 22 reponses I got. Would you say the less TWR below 0.5g (I assume Kerbin g's) makes it less and less practical, especially with more demanding (dV-wise) transfers?

[George van Doorn]

 

8 minutes ago, Ohm is Futile said:

I think this does a better job of telling me what the issue is with my own project in a single line than the 22 reponses I got. Would you say the less TWR below 0.5g (I assume Kerbin g's) makes it less and less practical, especially with more demanding (dV-wise) transfers?

Even 0.01, Ion-like can make sense if you plan your escape correctly. It's really paramount to see the big picture here. What's the objective? A certain solar orbit that will get you places, right? What's the duration of the solar orbit? A year more or less, depending if you go low or high. What's your burn time in comparison to that? 1 hour? That can all be accomplished in Kerbin's SOI even in one burn. The most agro exits are done in a day or so. OK, fine, you don't make use of the Oberth effect as much, so what? You must have 10 000 - 15000 m/s + on that baby.

Offtopic: BTW, your nickname is wisely crafted, you gotta think a bit to realize what you mean ;).

[DerekL1963]

1 hour ago, Ohm is Futile said:

Would you say the less TWR below 0.5g (I assume Kerbin g's) makes it less and less practical, especially with more demanding (dV-wise) transfers?

Nope, you just need to plan accordingly, I've gone pretty much everywhere outside of Kerbin's (Kerbolar) orbit with TWRs in the .10-.30 range.  'Planning accordingly' means ensuring your departure orbit is high enough that your burn time is around 1/8 (or less if possible) of your orbital period to minimize the time and angle spent burning off-prograde.  With a TWR of .14 and a 25 minutes ejection burn, my Jool-5 mothership departed Kerbin from a 3000 mile orbit (burn time=1/12 of the orbital period) - and my maneuver node was on the circle of the prograde marker at the start of the burn.  (I.E. barely off prograde.)

There's a school of thought that prefers burning much lower (to take advantage of the Oberth effect) and splitting the burn across multiple nodes (to minimize the time spent burning off-prograde), but I've never been fond of that approach.  Any error in the earlier burn(s) propagates forward, and without recalculating between each burn you end up more and more off target.  (Plus, if any of your burns have an Ap near or outside of the Mun's orbit, avoiding the mun constrains your burn times and orbits.)  I prefer one-and-done.

[GoSlash27]

1 hour ago, Ohm is Futile said:

I think this does a better job of telling me what the issue is with my own project in a single line than the 22 reponses I got. Would you say the less TWR below 0.5g (I assume Kerbin g's) makes it less and less practical, especially with more demanding (dV-wise) transfers?

It definitely gets less convenient if you get much below .5g, because you may need to do multiple passes. I personally don't find a need to go under .5G because that moves way more payload than I have a need to move.

HTHs,
-Slashy

[Jestersage]

When you guys say payload, is that the crew cab and cockpit and lab, or those three plus the fuel?

[GoSlash27]

1 hour ago, Jestersage said:

When you guys say payload, is that the crew cab and cockpit and lab, or those three plus the fuel?

Jestersage,

 When I speak of "payload", it's anything that's not engines, fuel, and tanks meant for use by the stage.

Best,
-Slashy

[Jestersage]

Okay, I have bumped to deltaV to 4000 by adding more fuel. You guys are right: I still have ways to go.

[GoSlash27]

7 hours ago, Jestersage said:

Okay, I have bumped to deltaV to 4000 by adding more fuel. You guys are right: I still have ways to go.

Conversely, you can also increase the DV by switching to a lighter and less powerful engine. It's all in what you're trying to do. I generally look at how much DV I need for a mission and design the lightest and cheapest stage that'll get the job done. A bunch of extra DV doesn't help if you never wind up using it. It's just a lot of dead weight you end up heaving into orbit and dragging around the solar system.

 Glad you got it sorted out and give 'em hell,
-Slashy