How can I squeeze an extra 1k dV out of this mining-equipped Mk3 SSTO

[tibbe]

As a fun challenge for myself, I'm trying to put together an SSTO that can go to Laythe and back by mining resources on the way. The constraints I set myself are as follows:

• The craft should look like a plane! There are lots of powerful but not very plane-like SSTOs out there. I want this to be simple (fewer parts) and plane-looking SSTO.
  
• Needs to include the equipment needed for mining (drill, converter, and ore tank).
  

I put together a plane that fits the bill. The only issue is that it lacks the dV to get to Minmus for the first refueling. I'm about 1k short if I want some dV to spare for what's likely to be a shoddy landing. Can this SSTO be optimized will still stay with the current look-n-feel? Making it a bit longer is OK.

http://kerbalx.com/tibbe/Mining-SSTO-Mk3

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Edited September 28, 2015 by tibbe

[GoSlash27]

Tibbe,

Probably not.

DV is a function of your wet/ dry ratio and specific impulse. You're not just going to find another km per second DV by optimizing.

You will either need a lot more of your spaceplane to be fuel or else you will need a much more efficient rocket.

This will require a wholesale redesign.

OTOH, you could stick with this design and simply refuel it in LKO before moving on.

Best,

-Slashy

[SpaceplaneAddict]

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What he said. Also, stick on a nuclear engine, some liquid fuel, and voila, instant DeltaV. Don't know how much though.

[Clipperride]

I'd say your options are: either of the above or drop the single stage requirement and add drop tanks and/or a SRB to help you get up to speed.

There doesn't seem to be much room for reducing drag certainly not enough to make up the difference you're looking for.

The only other area you might be able to improve is the flight profile up to orbit. My current space plane ascent is; get to over 400m/s close to sea level then climb (add 5° then wait for speed to start increasing again before increasing the angel) then level off to accelerate at 10km. You might find the Rapiers are still accelerating you up to ~20km, so use manual mode switching (apologies if that's old news!).

My vote would be a refuel in LKO.

[Nothalogh]

DV is a function of your wet/ dry ratio and specific impulse. You're not just going to find another km per second DV by optimizing.

Ain't that the truth

[Justy]

My vote: don't.

I mean, keep your spaceplane -- it looks great! But use it as a spaceplane. Why haul wings and jet engines to Minmus? Make one of your spaceplane's earliest payloads a nuclear or ion powered tug. Then, future flights bring payloads up to be picked up by the tug and hauled to Minmus.

This was the real job of NASA's Space Shuttle. We call the shuttle "STS", Space Transportation System, but shuttle was only supposed to be a part of a Space Transportation SYSTEM that would also include chemical-powered tugs (with heat shields for aerobraking) for Earth/Moon work, nuclear-powered tugs for deeper space work, and a maintenance dock at what was then imagined to be Space Station Freedom.

The only optimizing I can see doing is to ensure your fuels are in proper balance. Once you get to orbit, you're on rocket power only, so you should have the right balance of liquid fuel & oxidizer for rockets, any extra liquid fuel beyond what you have oxidizer to burn with is dead weight from then on. There might be room to reduce some of the liquid fuel carried; between the wings and engine pods, you have a lot of storage there. Conversely, if you spend so much time on jet power that you reach orbit with an overabundance of oxidizer, that's dead weight too, and you should add more liquid fuel tankage so you can take advantage of it.

[GoSlash27]

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What he said. Also, stick on a nuclear engine, some liquid fuel, and voila, instant DeltaV. Don't know how much though.

Spaceplanes that are designed that way from the outset fare very well, but in the main spaceplanes don't respond well to unplanned additions. It usually requires rebalancing and scaling up the rest of the aircraft to accommodate it.

My back- of-the-envelope says that a RAPIER powered rocket needs to be 28% fuel by mass *when in orbit* in order to have 1 km/sec DV. This is not counting the fuel that's required to get it up there.

If you incorporate NERVs, this percentage drops to 12%, which is a whole lot more manageable. The trouble is that NERVs are so heavy that the engine itself takes up a large portion of the total mass when built powerful enough to have reasonable burn times.

IME NERVs really aren't worth the mass penalty on trips requiring less than 2km/sec DV.

*edit* And I'm with Justy on this one. Why haul a mostly- empty airplane all the way to Minmus and back? It makes a lot more sense to transfer crew in LKO to a smaller and much more efficient fully- fueled spaceship for that leg of the journey.

I think of a spaceplane like an old WWII "Duck".

It's useful for ferrying troops and supplies between ship and shore, but you wouldn't want to cross an ocean in one. That's what ships are for.

Best,

-Slashy

Edited September 27, 2015 by GoSlash27

[Nothalogh]

shuttle was only supposed to be a part of a Space Transportation SYSTEM that would also include chemical-powered tugs (with heat shields for aerobraking) for Earth/Moon work, nuclear-powered tugs for deeper space work, and a maintenance dock at what was then imagined to be Space Station Freedom

Now you're just trying to depress us all

[Mastikator]

Replace the RAPIER at the back with an aerospike thruster. When in space only use the aerospike, use the rapiers only for safe landing if needed. If it's a mining ship then losing all fuel on landing is fine.

[panzer1b]

http://dukws.co.uk/dukw%2030.JPG

It's useful for ferrying troops and supplies between ship and shore, but you wouldn't want to cross an ocean in one. That's what ships are for.

Then why have i spent so much time trying to get ships with 7K dV in LKO, all while carrying 1.5t of ordinance?

Anyways, for all seriousness, there are 3 major ways to increase dV of any ship, and these are the only 3 ways to do it (without cheats or mods):

*Add more fuel (well, extra fuel always adds dV provided you can actually drag it up to orbit without spending excessive extra fuel in the ascent).

*Remove dry mass (anything that isnt fuel will cut down on dV).

*Switch to a higher dV engine system (i like to go with nukes, but if you want ions give the absolute best range period if you consider their mass at teh cost of abyssmal TWR).

Aside from these options, there is no way to really increase dV, either increase the fuel to dry mass ratio (either increase amount of fuel or decrease stuff that isnt fuel), or pick a engine that has better ISP. You cannot do anything else really.

If you want to keep the shape, consider part clipping for fuel tanks (there are many opinions on this, some people consider it normal, others consider anyone who clips at all a cheater). Myself, i use clipping when i insist on a certain appearance, in particular when im trying to make a very minimalistic vessel. Just place a few fuel tanks inside a bomb bay, or even add them to the fuselage (since you dont mind making it longer).

There are many other options if you insist on keeping a certain look, but overall, you need to add more fuel, remove mass, or switch to higher ISP propulsion.

[Yakky]

The responses above about not hauling a bunch of dead mass are basically right. However, in the spirit of trying to answer your question the way you intended it, here are the things I would try. Not sure if they will be able to get you 1k dV but they should at least be good for a few hundred.

1. You've chosen a very heavy cockpit, which is basically dead weight. Do you need a 4-seater? Try replacing the cockpit with some fuel and a smaller cockpit. That will get your mass fraction up a little.

2. Can you delete the 5th engine entirely and still have enough atmospheric performance? More eliminated mass = slightly higher mass fraction.

3. Don't go with auto-switching to closed cycle on your Rapier engines. Use manual switching in order to milk every last m/s out of the air breathing side of the engines. Play with your ascent profiles to try to hit at least 1400 m/s, ideally more, before you switch to closed cycle.

4. Carefully optimize how much liquid fuel vs. oxidizer you have so that you aren't lugging excess liquid fuel after you get out of the atmosphere.

5. Use small landing gear. That will save you more than half a ton of dead mass.

6. Drain the monopropellant out of your cockpit since it doesn't look like it's needed.

7. Failing all else, consider ion engines to boost to Minmus encounter after you reach orbit. Tedious, but very high dV. Bring along a good book to read, and save a little rocket fuel for the Minmus landing.

Hope these help. Other than ion engines, I doubt they will get you a full 1 km/sec, however.

Edited September 28, 2015 by Yakky