long range ferry craft

[AHeroReborn]

I have tried every engine combination I could think of. Dozens of nerves. Lots and lots of ion engines, lots of rhinos. I Just can't seem to make a vehicle capable of more than 10,000 m/s delta v. How would one make such a vehicle and still have a decent thrust to weight ratio?

[Guest]

1 hour ago, AHeroReborn said:

I have tried every engine combination I could think of. Dozens of nerves. Lots and lots of ion engines, lots of rhinos. I Just can't seem to make a vehicle capable of more than 10,000 m/s delta v. How would one make such a vehicle and still have a decent thrust to weight ratio?

Staging. You can make the stages recoverable, or even so that the later stages RV with the earlier ones and reassemble.

Very high dV craft will have very low TWR. There's simply no way around it. There's a loop of diminishing returns when piling on more engines and more tanks, because fuel tanks also add dry mass. Somebody did the numbers and a Nerv with infinite fuel tanks will give you 17,000 m/s with a TWR asymptotically approaching zero. It's the only engine (other than the Dawn) capable of cracking the 10k barrier under any circumstances and that requires a ca 75% fuel fraction; you can calculate yourself what the TWR on that would be, but it won't be much.

Ion engines will get you past 17k but again, always with very low TWR. 

Edited February 16, 2018 by Guest

[Foxster]

3 hours ago, AHeroReborn said:

How would one make such a vehicle and still have a decent thrust to weight ratio?

You pretty much can't, or at least not for anything practical. 

You have to live with and manage long burn times and multiple stages for really high dV craft (Alt > helps a lot). Or do the trip in multiple hops. 

One important thing is to have the payload at the absolute minimum. Don't be tempted to throw on even one single item unless it is essential. With just a-man-in-a-can I completed a trip to Eeloo and back with a single fuel tank.  

You can create modular craft like this:

This uses fuel cells (and their needed LF+O) to power ion engines, so is handy for the outer planets where sunlight is weak. You burn an ion tank and the attached four LF+O tanks and then stage them, repeat, then burn and drop the stage that also has 4 ion engines and 2 large fuel cells, rinse and repeat until all the blocks have been used. It's a kind of vertical asparagus staging that works because ion engines' exhaust is not blocked by other parts. 

Edited February 16, 2018 by Foxster

[bewing]

Use a big engine, grab an asteroid, and refuel.

[Snark]

12 hours ago, AHeroReborn said:

I Just can't seem to make a vehicle capable of more than 10,000 m/s delta v. How would one make such a vehicle and still have a decent thrust to weight ratio?

Rocket engineering is all about tradeoffs.  You can't have everything.  Essentially, what you're asking for, here, is:

• high dV
• high TWR
• few stages (you don't actually say this, but implicitly you're asking for a single stage)

You can't have all three.  Any time you improve one of these three things, it hurts the others.

• High dV means low TWR, and/or more stages.  That's because getting a high dV means you have to use high-Isp engines (which typically have low thrust), and also means you have to absolutely minimize dead weight (which means few engines, and discarding empty fuel tanks).
• High TWR means low dV, and/or more stages.  That's because getting a high TWR means using high-thrust engines (usually lower Isp), and having lots of engines (lots of dead weight), both of which lower your dV.  And getting a high TWR also means shedding dead weight other than engines, which means multiple stages.
• Having a ship that's only a single stage means you can't ditch the dead weight of empty fuel tanks and unused engines.  This means you're hauling a lot of dead weight and will get absolutely clobbered by the rocket equation (lowering your dV) and also by Newton's second law (lowering your TWR).

So. You can't have all three, which means you have to pick what's actually important to you, optimize for that, and accept that you will get that by sacrificing everything else.  For example, if what you absolutely have to have is a high-dV interplanetary tug that's fully reusable (and therefore can't shed stages)... well, that means you have to accept that you'll have an absolutely abysmal TWR.

 

The other thing we haven't mentioned here is refueling.  If you set up ISRU refining/refueling infrastructure at each of your destinations, then you make it easier to get where you need to go.  You can't mine xenon, which means in this case the best Isp you can get is LV-N.

So, let's do the math and see what your ship would have to look like, shall we?  You've said you want 10 km/s dV on your ship, and we know the Isp of an LV-N, so plug those numbers into the rocket equation.  10,000 m/s, divided by 9.8 m/s², divided by 800 s (the Isp of an LV-N), then raise e to that power... and we get a minimum required mass ratio of 3.58.

That is, the total mass of your ship with a full load of fuel, divided by the dry mass (after it's empty of fuel), can't be any lower than 3.58.

• Your ship consists of:  engine, payload, fuel tanks, fuel.  (I'm defining "payload" as "anything other than engine and fuel tanks".)
• It needs to be the case that (engine + payload + tanks + fuel), divided by (engine + payload + tanks), is no lower than 3.58.
• Abbreviating, (E+P+T+F) / (E+P+T) >= 3.58.
• For standard stock fuel tanks, the mass of the tank is 1/8 the mass of the fuel it contains.  So, we can say T = 0.125*F.
• And in this case, the engine is an LV-N, which has a mass of 3 tons.
• So, this gives us (3t + P + 1.125*F) / (3t + P + 0.125*F) >= 3.58
• 3t + P + 1.125*F >= 10.74t + 3.58*P + 0.4475*F
• 0.6775*F >= 7.74t + 2.58*P
• F >= 11.42t + 3.81*P

So.  If you want to have a single-stage ship, powered by a single LV-N, that has at least 10 km/s of dV ... then the amount of fuel it has to carry will equal 11.42 tons, plus 3.81 tons of fuel for every ton of payload (where "payload" is defined as "anything other than the LV-N and fuel tanks").

Which means you're going to have a very low TWR.  For example, even if your payload is a paltry 8 tons... that means you'd need to carry 42 tons of fuel, with a total ship mass of 58 tons.  Which means, with a single LV-N, it would be able to accelerate at just barely over 1 m/s².  And not a whit more.

And if you want more payload than 8 tons... the TWR would have to be even less.

 

[GarrisonChisholm]

This rather old thread none-the-less has some pleasantly evolved High DV probe conversation, and a land-mark submission by @Archgeek which I unfortunately broke while deploying.

 

Edited February 16, 2018 by GarrisonChisholm

[A_name]

11 hours ago, Brikoleur said:

Staging. You can make the stages recoverable

 

If you're going interplanetary, when ditching the stages you can leave some extra fuel enough to alter their course so that they encounter Kerbin in the next solar orbit for recovery, which IMO is pretty badass.

 

Also, you could make it landable (or have it carry a lander) to refuel using ISRU.

[Archgeek]

13 hours ago, Brikoleur said:

Somebody did the numbers and a Nerv with infinite fuel tanks will give you 17,000 m/s with a TWR asymptotically approaching zero. It's the only engine (other than the Dawn) capable of cracking the 10k barrier under any circumstances and that requires a ca 75% fuel fraction; you can calculate yourself what the TWR on that would be, but it won't be much.

Ion engines will get you past 17k but again, always with very low TWR. 

Indeed, Ions can crack the 100k barrier, if you're patient; and they can crack 60k for under 130 tonnes, or 50k for less than 50 tonnes.

[ajburges]

6 hours ago, A_name said:

If you're going interplanetary, when ditching the stages you can leave some extra fuel enough to alter their course so that they encounter Kerbin in the next solar orbit for recovery, which IMO is pretty badass.

More badass is a booster tug that stages before you go hyperbolic and aerobrakes back to a parking orbit.

Alternatively, have it burn retrograde after boost to stay in SoI!

Edited February 17, 2018 by ajburges