Cheapest 50 tonnes to orbit?

[paul23]

Well I set myself the following challenge: get a 50 tonnes ship into orbit, for an as small as possible cost. Currently I don't have the mainsail unlocked, and with skippers + boosters I manager to get the ship in orbit for roughly 40k.

But even that is too expensive for my missions (playing at 10% funds, the ship requirement came from USI life support to have a minmus lander for 3 kerbals). So does anyone have a design that costs less? at the 160 science barrier.

[CBase]

Is cheapest measured by launch costs or after recovery ?

[paul23]

4 minutes ago, CBase said:

Is cheapest measured by launch costs or after recovery ?

I currently measure it by launch cost, though for me obviously recovery is most important.

[RizzoTheRat]

50 tonnes payload to LKO?  That's a pretty big ask at your tech level even without trying to keep the costs down.

Assuming a significant proportion of the payload mass is fuel for the follow-on mission, you might find it's cheaper to do 2 x 25 tonne launches, one with the ship and the second one with the fuel.

Your biggest cost saving would be to try and reduce the weight of the payload.  A 3 crew capsule and an empty hitchhiker should give you plenty of habitation to make it to Minmus, and you should only need a few minipacks of supplies.  No need for recyclers and other heavy USI bits if you're only planning a single landing.  Another option is multiple launches again, one for the lander/re-entry capsule and one for a module that stays in Minmus orbit with extra habitation and life support.

I'm currently putting an 18 tonne payload in to LKO for about 15000 kredits (830 kredits per tonne, slightly more expensive per tonne than your launcher!) with a reusable VTOL SSTO, but using the higher tech level Vector engines as I find Mainsails and Twin Boars tend to overheat on re-entry.  The trouble with VTOL SSTO's is they don't scale all that well and launch cost per tonne tends to increase with heavier loads, so I'm launching stuff in smaller modules and docking it together in orbit.

HTOL SSTO's will generally be cheaper again, but are more of a design challenge and can struggle with odd shaped payloads.  Not sure which engines you have available at your tech level but I'd guess it would be a major challenge to get something that big in to orbit that early in the game.

[CBase]

I did tried a little, but at 40k launch costs you are already 1k cheaper than my best effort at throw away rocket design, SSTO of course being even more expansive.

Spaceplane lifter without at least panther engines is not gonna work.

20 hours ago, RizzoTheRat said:

The trouble with VTOL SSTO's is they don't scale all that well and launch cost per tonne tends to increase with heavier loads, so I'm launching stuff in smaller modules and docking it together in orbit.

Actually the rocket equation does scale: double dry mass with same Isp requires double wet mass. And on my SSTO lifters are some parts that I don't scale (parachutes, probe core, landing legs) so launch costs tend to decrease for me with bigger cargo. Finally drag scales mainly with front area, but mass with volume, again bigger is better.

So to really bring down costs I only see Rizzos first question to really think about:

20 hours ago, RizzoTheRat said:

50 tonnes payload to LKO? 

 

Edited November 14, 2019 by CBase

[RizzoTheRat]

2 hours ago, CBase said:

Actually the rocket equation does scale: double dry mass with same Isp requires double wet mass. And on my SSTO lifters are some parts that I don't scale (parachutes, probe core, landing legs) so launch costs tend to decrease for me with bigger cargo. Finally drag scales mainly with front area, but mass with volume, again bigger is better.

I must admit I hadn't thought about that as well as I should have done, I was thinking in terms of extra weight needing extra fuel, and the extra fuel needing extra fuel etc.  But you're right about the rocket equation and I hadn't thought about the drag difference.  I scaled up my SSTO to put 42 tonnes in to a 100km orbit last night and it worked out a bit under 800 credits per tonne.  That's with 6 vectors, I guess I could squeeze a 7th on the engine plate but after that I'd need to increase diameter which will mean more drag.  I could probably get the current design a bit cheaper if I used parachutes rather than saving about 600m/s for deorbit and landing, but where's the fun in that?  

A 2 stage recoverable system should be cheaper still but that means messing about with mods that tweak save files so you can fly them both.

[CBase]

7 hours ago, RizzoTheRat said:

  I scaled up my SSTO to put 42 tonnes in to a 100km orbit last night and it worked out a bit under 800 credits per tonne. 

Is that per tonne orbital payload as launch cost or after recovery ?

I just clicked together a 50t SSTO with 7 Vector, 5m spacey tanks and thrustplate and the SSTO lifter is 200000 funds of which I can recover 185000. So total costs per tonne payload to orbit is 540 funds but requires nearly 4000 per tonne launch invest for a perfect landing. At 3800+ dV at start I can return with 200-300 dV using two procedural canards at both ends to balance reentry pitch of about 10°. A single drag and radial parachute get speed from 700 m/s to less than 50 m/s so that 100 m/s dV are enough for touch down, rest is spent to target KSC.

 

[RizzoTheRat]

After recovery. I'm doing a power only landing so need something like 400m/s at the end which will bump tbe cost up a chunk.   It started as an experiment to reliably land on the pad using KOS but I landed it on the runway at the moment as getting the range alignment to drop it on the pad needs a lot of fine tuning.

[Pecan]

12 hours ago, RizzoTheRat said:

landed it on the runway at the moment

Seems like a very reasonable result to me, there's no benefit to a pad-landing so no reason to make it harder than it needs to be.

[RizzoTheRat]

Other than fact it looks so freaking cool! 

Interestingly landing near the KSC claims to give a 98% return on funds, but actually gives 100%.  Not sure how far away you have to land before it drops below 100%

[TheFlyingKerman]

The biggest limitation of a rocket SSTO is that the engine is too powerful for most of the flight. So instead I used a weaker Skipper added moar boosters to help it off the pad.

It delivers 9.5t to orbit. Fuel cost is around 3500 funds. The boosters fall back to KSC before they get deleted.

 

[paul23]

I'm not looking into ssto at all? Using stage recovery to recovery with parachutes.

[CBase]

Ahh you never mentioned that you are using stage recovery mod. Anyway I think you will hardly get below 40k launch costs for this big payload.

[Pds314]

Why do you need a 50 tonne payload to LKO for as three Kerbal Minmus landing? Does USI really increase the craft mass by THAT much? In stock, a lander can plus a mk2 lander can should be 1960 kg sans monoprop. 1800 if you just use three lander cans.

Orbital insertion, construction, Minmus landing, return to orbit, and return to Kerbin requires 660 Delta-V by the community map. Although realistically it's much less. So that means you need a rocket motor. Any size will probably do just fine because the gravity is so low. So we'll use a single Spark. This gives us 20 kN of thrust, which is completely excessive for purpose but will make nice, crisp, efficient burns. It weighs 130 kg so we're up to 1930 kg if we used three Mk1 lander cans. Next we need to be capable of actually landing the vehicle on something. Given the wide base I've just described and microscopic gravity, I think three micro lander legs is fine. That's 60 kg. Our lander is now 1990 kg. Next we need to bring it back to Kerbin. This requires a heat-tolerant part. I would go with a heat shield, but honestly the 2.5m service bay is more than enough, and 200 kg lighter than an empty 2.5m heatshield. Ok. Cool. We're at 2290 kg. But we need parachutes if we want to land the thing relatively intact. Based on the service bay's crash tolerance, one small chute is probably more than sufficient and we're at 2390 kg.

Ok, but we probably want to do things while we're on Minmus, so let's give it a whopping 51% mass increase to allow for scientific mission payload. That's 1210 kg of returnable science payload.

Now we need our fuel tank. We need about 25% of our mass to be fuel if we want 700 Delta-V. A bit much but hey, only time you can have too much fuel is if you're on fire. So in addition to 3600 kg, let's add an FLT-200. This brings our dry mass to 3725 and total mass to 4725. Our Delta-V is now 746 m/s. Probably 20% greater than necessary if not more.

Ok. Let's add more tanks now. We can add three FLT-100s and an Oscar-B on a 10 kg decoupler. We don't need another engine because we can just use what we have.

Total mass to trans-minmus intercept: 4947.5 kg.

Payload mass to LKO: 6647.5 kg.

Keep in mind that this is by no stretch of the imagination a barebones, totally optimized design. You could reduce the mass significantly. I don't understand where the other 43 tonnes in LKO is coming from. Surely USI doesn't require it for a Minmus lander, no?

Edited November 17, 2019 by Pds314