An Air-Breathing SSTO in Real Solar System(KSP mod)

[DA299]

Hello Everyone,

First of all, I apologize if this isn't in the right forum. With that out of the way, let's get started;

This is (as of now) just a feasibility study, but I want to be able to build a methalox fueled airbreathing SSTO in Real Solar System and Realism OverHaul.

To orbit Earth a velocity of 7.8 Kmps and an DeltaV of almost 9000 m/s is required.

I will assume a 100 tons total mass for the total vehicle, as it is a nice round number.

I want the air-breathing mode of the engines( 'jet' mode ) to get me up to around 3500 m/s or Mach 10.5. I assume this is realistic because scramjets have already been built that can operate at Mach 9 or whereabouts.

I will assume a (averaged over the entire burn)Specific Impulse of 2200 sec for the Airbreathing engine mode. This is considerably higher than the 1500-1800 which is quoted as the max for kerosene fueled ram and scramjets... but using Methane gives us some performance boost.( this assumption may or may not be realistic... you are free to critique me on this)

Next, The specific impulse of the Closed-cycle (rocket mode) is assumed to be 380 sec... which is well within the realms of reality for a methalox engine (Perhaps I could up this number, Please give me some info. about the vacuum performance of methalox engines.)

The TWR(at Takeoff, in jet mode) is assumed to be 0.4. This is reasonable as Airbreathing jets have a very low TWR. The total thrust in airbreathing mode is 392 kN. I will be using two engines, so each engine actually produces 196 kN.Throughout the Airbreathing phase, I will assume an avg. Acceleration of around 1.75 m/s^2. Even though the vehicle gets lighter, the engines also lose TWR as they go faster, as well as the effects of drag; so I figure 1.75 m/s^2 is reasonable.(Again it's open to critique.) 

Accelerating at a constant 1.75 m/s^2 ( again this is a gross over-simplification), it takes around 2000 seconds for the vehicle to reach 3500 m/s.

In these 2000 sec, the engines produce an avg 392 kN of thrust.. and consume around 35 tons of methane as propellant.

So now we are going at 3500 m/s, around 30 kms up, and weigh around 65 tons, we need around 5000 m/s more at this point to get into orbit. Before we calculate the payload this thing can haul into orbit.. we need some more calculations- such as the vehicles dry and wet mass.

Now Air-Breathing Engines- at best- have a TWR of around 8... I will assume 7 for the ones I'm using... this makes their weight around 5.7 tonnes. At minimum, the entire empty vehicle must weight around 5 tonnes(without engines), and this is assuming that the vehicle is practically one big fuel tank. So the empty weight of the vehicle is at minimum 11 tons. So can we even get into orbit without any payload at all? Using these numbers and inputting them into the Tsiolkovsky Rocket Equation, along with a specific impulse of 380 s, we get a total 'rocket' mode DeltaV of around 6500 m/s... this is way more than the 5000 m/s we need... so again doing some maths with the Tsiolkovsky Rocket Equation reveals that we can take almost 5 tons into LEO. This may not sound like much, but it is a payload fraction of 5 %, which is the same as a high performance staged rocket.

 

So, what do you guys and gals think of my analysis? I've tried to be as reasonable and as conservative as possible... Anyways do you think such a vehicle is possible to build in KSP-RO and/or real-life??

Edited September 22, 2020 by DA299