I fail to see how vertical or horizontal makes any difference there. The problem is the same.
12 Raptor 80 how much propellant mass were you using?What is the maximum propellant mass for the tanker for their propellant tank size?SX PDF slide 36 show 2,500 mt propellant.And shows 380 mt propellant delivered to orbit. I assume that was part of the 2,500 mt and not extra.
I'd argue that Airlaunch (horizontal takeoff TSTO) is different enough from traditional rocketry to merit a distinction, but that Horizontal SSTO suffers from the worst of both worlds.
Now, if we could get an airllaunch stage that could compete with the capability of even a RTLS Falcon 9 first stage... Upper atmoshphere with a high mach number, and all that. SCRAMjet and SABER may be possibiities in the future... as is, amusingy enough, certian blimp concepts.
Blimp concepts ( historically called rockoons) suffer from the problem that once the rocket separates you have a very short time to either vent the lift gas or ("magically") compress (or liquefy) it before your balloon rises so high the pressure differential splits it (or burst height).
Nothing that HMXHMX hasn't talked about numerous times.
Here are the Raptor figures I'm using in SpaceSim. Most of these figures have been published by SpaceX or mentioned in the Reddit AMA. I've simply interpolated to get my Raptor 80 figures. The mass estimates are envy's, they sound pretty reasonable to me. Cycle Full-flow staged combustion Oxidiser Subcooled liquid oxygen Fuel Subcooled liquid methane Chamber pressure 300 bar or 30 MPa Throttle capability 20% to 100% thrust Sea-level Nozzle Expansion Ratio 40 Thrust (SL) 2,842 kN Thrust (Vac) 3,061 kN Isp (SL) 333s Isp (Vac) 360s Diameter 1.51m Mass 2400 kg Expansion Ratio 50 Thrust (SL) 3,094 kN Thrust (Vac) 3,333 kN Isp (SL) 334s Isp (Vac) 361s Diameter 1.78m Mass 2500 kg Expansion Ratio 80 Thrust (SL) 3,200 kN Thrust (Vac) 3,400 kN Isp (SL) 336s Isp (Vac) 368s Diameter 2.4m Mass 2800 kg Vacuum Nozzle Expansion Ratio 200 Thrust 3,500 kN Isp 382s Diameter 3.8m Mass 3000 kg
I don't know from where you got your Raptor 40 performance numbers.
Plus your nozzle diameter is also too small: it should be 1.7m to be consistent with the Raptor 200 and Raptor 80 nozzles. Although I admit it is inconsistent with measurements on the SpaceX slides/renders and 42 of those wouldn't fit under the booster... but it is better to be internally consistent with your numbers IMHO. The booster seem to be using a Raptor 32 while the BFS a Raptor 44...
Simple interpolation for getting Raptor 80 numbers is wrong, as it would be working much more over-expanded than the Raptor 40, that is already slightly over-expanded at SL. Envy887's numbers seem much better, but you seem to have used only his diameter and mass numbers, not the SL performance ones:SL: 3005 kN at 323.2 sVacuum: 3479 kN at 372.2 s
I also don't know from where you got your Raptor 50 numbers, but they are suspicious as well, as any ER higher than about 32 should have smaller SL thrust and isp. Correct me if I'm wrong.
Plus, I'm doubtful if the BFS can throttle enough for landing with just Raptor 80 engines. I would leave the center cluster alone in those modifications.
From the Reddit AMA, which was more recent than the IAC Mexico presentation. In the AMA Elon said the Raptor 40 has 290mT thrust at SL, less than the figure from the IAC presentation.
If the slides are from the CAD drawings, why would they be incorrect? It may be that the Raptor 40 has a slightly smaller throat, and the 1.51m Ø and 40:1 ratio are correct.
The Raptor 50 numbers are from the IAC Mexico slides. Where does your 32:1 figure come from? Wouldn't chamber pressure effect the point at which a nozzle becomes overexpanded?
Quote from: Manabu on 03/05/2017 03:26 AMPlus, I'm doubtful if the BFS can throttle enough for landing with just Raptor 80 engines. I would leave the center cluster alone in those modifications.Leaving the centre three engines as Raptor 50s could certainly work, the difference in performance would be minimal. How many percent do you estimate the Raptor 80 could throttle to?
Increasing expansion does eventually lead to seperation at sea level, at about 140:1, but SL thrust drops to 2715 kN per engine and ISP drops to 292s. On a thrust-limited design this is a poor trade-off, even though vacuum ISP is close to 380.However, instead of nine 80:1 nozzles it might make sense to have a mix of 50:1 and 140:1. The former have much higher thrust and ISP at SL, and the latter have higher thrust and ISP in vacuum, but both are operable (if needed) across the entire flight. The 50:1 engines could be shut down to improve overall ISP once the vehicle is high enough (and light enough) that the 140:1 nozzles can maintain optimal acceleration.
Envy887's numbers seem much better, but you seem to have used only his diameter and mass numbers, not the SL performance ones:SL: 3005 kN at 323.2 sVacuum: 3479 kN at 372.2 s
I got 3573 kN for the Raptor 200, because I was basing this on the R40 engine performance and not trying to match both... They aren't quite consistent with each other.
... I'm just trying to contribute to better simulations and learn in the process.