A few days ago there was a picture showing six Raptors in a tent. https://twitter.com/delta_v/status/1379912546430418945If I saw correctly are new generation raptor (those compatible with sn15+). Are there more old generation raptors? If yes, can they being upgraded or it isn't worth it, even if possible?

The problem is thrust. With the 28 standard sea-level raptors at 25 MPa the "lift off" TWR is 0.91. Even at 30 MPa the TWR is only 1.09 which also helps explain why SpaceX is willing to use a worse expansion ratio in order to get more thrust with the non-throttling raptors. The full stack can achieve a decent TWR of 1.20 using the current 25 MPa sea level raptors, but it would require 37 engines, literally the maximum number of 1.3 m diameter nozzles you can fit in a 9 m diameter circle. _{That packing would have a rather attractive C6 symmetry group though, and we could talk about the chirality of Super Heavy...}

It seems to me that the drive for the 30 MPa chamber pressure is almost entirely an issue of thrust, not ISP. Vacuum raptors, with their space constrained expansion ratio barely gain any Isp (<1 s) in the shift from 25 MPa to 30 MPa.

It seems to me that the drive for the 30 MPa chamber pressure is almost entirely an issue of thrust, not ISP. Vacuum raptors, with their space constrained expansion ratio barely gain any Isp (<1 s) in the shift from 25 MPa to 30 MPa. Sea level raptors gain more on account of their expansion ratio being tied to an exit pressure of ~0.8 atm, but it's only ~3 s. If the full stack can reach a target orbit with 100 tonnes of payload when running at 30 MPa it can reach the same orbit running at 25 MPa with 96 tonnes of payload, even in the worst case scenario where all three SL raptors on Starship have to be used for the entire burn.Chamber pressure is even less of a factor for the 6 month transit time Mars burn of ~4.5 km/s. Again assuming the worst case, that all three SL raptors need to be used to get to the propellant transfer orbit and one SL raptor needs to be used on the Mars burn for gimbal control, it only takes one additional tanker of fuel for the 25 MPa vs the 30 MPa, and that additional tanker would have 73 tonnes of spare payload.Heck, if you're only concerned with Isp you can go as low as the BE-4 at 13 MPa and still make it to orbit with 78 tonnes of payload, and to Mars with 8 tanker fill ups! (vs 6 for 30 MPa)The problem is thrust. With the 28 standard sea-level raptors at 25 MPa the "lift off" TWR is 0.91. Even at 30 MPa the TWR is only 1.09 which also helps explain why SpaceX is willing to use a worse expansion ratio in order to get more thrust with the non-throttling raptors. The full stack can achieve a decent TWR of 1.20 using the current 25 MPa sea level raptors, but it would require 37 engines, literally the maximum number of 1.3 m diameter nozzles you can fit in a 9 m diameter circle. _{That packing would have a rather attractive C6 symmetry group though, and we could talk about the chirality of Super Heavy...}

Quote from: Pueo on 04/11/2021 01:52 amThe problem is thrust. With the 28 standard sea-level raptors at 25 MPa the "lift off" TWR is 0.91. Even at 30 MPa the TWR is only 1.09 which also helps explain why SpaceX is willing to use a worse expansion ratio in order to get more thrust with the non-throttling raptors. The full stack can achieve a decent TWR of 1.20 using the current 25 MPa sea level raptors, but it would require 37 engines, literally the maximum number of 1.3 m diameter nozzles you can fit in a 9 m diameter circle. _{That packing would have a rather attractive C6 symmetry group though, and we could talk about the chirality of Super Heavy...}Exactly this. Having played around with numbers, starship looks pretty good with a thrust/weight of 0.94 with 100 tons of payload, plenty for a second stage (I have Falcon 9 at 0.77 for a starlink launch). But SH + Starship (100 ton payload) is just too heavy for 28 engines. I was getting a 1.12 with my estimates, but that's just too low - at a low thrust/weight you lose so much due to gravity losses. My Falcon 9 first stage numbers say T/W of 1.34 for a Starlink launch, and that's the range I'd expect them to do. The "super raptor" variant for the outer 20 engines would give a T/W of 1.48. That would be spicey.

According to a cursory Google search, the Saturn V had a 1.2 T/W. So my question is how low is too low? I thought anything over 1.0 was acceptable depending on what you are trying to optimize for.

Quote from: schuttle89 on 04/13/2021 02:06 pmAccording to a cursory Google search, the Saturn V had a 1.2 T/W. So my question is how low is too low? I thought anything over 1.0 was acceptable depending on what you are trying to optimize for.Depending on ISP (which translates into how fast you’re moving propellant mass through the engine), “too low” is the point at which your vehicle loses more delta-V to gravity than it can afford to lose. Launch from Earth and you are fighting two things: gravity and atmospheric drag. If your T/W ratio is too low, you’re wasting prop by not accelerating quickly. Too high and you end up with huge drag losses and aerothermal heating in the dense lower atmosphere. Like all engineering, there’s a lot of balancing and trade-offs involved.

Quote from: Herb Schaltegger on 04/13/2021 02:41 pmQuote from: schuttle89 on 04/13/2021 02:06 pmAccording to a cursory Google search, the Saturn V had a 1.2 T/W. So my question is how low is too low? I thought anything over 1.0 was acceptable depending on what you are trying to optimize for.Depending on ISP (which translates into how fast you’re moving propellant mass through the engine), “too low” is the point at which your vehicle loses more delta-V to gravity than it can afford to lose. Launch from Earth and you are fighting two things: gravity and atmospheric drag. If your T/W ratio is too low, you’re wasting prop by not accelerating quickly. Too high and you end up with huge drag losses and aerothermal heating in the dense lower atmosphere. Like all engineering, there’s a lot of balancing and trade-offs involved.Propulsive landings change the dynamics too. For Falcon 9 or Super Heavy, any extra tankage is all the more mass you have to turn around and bring back to the landing site. This is very different from the expendable model where you can tolerate 80% gravity losses for a portion of the flight and still come out ahead because the dry mass for a booster doesn't matter so much - it will always have a heavy second stage on top of it.So the dry mass of Super Heavy matters a lot more than an expendable stage, and thus they can't go as far down the road of diminishing gains lengthening the stage to gain a small bit of extra boost at the cost of low TWR wasting most, but not all, of the gains.

Quote from: Keldor on 04/13/2021 04:58 pmQuote from: Herb Schaltegger on 04/13/2021 02:41 pmQuote from: schuttle89 on 04/13/2021 02:06 pmAccording to a cursory Google search, the Saturn V had a 1.2 T/W. So my question is how low is too low? I thought anything over 1.0 was acceptable depending on what you are trying to optimize for.Depending on ISP (which translates into how fast you’re moving propellant mass through the engine), “too low” is the point at which your vehicle loses more delta-V to gravity than it can afford to lose. Launch from Earth and you are fighting two things: gravity and atmospheric drag. If your T/W ratio is too low, you’re wasting prop by not accelerating quickly. Too high and you end up with huge drag losses and aerothermal heating in the dense lower atmosphere. Like all engineering, there’s a lot of balancing and trade-offs involved.Propulsive landings change the dynamics too. For Falcon 9 or Super Heavy, any extra tankage is all the more mass you have to turn around and bring back to the landing site. This is very different from the expendable model where you can tolerate 80% gravity losses for a portion of the flight and still come out ahead because the dry mass for a booster doesn't matter so much - it will always have a heavy second stage on top of it.So the dry mass of Super Heavy matters a lot more than an expendable stage, and thus they can't go as far down the road of diminishing gains lengthening the stage to gain a small bit of extra boost at the cost of low TWR wasting most, but not all, of the gains.Also, with high T/W, the staging happens faster and you have moved less horizontal distance when that staging happens, while you still have about as much velocity, so you stay in the air for equal total time (more remaining time)And when you have less distance but more time for your way back, that means you can travel back at considerably slower horizontal speed => smaller boostback burn needed, less fuel spent for the boostback burm.

There was a post, that I can't find, in which was sais that a wors expantion ratio could be a option, maybe temporaney, to gwt more theust from a Raptor. "Worse" means higher 1:e (e is the exit area in throat area unit) or lower? Why is considerable worse?

Quote from: Alberto-Girardi on 04/13/2021 06:58 pmThere was a post, that I can't find, in which was sais that a wors expantion ratio could be a option, maybe temporaney, to gwt more theust from a Raptor. "Worse" means higher 1:e (e is the exit area in throat area unit) or lower? Why is considerable worse?So, a higher expansion ratio is, absent other concerns, better, so a lower expansion ratio is worse. There are trade offs to making engine bells bigger and it is not always possible to do, etc, but a higher expansion ratio extracts more thrust from the exhaust. In atmosphere, maximum expansion ratio is severely restricted by the atmosphere, but in vacuum in theory an engine with an *infinite* expansion ratio would be best.