I did a post on this a few years back, but couldn’t find it to repost.
A commentor in response to one of Rand Simbergs’ articles brought up (again) the old orbital is 8 times the velocity of suborbital and therefore 64 times as hard because energy goes as the square of velocity. Where to begin arguing this again?
The argument was that suborbital is Mach three vs orbital being over Mach twenty-four. Mach three won’t get you to space. 900 meters per second (~Mach 3) vertical velocity will get you 90 seconds of climb at an average of 450 meters per second, or about 40.5 kilometers of altitude. That doesn’t include drag, gravity losses during acceleration or engine back pressure losses. Mach 4 gets you 72 kilometers if you are in vacuum the whole time. It would take nearly Mach 5 to reach the Von Karmin line from sea level if you were in vacuum the whole way. 1,430 meters per second is considerably more than the 900 meters per second implied by the comment.
That was a straight up and down in vacuum from sea level. The flight doesn’t start in vacuum. Back pressure losses on a rocket engine gives 5-20% less performance at sea level than in vacuum. That is a major Isp hit on performance to a vehicle with a finite propellant capacity and facing exponential weight gain for excess mass. That 5-20% more propellant for a given thrust must be lifted by even more propellant. That extra propellant requires a larger airframe and engine capacity. The larger engine and airframe requires more propellant and so on. A suborbital vehicle has just as much back pressure loss as an orbital vehicle .
The vehicle has drag while it is accelerating toward space. The drag costs more propellant equals more engine and airframe, equals even more propellant and so on. If too much acceleration is done too early drag goes up by several more factors which would cost more propellant etc… Another aspect of the drag is heating if too much time is spent at high mach during the climb in the atmosphere. Thermal protection can get up there fast with a poor design. A suborbital vehicle has just as much drag loss in the climb out as an orbital vehicle.
There are gravity losses during the climb out. A suborbital vehicle has just as much drag loss as an orbital vehicle.
Actual mass ratios for suborbital above the Von Karmin line tend to be in the 3 to 4 range. If Mach 3 in vacuum was the whole story, then mass ratios would be in the 1.3 to 1.5 range. Orbital mass ratios tend to be in the 10 to 20 range depending on propellant choices and engine efficiency. The difference in mass ratios is about 4 to 5 with similar propellants. That is undeniably a major difference and order of difficulty. Just not 64 times the difficulty, especially with staging.
Life support is clearly different for manned vehicles. Bottled air for 30 minutes is different than 30 days for one. Food, toilet facilities, and creature comforts are quite different as well. This particular aspect may well be 64 times as hard. I doubt it, but it is possible.
Acceleration couches, controls, and displays will be about the same. More propellant for the RCS with the same number of thrusters is not that much more difficult. Communications will be more complex if the mission requires, though not an order of magnitude, and certainly not 64 times as hard.
Unmanned vehicle payloads have even less need for the 64 times as expensive meme. The energy supply is about the only thing that is clearly in a different league, though solar panels are getting to be quite well known even there.
Look at the historical record for more data. Working backwards from Elons’ billion investment in orbit with the 64 times as hard, any suborbital company spending more than 15 million should have been providing reliable service. Count for yourself the ones that have spent over 15 million and still don’t have an operational vehicle.
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