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Interesting. So Wiki says a fully-loaded B-52 can carry 35 tons of bombs. Everybody knows 1) re-entry after achieving orbital speeds is non-trivial, and 2) do

Interesting. So Wiki says a fully-loaded B-52 can carry 35 tons of bombs. Everybody knows 1) re-entry after achieving orbital speeds is non-trivial, and 2) doing a stationary drop from orbital heights is possible. There is even some talk of "skydiving from orbit" So you take a B-52 out over the ocean. At 50,000 MSL, you extend two rocket pods, point the nose straight up, and take yourself out of the atmosphere post-haste. Somewhere around 200-400,000 MSL, you roll over into a standard orbit configuration. Burn about half your fuel. Once you get to where you want to go, burn the other half, reach a dead stop, then parachute into the lower atmosphere, do an engine re-start, and be on your way. I'm not saying it wouldn't be a helluva ride, but I'm failing to see the part where we couldn't have done this 20 or 30 years ago. Perhaps the problem was that folks tried to imagine some sort of new propulsion system -- and the numbers just don't work out. Instead of solving SCRAMJET, maybe that money would have been better spent on truly reusable rockets and high-performance, plane-rated parachutes? ADD: This comment was meant to address the problem of sub-orbital flight from point A to point B, not getting completely to orbit. Both SSTO and suborbital flight were mentioned in the article. My point was that suborbital flight doesn't have as many problems as we might think it does.

TeMPOraL 11y ago
> Once you get to where you want to go, burn the other half, reach a dead stop, then parachute into the lower atmosphere, do an engine re-start, and be on your way. I think that would require a shit ton of fuel. If you want to actually achieve orbit, that's at least 7.8 km/s of Δv to do it, and that much again to reach "dead stop". Your B-52 would have to be carried on a booster rocket, as opposed to carrying one. A suborbital trajectory would make more sense, but that's generally what the US has ICBMs for.
- deleted 11y ago
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mnw21cam 11y ago
The amount of energy required to lift an object to 50,000 feet altitude under mach 1 is insignificant compared to the amount of energy required to achieve orbit. You're basically talking about making a rocket, but saving a very small amount of fuel by starting a little higher in the atmosphere. Then you double the delta-v required by having a dead stop in orbit. That much delta-v would require a very large multi-stage rocket indeed, which defeats the object of your exercise.
- DanielBMarkham 11y ago
  Yes. Apologies. I will make an addition to my comment. I was only addressing sub-orbital flight, not orbital flight. The initial article was about the old NY-to-Tokyo in 2 hours thing we've been reading about for ages. You cannot make orbit, but you should be able to go a long ways ballistically. Add: A great way to learn about the energies involved is to create your own simulator in your programming language of choice. The ballstic/orbital math isn't that hard, and you don't run into a huge number of problems just modeling how to get to orbit. I did this many years ago, and concluded that for non-biological items, mass drivers win hands-down. Unless the magic of field propulsion finally happens, taking your own fuel along with you sucks.
rayiner 11y ago
In your hypothetical, the initial flight is doing almost none of the real work and you might as well just launch that booster from the ground. Look at the rocket equation. You have to achieve a given delta-v to get into orbit. That drives the required mass ratio of the rocket. Getting up to 500 mph or whatever a B-52 can do basically has negligible impact on the required delta-v, so the require rocket size is no smaller. SCRAMJETS are interesting because they can get up to a significant fraction of the required delta-v while in air-breathing mode. They can do that because unlike a traditional jet engine, they don't have to slow air down to subsonic speeds for combustion. That's what drives the maximum speed of traditional jet engines. Edit: saw your other response. Delta-v is still an issue for suborbital trajectories. Hitting 100 km altitude requires a speed of about 3,200 mph. If you've got a subsonic air breathing stage, you're still not getting much benefit. The big benefit of SCRAMJETS for suborbital flight is that they can hit the required speeds in air-breathing mode the whole way. No need to carry around heavy oxidizer.
ashark 11y ago
It takes way more fuel to reach a dead stop in orbit than it does to just nudge yourself down into the atmosphere and let friction do the work for you. Waaaay more. And you have to lift all that extra fuel into orbit with you, which means even more fuel.
jessriedel 11y ago
"Suborbital" refers to a ballistic trajectory that is "close" to an orbital trajectory, so that the projectile follows an unpowered arc, but one that is not wide enough to achieve orbit. The energy required for suborbital trajectories is still in the same neighborhood as orbital flight. The reason your proposal doesn't work is that during the portion of the trajectory outside the atmosphere, you either need to be ballistic (and therefore suborbital if you're traveling intercontinental) or you need to be constantly directing a portion of your rockets thrust downward. The latter is fantastically inefficient, which is why planes operate in the atmosphere (where they can use air to convert efficient horizontal thrust to lift) and why suborbital rockets are ballastic for the large majority of their flight.
- mikeash 11y ago
  Suborbital just means you went to space but didn't achieve orbit. There's no requirement to be "close" to an orbital trajectory. Blue Origin's New Shepard and Virgin Galactic's SpaceShipOne are classic examples of suborbital spacecraft, and neither of those came anywhere near orbital speed.
- vpribish 11y ago
  Appreciate that you are trying to help here, but there is a lot that is simply wrong or confused in your comment. Suborbital does not imply "close to orbital", and does not require an amount of energy in the neighborhood of that for orbital flight, and ballistic/intercontinental/efficient-horizontal-thrust is just a very muddled way to think about this.
mikeash 11y ago
An empty B-52 is about 83 tons. Add in 35 tons of rocket propellant (assuming liquid hydrogen, others will be worse) with no additional tankage or engine weight and you get about 3.7km/s of delta-v. Expend half of that on a suborbital trajectory, and you get about an 85km trip. Not terribly useful.
Merad 11y ago
Your hypothetical is missing the part where the rocket pods tear the B-52 apart and a flaming ball of debris falls to earth. You see, to accelerate straight up (or nearly so) your thrust must be greater than your weight. To get enough acceleration to be useful, you usually need an amount of thrust that's more like 1.2x your mass. For example, the fully loaded Saturn V weighed about 6,500,000 lbs and the first stage had 7,650,000 lbs of thrust. So your fully loaded B-52 (~265,000 lbs) needs around 320,000 lbs of thrust from the rockets. This is more than double the amount of thrust produced by the jet engines. By the time you've accounted for your rocket engines, strengthened the airframe to survive the rocket thrust, and added a reaction control system (to turn around for deceleration), you've lost a good chunk of your 35 tons of rocket fuel. It's about this time you realize that even 35 tons of rocket fuel is a laughably small amount. A single space shuttle main engine is a bit more powerful than what you'd need (~420,000 lbs of thrust), but it's good enough for ballpark numbers. It will consume 35 tons of fuel in about a minute... meaning your mission profile is probably something like 30 seconds of vertical acceleration, 15 second horizontal acceleration, 15 seconds horizontal deceleration. Not enough to take you very high or very far. It's not that we can't do it, it's that it's woefully inefficient.

TeMPOraL 11y ago

> Once you get to where you want to go, burn the other half, reach a dead stop, then parachute into the lower atmosphere, do an engine re-start, and be on your way. I think that would require a shit ton of fuel. If you want to actually achieve orbit, that's at least 7.8 km/s of Δv to do it, and that much again to reach "dead stop". Your B-52 would have to be carried on a booster rocket, as opposed to carrying one. A suborbital trajectory would make more sense, but that's generally what the US has ICBMs for.

deleted 11y ago
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mnw21cam 11y ago

The amount of energy required to lift an object to 50,000 feet altitude under mach 1 is insignificant compared to the amount of energy required to achieve orbit. You're basically talking about making a rocket, but saving a very small amount of fuel by starting a little higher in the atmosphere. Then you double the delta-v required by having a dead stop in orbit. That much delta-v would require a very large multi-stage rocket indeed, which defeats the object of your exercise.

DanielBMarkham 11y ago
Yes. Apologies. I will make an addition to my comment. I was only addressing sub-orbital flight, not orbital flight. The initial article was about the old NY-to-Tokyo in 2 hours thing we've been reading about for ages. You cannot make orbit, but you should be able to go a long ways ballistically. Add: A great way to learn about the energies involved is to create your own simulator in your programming language of choice. The ballstic/orbital math isn't that hard, and you don't run into a huge number of problems just modeling how to get to orbit. I did this many years ago, and concluded that for non-biological items, mass drivers win hands-down. Unless the magic of field propulsion finally happens, taking your own fuel along with you sucks.

rayiner 11y ago

In your hypothetical, the initial flight is doing almost none of the real work and you might as well just launch that booster from the ground. Look at the rocket equation. You have to achieve a given delta-v to get into orbit. That drives the required mass ratio of the rocket. Getting up to 500 mph or whatever a B-52 can do basically has negligible impact on the required delta-v, so the require rocket size is no smaller. SCRAMJETS are interesting because they can get up to a significant fraction of the required delta-v while in air-breathing mode. They can do that because unlike a traditional jet engine, they don't have to slow air down to subsonic speeds for combustion. That's what drives the maximum speed of traditional jet engines. Edit: saw your other response. Delta-v is still an issue for suborbital trajectories. Hitting 100 km altitude requires a speed of about 3,200 mph. If you've got a subsonic air breathing stage, you're still not getting much benefit. The big benefit of SCRAMJETS for suborbital flight is that they can hit the required speeds in air-breathing mode the whole way. No need to carry around heavy oxidizer.

ashark 11y ago

It takes way more fuel to reach a dead stop in orbit than it does to just nudge yourself down into the atmosphere and let friction do the work for you. Waaaay more. And you have to lift all that extra fuel into orbit with you, which means even more fuel.

jessriedel 11y ago

"Suborbital" refers to a ballistic trajectory that is "close" to an orbital trajectory, so that the projectile follows an unpowered arc, but one that is not wide enough to achieve orbit. The energy required for suborbital trajectories is still in the same neighborhood as orbital flight. The reason your proposal doesn't work is that during the portion of the trajectory outside the atmosphere, you either need to be ballistic (and therefore suborbital if you're traveling intercontinental) or you need to be constantly directing a portion of your rockets thrust downward. The latter is fantastically inefficient, which is why planes operate in the atmosphere (where they can use air to convert efficient horizontal thrust to lift) and why suborbital rockets are ballastic for the large majority of their flight.

mikeash 11y ago
Suborbital just means you went to space but didn't achieve orbit. There's no requirement to be "close" to an orbital trajectory. Blue Origin's New Shepard and Virgin Galactic's SpaceShipOne are classic examples of suborbital spacecraft, and neither of those came anywhere near orbital speed.
vpribish 11y ago
Appreciate that you are trying to help here, but there is a lot that is simply wrong or confused in your comment. Suborbital does not imply "close to orbital", and does not require an amount of energy in the neighborhood of that for orbital flight, and ballistic/intercontinental/efficient-horizontal-thrust is just a very muddled way to think about this.

mikeash 11y ago

An empty B-52 is about 83 tons. Add in 35 tons of rocket propellant (assuming liquid hydrogen, others will be worse) with no additional tankage or engine weight and you get about 3.7km/s of delta-v. Expend half of that on a suborbital trajectory, and you get about an 85km trip. Not terribly useful.

Merad 11y ago

Your hypothetical is missing the part where the rocket pods tear the B-52 apart and a flaming ball of debris falls to earth. You see, to accelerate straight up (or nearly so) your thrust must be greater than your weight. To get enough acceleration to be useful, you usually need an amount of thrust that's more like 1.2x your mass. For example, the fully loaded Saturn V weighed about 6,500,000 lbs and the first stage had 7,650,000 lbs of thrust. So your fully loaded B-52 (~265,000 lbs) needs around 320,000 lbs of thrust from the rockets. This is more than double the amount of thrust produced by the jet engines. By the time you've accounted for your rocket engines, strengthened the airframe to survive the rocket thrust, and added a reaction control system (to turn around for deceleration), you've lost a good chunk of your 35 tons of rocket fuel. It's about this time you realize that even 35 tons of rocket fuel is a laughably small amount. A single space shuttle main engine is a bit more powerful than what you'd need (~420,000 lbs of thrust), but it's good enough for ballpark numbers. It will consume 35 tons of fuel in about a minute... meaning your mission profile is probably something like 30 seconds of vertical acceleration, 15 second horizontal acceleration, 15 seconds horizontal deceleration. Not enough to take you very high or very far. It's not that we can't do it, it's that it's woefully inefficient.