29 ms·

I don't understand the math of this sort of launch vehicle. Any rocket launched at a high altitude still needs to accelerate up to orbital velocity, which is o

• proaralyst 8y ago
  I thought the argument was that rockets spend a lot of fuel just getting through the thick low part of the atmosphere, which is the same point at which they're heaviest. Skip the start & you need less fuel.
  • Quequau 8y ago
    But it's not just altitude, right? Acceleration and Speed are important factors to getting to orbit (not that I'm a rocket surgeon).
    • alonmower 8y ago
      Drag at low altitudes is much worse, so perhaps just bringing it to a higher altitude is that much more efficient?
      • sitkack 8y ago
        Build a 3 mile tall launch platform that is also an EM rail. Or better yet, BORE a tunnel through a really tall mountain. https://en.wikipedia.org/wiki/Pico_de_Orizaba https://en.wikipedia.org/wiki/Pico_de_Orizaba
    • throwanem 8y ago
      Right. The closer to sea level you are, the denser the atmosphere you're pushing out of the way as you accelerate - thus, the more force you need to use in doing so, and the lower the acceleration you obtain from the propellant you expend. So if you can get above the thickest part of the atmosphere, say attached to a giant goofy-looking airplane, before you light off your engine - rather than having to push your way through that same thick air on your own - you need less propellant, overall, to reach orbit, which means you can devote a larger fraction of your launch vehicle's mass to payload.
      • Retric 8y ago
        Rockets are not moving very quickly in the lower atmosphere, which makes a real difference. Space shuttle was 3 g - 1 from gravity = 2g net acceleration, but that's kept low for passengers. Even then 2 g net ~= + 44 MPH every second. So, first 15 seconds your going under 660 MPH which is not that fast, but you get ~1.4 miles up. Because drag increases with speed very quickly the next bit is harder, but you very quickly get above what an aircraft could take you with ~5.6 miles at 30 seconds. Note: Higher g's mean more drag in denser atmosphere, but less gravity drag it's a meaningful trade-off. However, aircraft's velocity is very useful essentially saving those first 15 seconds of full burn, but comes at the price of needing more structural elements to support the hanging rocket.
  • sandworm101 8y ago
    >> a lot of fuel just getting through the thick low part of the atmosphere That statement is misunderstood. Google "rocket ISP". Rocket engines work less efficiently at lower altitudes, and cannot be properly optimized due to the rapidly changing pressures between sea level and the near-vacuum at altitude. They burn extra fuel due to this inefficiency. And since rockets are at their heaviest in the lower atmosphere, even little inefficiencies really add up. Atmospheric drag is next to nothing in the equation. Even speed isn't the issue. It is about how they cannot design an efficient engine bell to cover the initial climb.
  • fermienrico 8y ago
    I've also heard that fuel is not the main cost contributor to a rocket launch. In fact, it is a really insignificant portion of the cost. It is the vessel that contains giant amounts of fuel that is costly. Hence reusability from SpaceX makes sense and so does launching the rocket from high altitude to avoid building the first stage all together.
• wanderingjew 8y ago
  If you're above the the weather, you can launch any time.
  • maxxxxx 8y ago
    This plane doesn't look like it can handle bad weather well either.
    • bdamm 8y ago
      The idea would be to launch somewhere other than the bad weather. Airplanes can move around to avoid weather. Another advantage would be more flexibility in launch window; the airplane can launch and get high up, then linger into a specific launch timeframe at a specific place, rather than being fixed in place and only changing launch time.
      • elteto 8y ago
        This doesn't make much sense... yes, you can have two fully operational launch sites in different geographical locations but the payload can't be in two places at once. If you have bad weather where the payload is then you won't launch until after is passed.
        • bdamm 8y ago
          The plane can take off ahead of the bad weather and go to where weather is predicted to be better. Or, the plane can take off, go to where weather is better, then take off and fly above the troposphere in the original location and launch on time and on place.
    • presscast 8y ago
      Why?
      • maxxxxx 8y ago
        It just looks fragile to me. I may be wrong.
• mikeash 8y ago
  The rocket equation is nasty. Adding a bit more energy requires exponentially more effort. This is why orbital rockets are staged: past a certain point, you hit diminishing returns with making a single stage bigger and bigger, and it works out better to stack it on top of another stage that does the extra work. Think of the airplane part of this as an efficient, highly reusable first stage with poor performance.
  • bdamm 8y ago
    Maybe the first 1/2 stage.
  • theothermkn 8y ago
    The converse of this is that the first bit of velocity you save pays off handsomely. Every multiple of the exhaust velocity that you save nets you a factor of e savings in mass ratio. Additionally, getting out of the soup of the lower atmosphere improves the average efficiency of your engine. Again, this is a bigger deal, relatively speaking, when you’re saving the first bit of velocity off the top, as you’re multiplying on a larger launch vehicle.
• dexen 8y ago
  Air launch (to orbit)[1] has been a thing for a good while now. As others have stated, fuel savings are enormous; jet engines have orders of magnitude higher efficiency than rocket engines when taking fuel tankage into account. Air resistance is barely an issue, and same for the gravity drag. But most importantly, launch mass is greatly reduced in part thanks to not having to carry oxidizer for the first stage. Another factor, rocket's first stage must have powerful, thus heavy, engines with enough thrust to both counter gravity and impair acceleration; an orbital insertion stage can do with much lower TWR, thus again mass savings for air launch. Point in case, note the size of the Pegasus orbital rocket [2]. Or the orbital anti-satellite missile, sized like a typical cruise missile, all thanks to air launch [3] from a typical fighter jet. Now that SpaceX is able to recover the first stages from some launches, the cost of first stage is a bit less of a factor, but still there are other considerations, like ability to launch from any decent runway vs having to maintain a dedicated facility, unusable for any other commercial purposes, far away from humans due to incredible amount of noise rocket engines create. [1] https://en.wikipedia.org/wiki/Air_launch_to_orbit https://en.wikipedia.org/wiki/Air_launch_to_orbit [2] https://en.wikipedia.org/wiki/Pegasus_(rocket) https://en.wikipedia.org/wiki/Pegasus_(rocket) [3] https://en.wikipedia.org/wiki/ASM-135_ASAT https://en.wikipedia.org/wiki/ASM-135_ASAT
  • greglindahl 8y ago
    As you can tell from air launch not dominating the market, there are minuses to it, too. Pegasus has extra mass because it's got aerodynamic surfaces that traditional rockets don't need. And it takes a lot of gear at the remote airstrip to support the launch. https://spaceflightnow.com/2018/06/08/airplane-with-pegasus-rocket-returning-to-california-postponing-nasa-satellite-launch/ https://spaceflightnow.com/2018/06/08/airplane-with-pegasus-...
• sandworm101 8y ago
  Air pressure. Google "ISP". Rockets are more efficient at lower air pressures, preferably vacuum. But a rocket (really just the bell/nozzle bit where the fire comes out) must still be designed to operate in a particular air pressure. The problem with the first stage of every rocket is that is must travel between sea level a near-vacuum at high altitude. So you cannot ever reach proper efficiencies. By launching at altitude they can optimize for vacuum operation without worry about sea level performance. We are talking about roughly 10~% margins here, but rocket companies can spend billions on 1-2% improvements. Also, by launching from an aircraft over an ocean one can avoid many of the ground realities of rocket operations. Land-based sites much deal with shipping notices and noise issues. Stratospheric can fire these things in different directions, to different orbits, without worry. They might also be able to launch multiple rockets on a single flight, a totally new service that might hit an unexplored market. Lastly, politics. Stratolaunch could travel the world to launch rockets from basically any big airport. So if the Saudis want to launch something without the hassle of US/Russian security checks, they can have Stratospheric show up to handle the launch locally. There are probably enough countries wanting to launch something locally for prestige alone. Or... someone just wants to have the biggest airplane in the world (paging Dr. Freud) and this is the way to make that happen.
  • walrus01 8y ago
    > . So if the Saudis want to launch something without the hassle of US/Russian security checks, they can have Stratospheric show up to handle the launch locally. No, because stratolaunch as a US company is subject to ITAR, and its rockets will be built with ITAR restricted tech.
    • sandworm101 8y ago
      SA is a regular importer/exporter of ITAR stuff, including weaponry. I'm sure they would have no problems getting Stratolaunch to show up for a few launches. But getting a Saudi sat, a defense-related sat, onto a US rocket would take years, perhaps a decade.
• mabbo 8y ago
  A few points, from a mostly-laymen point of view. When you watch a SpaceX launch they actually have to really dial down their acceleration to get through Max-Q, the highest dynamic pressure, due to air resistance. Air resistance is a function of speed SQUARED and the density of air. By starting the rocket that much higher, I believe they are essentially removing that inefficiency- by the time they're moving fast enough for it to matter, there isn't enough air for it to matter. There's also the whole tyranny of rocketry equation. The first foot of elevation gained is the hardest and the first mile per hour of speed is the hardest. Starting even a little bit higher and faster is a big win over taking off from sea level. And as others have said, they've got incredible flexibility here. Whatever orbit you want to be in, the launch will take place in that orbit. A lot of traditional rocket launches have 'instantaneous' launch windows where missing by 1 second is a no-go because the ground is only in the right spot at one moment. Most important of all: let them try! If this whole thing is a bust, it's private enterprise and you and I lose nothing over it while getting to see an enormous jet. And if it does work, we all win from the increase in availability of satellites and access to space.
• InclinedPlane 8y ago
  There are legitimate advantages to high altitude launch. A big one, perhaps the biggest, is that exhaust pressure of a rocket needs to be at least as high as the ambient atmospheric pressure at launch. This limits the expansion of exhaust and the sizes of nozzles for rockets that are launched at sea level. A notable example is the Falcon 9 launcher, which has 9 engines on the first stage and one similar engine with an engine bell that is almost the same diameter as the entire rocket for the second stage which only operates in a near-vacuum environment. Higher expansion translates to higher exhaust velocity (Isp) for identical propellants. Since the rocket equation is exponential with respect to the ratio of delta-V and exhaust velocity, even slightly higher exhaust velocity can translate to significant increases in overall stage performance. To use the example of the Falcon 9 again, consider the second stage with a dry mass of 4 tonnes, 107.5 tonnes of propellant, and an exhaust velocity of 3.41 km/s. With a 5 tonne payload the stage can achieve a delta-V of: ln((107.5 + 4 + 5)/(4 + 5)) * 3.41 km/s = 8.7 km/s. Now, if we substitute the first stage sea level exhaust velocity of just 2.76 km/s the overall delta-V is 7.1 km/s, and to get to the same delta-V (8.7 km/s) now requires a mass ratio of 23.4:1, resulting in an effective payload of just 0.8 tonnes, less than 1/6th as much as the original payload! In actuality the overall performance differences won't be quite as severe but this does point out the potential for a rocket launched from higher altitude to have equivalent performance to a much larger rocket launched from sea level. Another major advantage is operational flexibility. You can launch from a wider variety of launch locations, which might make it easier to launch in a more timely manner, or could make it easier to launch certain kinds of payloads (which don't need to be shipped around the world, the launcher can come to you). And potentially it makes it possible to launch from more equatorial locations, gaining a slight orbital mechanics advantage. However, all of this comes at some pretty steep costs. With this design it's harder to make incremental changes to the vehicle. One of the classic mechanisms for improving launcher performance is stretching stages, but that's out of the question here. You can't make the rocket bigger without making the carrier aircraft bigger as well. Additionally, and most damning, the tradeoff here is a bad one. On the one hand you have a sea level launched rocket which needs to expend more propellant to reach orbit. However, propellant is dirt cheap, it's the cheapest part of a launch pretty much. Especially for a LOX/Kerosene rocket the propellant isn't even going to crack 7 digits in terms of cost, even for a big rocket. With high altitude launch some dirt cheap propellant is saved at the cost of an inordinately expensive carrier aircraft, and a very complex and dangerous flight profile (you need to have a piloted aircraft takeoff with a fully fueled rocket). Moreover, you've now cut off all of the most straightforward avenues for incremental improvements of the launch system: you can't make any part of it bigger without starting from square one with a new iteration. And in a world where reusable rockets are becoming ever more commonplace this makes even less sense.