6 ms·

There are several advantages for airplanes as first stages. Less air drag on the higher altitude and more efficient engines, as they are optimized for smaller e

by avmich 8y ago

There are several advantages for airplanes as first stages. Less air drag on the higher altitude and more efficient engines, as they are optimized for smaller external air pressure. The speed of aircraft adds to the speed of the rocket. Another plus is that with aircraft you don't need to build a launch pad, the rocket is just released in mid-air. Yet another benefit is that aircraft can fly to a more preferable point for the launch - for example, the point where it is convenient to drop the first stage of the rocket, or the point closer to the equator so the satellite won't need to spend as much fuel to change the orbital plane. The airplane first stage can potentially fly to very different launch point, in many different countries, which can make logistics simpler. Those advantages all add up economically.

duxup 8y ago
I hadn't thought of the fixed launch pad situation. Do they expend a lot of energy / cost moving a satellite from where a fixed launch pad might easily place the satellite vs it's proffered orbit?
- myself248 8y ago
  Mostly they have to pick their launch site so the flight path, headed into the right orbital plane, doesn't pass over populated areas. It would be rude to drop boosters and stages on people. https://history.nasa.gov/EP-165/p25.htm https://history.nasa.gov/EP-165/p25.htm
  - m4rtink 8y ago
    Well, while possibly rude it still happens sometimes: https://www.extremetech.com/extreme/263732-china-keeps-dropping-expended-rocket-boosters-near-villages https://www.extremetech.com/extreme/263732-china-keeps-dropp... Will likely no longer be an issue once the new chinese spaceport in the south fully comes online.
    - walrus01 8y ago
      Interesting fact, Israel is one of the few countries on earth that regularly launches into retrogade LEO (first stage flight path over ocean), because of the extreme political considerations around lobbing something that looks like an ICBM over their eastern neighbours. And the problem with dropping spent first stages on everywhere east of them.
      - myself248 8y ago
        Oh great, so if ASAT activity blows any of their stuff up, it'll have even more velocity relative to the other denizens of similar orbital planes.
  - duxup 8y ago
    If they were the self landing type and it landed in the local wall-mart parking lot.... would be kinda cool. The uncontrolled versions, less entertaining.
inamberclad 8y ago
I asked the engineers a while back, and they said the biggest gain was in potential energy that the rocket no longer has to make up.
ThenAsNow 8y ago
I've a detailed comment that addresses several of the points you're making at https://news.ycombinator.com/item?id=19660163 https://news.ycombinator.com/item?id=19660163 Is it true, strictly speaking, that you may use less propellant in the expendable stage due to air-drop? Sure. Although to be clear when talking about engine efficiency, it's really about being able to run a larger area ratio nozzle and getting a higher mission-averaged Isp. But ultimately when you consider all the "puts and takes" in launch system economics, those considerations are small potatoes (price out bulk LO2, and your choice of fuel). People (even university professors make this mistake) get so focused on minimizing propellant use that they fail to realize that complexity and dry weight are orders of magnitude more important. The only serious advantages to air-launch are the logistical ones you noted. Also note that for U.S.-based systems, the idea that you will be able to fly down to equatorial latitudes requires you have the range to carry the laden launch system (and top-off to contend with LOX boil-off) pretty far. Strato certainly doesn't have these kinds of legs. Virgin will probably be able to get more out of their system, but it's not going to scale significantly higher.
- nbadg 8y ago
  I agree that for traditional launch systems it doesn't make that much sense -- particularly in light of the recent development of reusable (or at least, partially reusable) rockets. That being said, I think the math changes somewhat for smaller launchers. The smaller the rocket, the worse the losses to the atmosphere get, and the more advantageous it is to escape atmospheric resistance in the soupy low atmosphere. So I think where air-launch systems could really shine, is by enabling low-cost launch of very small payloads. I am, however, not entirely convinced that a sufficient market for those kinds of dedicated launch services will emerge (as opposed to hitching a ride in spare fairing space of a separate launch). I could see this going either way; as much as I'd like to see easier space access for eg. university-funded nanosats, I'm not sure the timing will be right for something like Stratolaunch, because the new private aerospace (SpaceX, Blue Origin, etc) is focused so intensely on pushing down the cost of larger-scale rocketry, to a point where that same market would be better served by bulk launches using some kind of standardized dispenser.
  - 8y ago
afterburner 8y ago
> Another plus is that with aircraft you don't need to build a launch pad That one doesn't seem like an advantage, a launch pad is much cheaper than the world's largest really odd-looking and unique airplane.
- avmich 8y ago
  Yes, but for a launcher you may want to have several launchpads for different kinds of missions, say GEO and SSO launches. Strato can serve them all. And if something happens to launch pad after rocket ignition, you have to rebuild the pad, while with Strato if you released the rocket and flew away, rocket problems don't affect the plane.
razster 8y ago
They’re going to need to tighten up the landing. After touch down and braking, it was swing left and right. Seems kinda worrying
hawski 8y ago
How does a rocket deal with those side stresses while it's attached to the airplane? AFAIK rockets generally do not deal well with sideways forces, because a rocket is a giant can.
- mirimir 8y ago
  It's clear that solid-fuel rockets are OK with sideways forces, because they're transported that way. The can may be weak (except re internal pressure, obviously) but the fuel/oxidizer is ~like rubbery plastic. It's also clear that liquid-fuel rockets are OK with sideways forces, before they're loaded with fuel and oxidizer. Because they're so light. But once they've been loaded, they're very fragile re sideways forces. I've watched lots of rocket-failure video. A ~common scenario is loss of attitude control, for whatever reason, followed by crumpling and fireball. So I don't see how this thing could launch liquid-fuel rockets. Unless it could fly straight up, for long enough to transfer fuel and oxidizer. That'd be insane.
  - ThenAsNow 8y ago
    A liquid rocket vehicle would have to be designed to be carried and launched from a carrier vehicle like Strato has, but that's eminently doable. You design in the load paths suitable for carriage and also for engine-on ascent loads. There will be more structure attached to the cylindrical sections. But this added structural mass further detracts from the delta V advantages of being released from a "Stage 0" air platform at altitude and speed. And yes, you also design for the bending loads associated with acquiring the ascent flight path angle you need. Those bending loads will be more severe than what a well-guided ground-launched system will be designed to endure. Strato's aircraft undoubtedly pitches nose-up pretty hard before dropping the rocket, but it's definitely nowhere near vertical. Solid motor casings are inherently quite stiff, to deal with combustion pressures. There would be less attachment/carriage hardware mass penalty than with liquids. Bottom line, there is nothing inherent to Stratolaunch that makes it incapable of launching liquids, and even with the additional structural mass penalty, it's entirely possible for Strato to have lower cost per unit of mass to orbit via a liquid rocket stack than with solids (likely, in fact).

by avmich 8y ago

duxup 8y ago

I hadn't thought of the fixed launch pad situation. Do they expend a lot of energy / cost moving a satellite from where a fixed launch pad might easily place the satellite vs it's proffered orbit?

myself248 8y ago
Mostly they have to pick their launch site so the flight path, headed into the right orbital plane, doesn't pass over populated areas. It would be rude to drop boosters and stages on people. https://history.nasa.gov/EP-165/p25.htm https://history.nasa.gov/EP-165/p25.htm
- m4rtink 8y ago
  Well, while possibly rude it still happens sometimes: https://www.extremetech.com/extreme/263732-china-keeps-dropping-expended-rocket-boosters-near-villages https://www.extremetech.com/extreme/263732-china-keeps-dropp... Will likely no longer be an issue once the new chinese spaceport in the south fully comes online.
  - walrus01 8y ago
    Interesting fact, Israel is one of the few countries on earth that regularly launches into retrogade LEO (first stage flight path over ocean), because of the extreme political considerations around lobbing something that looks like an ICBM over their eastern neighbours. And the problem with dropping spent first stages on everywhere east of them.
    - myself248 8y ago
      Oh great, so if ASAT activity blows any of their stuff up, it'll have even more velocity relative to the other denizens of similar orbital planes.
- duxup 8y ago
  If they were the self landing type and it landed in the local wall-mart parking lot.... would be kinda cool. The uncontrolled versions, less entertaining.

inamberclad 8y ago

I asked the engineers a while back, and they said the biggest gain was in potential energy that the rocket no longer has to make up.

ThenAsNow 8y ago

I've a detailed comment that addresses several of the points you're making at https://news.ycombinator.com/item?id=19660163 https://news.ycombinator.com/item?id=19660163 Is it true, strictly speaking, that you may use less propellant in the expendable stage due to air-drop? Sure. Although to be clear when talking about engine efficiency, it's really about being able to run a larger area ratio nozzle and getting a higher mission-averaged Isp. But ultimately when you consider all the "puts and takes" in launch system economics, those considerations are small potatoes (price out bulk LO2, and your choice of fuel). People (even university professors make this mistake) get so focused on minimizing propellant use that they fail to realize that complexity and dry weight are orders of magnitude more important. The only serious advantages to air-launch are the logistical ones you noted. Also note that for U.S.-based systems, the idea that you will be able to fly down to equatorial latitudes requires you have the range to carry the laden launch system (and top-off to contend with LOX boil-off) pretty far. Strato certainly doesn't have these kinds of legs. Virgin will probably be able to get more out of their system, but it's not going to scale significantly higher.

nbadg 8y ago
I agree that for traditional launch systems it doesn't make that much sense -- particularly in light of the recent development of reusable (or at least, partially reusable) rockets. That being said, I think the math changes somewhat for smaller launchers. The smaller the rocket, the worse the losses to the atmosphere get, and the more advantageous it is to escape atmospheric resistance in the soupy low atmosphere. So I think where air-launch systems could really shine, is by enabling low-cost launch of very small payloads. I am, however, not entirely convinced that a sufficient market for those kinds of dedicated launch services will emerge (as opposed to hitching a ride in spare fairing space of a separate launch). I could see this going either way; as much as I'd like to see easier space access for eg. university-funded nanosats, I'm not sure the timing will be right for something like Stratolaunch, because the new private aerospace (SpaceX, Blue Origin, etc) is focused so intensely on pushing down the cost of larger-scale rocketry, to a point where that same market would be better served by bulk launches using some kind of standardized dispenser.
- 8y ago

afterburner 8y ago

> Another plus is that with aircraft you don't need to build a launch pad That one doesn't seem like an advantage, a launch pad is much cheaper than the world's largest really odd-looking and unique airplane.

avmich 8y ago
Yes, but for a launcher you may want to have several launchpads for different kinds of missions, say GEO and SSO launches. Strato can serve them all. And if something happens to launch pad after rocket ignition, you have to rebuild the pad, while with Strato if you released the rocket and flew away, rocket problems don't affect the plane.

razster 8y ago

They’re going to need to tighten up the landing. After touch down and braking, it was swing left and right. Seems kinda worrying

hawski 8y ago

How does a rocket deal with those side stresses while it's attached to the airplane? AFAIK rockets generally do not deal well with sideways forces, because a rocket is a giant can.

mirimir 8y ago
It's clear that solid-fuel rockets are OK with sideways forces, because they're transported that way. The can may be weak (except re internal pressure, obviously) but the fuel/oxidizer is ~like rubbery plastic. It's also clear that liquid-fuel rockets are OK with sideways forces, before they're loaded with fuel and oxidizer. Because they're so light. But once they've been loaded, they're very fragile re sideways forces. I've watched lots of rocket-failure video. A ~common scenario is loss of attitude control, for whatever reason, followed by crumpling and fireball. So I don't see how this thing could launch liquid-fuel rockets. Unless it could fly straight up, for long enough to transfer fuel and oxidizer. That'd be insane.
- ThenAsNow 8y ago
  A liquid rocket vehicle would have to be designed to be carried and launched from a carrier vehicle like Strato has, but that's eminently doable. You design in the load paths suitable for carriage and also for engine-on ascent loads. There will be more structure attached to the cylindrical sections. But this added structural mass further detracts from the delta V advantages of being released from a "Stage 0" air platform at altitude and speed. And yes, you also design for the bending loads associated with acquiring the ascent flight path angle you need. Those bending loads will be more severe than what a well-guided ground-launched system will be designed to endure. Strato's aircraft undoubtedly pitches nose-up pretty hard before dropping the rocket, but it's definitely nowhere near vertical. Solid motor casings are inherently quite stiff, to deal with combustion pressures. There would be less attachment/carriage hardware mass penalty than with liquids. Bottom line, there is nothing inherent to Stratolaunch that makes it incapable of launching liquids, and even with the additional structural mass penalty, it's entirely possible for Strato to have lower cost per unit of mass to orbit via a liquid rocket stack than with solids (likely, in fact).