Jet powered stage?

[Jas0n]

Would be feasible to have a rocket launcher powered part way by jet? Or is the max speed of traditional turbofan/turbojets too low for an orbital rocket? (I know that stratolaunch is a thing)

If a turbojet/turbofan wouldn't work, would a ramjet work? maybe have a small first stage to bring it up to functional speeds?

And if these are feasible, would it be more efficient than traditional rockets? 

 

[sevenperforce]

8 minutes ago, Jas0n said:

Would be feasible to have a rocket launcher powered part way by jet? Or is the max speed of traditional turbofan/turbojets too low for an orbital rocket? (I know that stratolaunch is a thing)

If a turbojet/turbofan wouldn't work, would a ramjet work? maybe have a small first stage to bring it up to functional speeds?

And if these are feasible, would it be more efficient than traditional rockets? 

 

Short answer: no.

Longer answer: also no.

Airbreathing turbine engines are very heavy, so they do not have a high enough thrust-to-weight to take off vertically, not with any appreciable payload. Thus, they need to take off horizontally, which means you're back to air-launch.

The thrust-to-weight ratio of a ramjet is a bit better, but then you have to deal with getting up to speed first, which probably means separate rocket engines. And even then your useful envelope is low, and you're accelerating more and more slowly as you climb, when you want to be accelerating faster and faster.

Airbreathing engines are good at efficient cruise, bad at continuous acceleration.

 

[Jas0n]

10 minutes ago, sevenperforce said:

Short answer: no.

Longer answer: also no.

Airbreathing turbine engines are very heavy, so they do not have a high enough thrust-to-weight to take off vertically, not with any appreciable payload. Thus, they need to take off horizontally, which means you're back to air-launch.

The thrust-to-weight ratio of a ramjet is a bit better, but then you have to deal with getting up to speed first, which probably means separate rocket engines. And even then your useful envelope is low, and you're accelerating more and more slowly as you climb, when you want to be accelerating faster and faster.

Airbreathing engines are good at efficient cruise, bad at continuous acceleration.

 

Ah I see. Thanks for the answer. 

[kerbiloid]

If the whole atmosphere were having the sea level density, it's height / thickness would be about 8 km.

So if accelerating vertically (as a rocket, rather than a plane) you have less than 8 km of air to breathe.

If your intakes total area is, say, 10 m², this means that total mass of the air on your way is ~8000 * 10 * 1.225 ~100 t, including just 20 t of oxygen.
Say, just 10 t of oxygen you can really collect. This makes no sense.

[p1t1o]

I supposed you could call the B-52 mothership of the Pegasus launch vehicle a "jet powered stage". Amongst various other similar arangements.

[NSEP]

You might be able to use it as a boost to get you to a slightlyl ower atmosphere though.

Edited August 18, 2017 by NSEP

[wumpus]

The reason this works in KSP is that Kerbin needs ~3km/s delta-v to get into orbit.  Earth needs ~9km/s to get into orbit.  Kerbal jets go faster than "real" jets, but the big reason KSP SSTO is a thing is that jets can get so close to orbital velocity on their own.

The SR-71 blackbird had more delta-v than any other air-breathing aircraft.  It also had a prototype means of launching a drone/rocket, but that was canceled after one death in two flights.  Even so, 1km/s is not all that significant when you need 9km/s delta-v.

The X-43 is my favorite example of at least a possibility for this type of thing.  The important thing to remember is that while the X-43 set some amazing speed records for maintaining air-breathing flight, it couldn't significantly accelerate at mach ~9, and I doubt it could get to mach 6 on its own (from anything less than mach 3, and even that is iffy).  Look at a picture of it: tiny little aircraft, massive solid rocket booster to get to speed.

KSP's favorite youtuber explains the benefits of mountain launch (oddly enough, altitude *does* matter for air-launch although not in the way most people expect).
https://www.youtube.com/watch?v=RsbDRDFVObE

 

[sevenperforce]

3 hours ago, wumpus said:

The reason this works in KSP is that Kerbin needs ~3km/s delta-v to get into orbit.  Earth needs ~9km/s to get into orbit.  Kerbal jets go faster than "real" jets, but the big reason KSP SSTO is a thing is that jets can get so close to orbital velocity on their own.

And even in KSP, it's barely break-even unless you're using the RAPIER engine.

Try using any jet engine other than the RAPIER for a first stage (vertical launch) and see whether you can get better overall mass ratios than an optimized equivalent-mass rocket stage.

Edited August 18, 2017 by sevenperforce

[magnemoe]

23 hours ago, sevenperforce said:

Short answer: no.

Longer answer: also no.

Airbreathing turbine engines are very heavy, so they do not have a high enough thrust-to-weight to take off vertically, not with any appreciable payload. Thus, they need to take off horizontally, which means you're back to air-launch.

The thrust-to-weight ratio of a ramjet is a bit better, but then you have to deal with getting up to speed first, which probably means separate rocket engines. And even then your useful envelope is low, and you're accelerating more and more slowly as you climb, when you want to be accelerating faster and faster.

Airbreathing engines are good at efficient cruise, bad at continuous acceleration.

This, an first stage typical go from 0 to 60 km who is pretty much space, here it moves at 2km/s who is twice ramjet speed.  
Air launch make sense for very small rockets, you can start with an vacuum nozzle and you are out of the high drag area. 

[kerbiloid]

18 hours ago, p1t1o said:

I supposed you could call the B-52 mothership of the Pegasus launch vehicle a "jet powered stage"

Unlikely 900 km/h of B-52 really add much for a 4-staged launch vehicle.

Pegasus looks like not a way to make the launch cheaper or so, but the way to launch satellites when launchpads are bombed out and gone.

[YNM]

Rocket and the jet stage staging :

[Jas0n]

Thanks for everyone's answers. My only experience with rocket science is in KSP where jet engines are way more powerful.

I thought the point of launching rockets from planes is so that the rocket itself doesn't have go up as far, and that it can be launched from any latitude?

[YNM]

8 minutes ago, Jas0n said:

Thanks for everyone's answers. My only experience with rocket science is in KSP where jet engines are way more powerful.

Finally ! Welcome to the wild world. 

KSP is a game, despite being "pretty realism-complete". Your standard jet engines IRL is only going up to like 30 km, and while they do about the same in-game, in-game atmo is only 70 km while IRL it's 100 km and Kerbin's atmo is 10% radii while ours is only 1.5% radii. There's not enough air for the engines to run into without making ProblemS for everyone.

8 minutes ago, Jas0n said:

I thought the point of launching rockets from planes is so that the rocket itself doesn't have go up as far, and that it can be launched from any latitude?

Yes. But that's it. They're barely helping with reaching orbital velocities. And height, in case of a converted or normal turbofan aircrafts.

Edited August 19, 2017 by YNM

[wumpus]

21 hours ago, YNM said:

Yes. But that's it. They're barely helping with reaching orbital velocities. And height, in case of a converted or normal turbofan aircrafts.

The really surprising thing is that the biggest benefit to altitude is effectively less "back pressure" (we see this in KSP as more thrust and thus higher Isp) as air pressure goes down.  An ideal vacuum nozzle is infinitely long, but real nozzles have to be chopped off at some point (although some are known to slide into position for an extension that interferes less with staging).  Ideally you would launch at a point that would fit your truncated vacuum nozzle, but I suspect that "near enough" will have to do (so your wings and turbofans still work).

Less aerodynamic drag can't hurt, but it turns out not to be as important as the air clogging up the nozzle.  Free altitude is effectively irrelevant, although if you compute the altitude's "delta-v", it might not be that bad (considering it counts for the fully fueled rocket), but both that and the few hundreds of m/s from the jet won't begin to justify the costs.

If you are launching a satellite into GSO (or otherwise really want the thing around the equator), then choosing your *latitude* is even bigger.  But I doubt that any LEO satellites need to be in equatorial orbit.

[sevenperforce]

Jet-drop staging can be good for polar orbits of small satellites that don't need to stay aloft for very long.

[magnemoe]

2 hours ago, wumpus said:

The really surprising thing is that the biggest benefit to altitude is effectively less "back pressure" (we see this in KSP as more thrust and thus higher Isp) as air pressure goes down.  An ideal vacuum nozzle is infinitely long, but real nozzles have to be chopped off at some point (although some are known to slide into position for an extension that interferes less with staging).  Ideally you would launch at a point that would fit your truncated vacuum nozzle, but I suspect that "near enough" will have to do (so your wings and turbofans still work).

Less aerodynamic drag can't hurt, but it turns out not to be as important as the air clogging up the nozzle.  Free altitude is effectively irrelevant, although if you compute the altitude's "delta-v", it might not be that bad (considering it counts for the fully fueled rocket), but both that and the few hundreds of m/s from the jet won't begin to justify the costs.

If you are launching a satellite into GSO (or otherwise really want the thing around the equator), then choosing your *latitude* is even bigger.  But I doubt that any LEO satellites need to be in equatorial orbit.

This you can start with an vacuum engine, this is nice as most of first stage flight is in effective vacuum. 
Air resistance is an huge factor for tiny rocets, its not an serious issue with large ones, This works well with that you can easy air drop an small rocket. 

[p1t1o]

On 8/19/2017 at 5:37 AM, kerbiloid said:

Unlikely 900 km/h of B-52 really add much for a 4-staged launch vehicle.

Pegasus looks like not a way to make the launch cheaper or so, but the way to launch satellites when launchpads are bombed out and gone.

Pegasus is not operated by the military. Turns out for small satellites (<500kg) it can be a fair bit cheaper, especially for launches into odd orbits.

Its not that 900kmh is particularly fast, but launching above most of the atmosphere gets you some significant benefit. 

[kerbiloid]

1 hour ago, p1t1o said:

Pegasus is not operated by the military

https://www.nasa.gov/centers/dryden/multimedia/imagegallery/Pegasus/EC91-348-4.html

https://www.google.ru/search?q=launch+vehicle+pegasus+b-52&newwindow=1&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiflMrzhOjVAhWodpoKHYtACtwQsAQIKA&biw=1920&bih=977

I remember this since late 1980s magazines, when B-52 was being mentioned as Pegasus' native carrier.

Edited August 21, 2017 by kerbiloid

[p1t1o]

22 minutes ago, kerbiloid said:

https://www.nasa.gov/centers/dryden/multimedia/imagegallery/Pegasus/EC91-348-4.html

https://www.google.ru/search?q=launch+vehicle+pegasus+b-52&newwindow=1&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiflMrzhOjVAhWodpoKHYtACtwQsAQIKA&biw=1920&bih=977

I remember this since late 1980s magazines, when B-52 was being mentioned as Pegasus' native carrier.

The B-52 in those pictures is operated by NASA, it doesnt mean that it is a military mission.

[kerbiloid]

39 minutes ago, p1t1o said:

it doesnt mean that it is a military mission.

(From "Foreign Military Review", 1990.)

Spoiler

Quote

РН «Пегас».
Разработка этого принципиально нового носителя была начата фирмами «Орбитал сайенсиз» и «Геркулес аэроспейс» в 1987 году.
В августе 1989 года на авиабазе Эдвардс (штат Калифорния) состоялась демонстрация опытного образца ракеты.
...
Запуск ракеты «Пегас» осуществляется с самолета В-52 на высоте 12,2 км при скорости М = 0,8.
В качестве первой полезной нагрузки для РН «Пегас» выведены в апреле 1990 года на орбиту спутник-ретранслятор «Гломар» в интересах ДАРПА (управление перспективных исследований МО) и спутник НАСА «Пегсат», состоящий из двух контейнеров с экспериментальным оборудованием.

 

_{More or less} translated:

Quote

LV "Pegasus".
Development of this brand new launch vehicle was started by "Orbital Sciences" and "Hercules aerospace" in 1987.
Its experimantal prototype was first demonstrated in August, 1989, on airbase Edwards (CA).
...
LV Pegasus gets launched from B-52 at an altitude of 12.2 km, at speed of Mach = 0.8.
As a payload in its first flight (April 1990) LV Pegasus has put into orbit a relay satellite "Glomar" in the interest of DARPA and NASA's satellite "Pegsat", with two containers of experimental equipment.

As I can understand, "Glomar" here is in fact GLOMR
http://www.daviddarling.info/encyclopedia/G/GLOMR.html

https://books.google.ru/books?id=a_Pa5ezo7soC&pg=PA83&lpg=PA83&dq=Global+Low+Orbiting+Message+Relay+Satellite&source=bl&ots=Xp3kWd0Pc0&sig=pMPGBrhCDDrZktk27LcGJbRUMSc&hl=ru&sa=X&ved=0ahUKEwjOuK3okujVAhUiJJoKHZq4Ae8Q6AEIRTAE#v=onepage&q=Global Low Orbiting Message Relay Satellite&f=false

(though sometimes it's mentioned as GLOMAR, too

http://acronyms.thefreedictionary.com/Global+Low+Orbiting+Message+Relay+Satellite

http://www.acronymfinder.com/Global-Low-Orbiting-Message-Relay-Satellite-(GLOMAR).html

)

and anyway, it's DARPA's.

This was a hot theme in 1970s-1980s when all were trying different ways to restore military sats using unusual ways: ICBM, SLBM, air launch.
In case launchpads are gone, but you still have a plane, a runway, and a storehouse of sats and pegasuses.
 

More recent continuation of the Pegasus' native purpose :

Spoiler

 

 

Edited August 21, 2017 by kerbiloid

[wumpus]

The Pegasus rocket was created and is run by Orbital Sciences Corp (now Orbital-ATK) and I think was the dawn of commercial satellite launches (back in 1990).  While they started with the B-52, Orbital now owns a L-1011 (bought in 1994) for launches.

It isn't exactly a popular launch system anymore, with 3 launches since 2008 and one scheduled for November.  However, with 43 launches (3 failures), it was more than enough to get Orbital in the space business.  A follow up (minotaur-C) replaces the air-launch with a surplus ICBM (Peacekeeper/MX) and provides three times the payload (but also has 3 failures in far fewer (6) launches).  Pegasus (at least the rockets, but without the wings) seems to be largely flown on top of surplus ICBMs and similar early stages (see Antares with NX-1 rockets).

Edited August 21, 2017 by wumpus
noted minotaur-C

[kerbiloid]

1 hour ago, wumpus said:

A follow up (minotaur-C) replaces the air-launch with a surplus ICBM (Peacekeeper/MX

And Minotaurs are also Orbital Sciences'.

Well, of course, "commercial Minuteman" & "civil Peacekeeper" would sound great, but one could name this somewhat like "a civil company employed to perform conversion and development tasks for military department".

So, the Scout's replacement, Pegasus, with its maiden flight from B-52 with DARPA's emergency relay sat on board, looks appearing between them not just coincidentally.
It's just a military project (emergency sats launch) being kept alive not for its commercial skills, but exactly for its military application. Otherwise why not just launch those 43 mini-sats between other "20 sats on one rocket".

Edited August 21, 2017 by kerbiloid

[wumpus]

21 hours ago, kerbiloid said:

 Otherwise why not just launch those 43 mini-sats between other "20 sats on one rocket".

They were launched over a 20 year period (a long time for a satellite to do nothing).  And I'm not sure that cheap rockets existed between 1990-2010 (Pegasus probably *was* the cheap solution).  And while it certainly requires a lot of military ties to use surplus missiles, it is pretty odd thinking of something that "beats swords into ploughshares" by using surplus ICBM stages to launch satellites as a "military program".

The satellites seem to be a mix of military, NASA-science, and commercial birds.  I suspect that the "ATK" bit of the company (I think bullets are their main product, although with big companies it is hard to tell) has a larger portion of sales to military.

[DerekL1963]

18 minutes ago, wumpus said:

And I'm not sure that cheap rockets existed between 1990-2010 (Pegasus probably *was* the cheap solution).

Pegasus is "cheap" taken in isolation (that is it's total cost to launch is less than other, larger, vehicles), but the last time I looked (it's been a few years) they were one of the more expensive vehicles on a per/kg basis.

[Steel]

4 hours ago, DerekL1963 said:

Pegasus is "cheap" taken in isolation (that is it's total cost to launch is less than other, larger, vehicles), but the last time I looked (it's been a few years) they were one of the more expensive vehicles on a per/kg basis.

All small launch vehicles have a high price/kg, that comes with the territory; you only get good $/kg when you want to launch tons at a time. For small sat launchers the total launch cost is all you really want to look at.