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I am wondering why we don't use jet engines as first stages. Most small rockets, like the Electron, can lift off with a small thrust. In the Electron's case, 192 kN. Why can't we replace the 9 Rutherford engines on the Electron with a/some jet engine(s), like a ram/scramjet with an equal amount of thrust? In a rocket/spaceflight simulator (KSP), I have tried replacing the first stage with a small first stage with a hybrid jet engine with 200 kN of thrust (excluding a horizontal takeoff/drop launch). This works, so why doesn't NASA or other aerospace companies use this?

peterh
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WarpPrime
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6 Answers6

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Take a look at the SABRE engine. The goal is to achieve single stage runway liftoff/land to/from orbit with a hybrid engine capable of breathing air at low altitude but switching to stored oxidizer and operating like a rocket when it is not practical to use ambient air.

The limitations of an air-breathing engine for space launch are that

  1. You can't go very high before the air gets very thin - not a lot of oxygen
  2. You can't go very fast before things start to get very hot from either friction or compression or both.

That said, the SABRE attempts to address these problems to a degree with some rather innovative ideas.

Anthony X
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    There's a third limit, which is that the system to cool the air down before it hits the fans needs to not get blocked with ice. That's actually been the biggest engineering issue with SABRE. The rest is fairly well-understood physics, but how they stop the precooler icing up is a very closely guarded secret. They've done tests to show that it does work, but they aren't telling anyone how they did it. – Graham Apr 16 '19 at 09:53
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    Comments are not for extended discussion; this conversation has been moved to chat. – called2voyage Apr 17 '19 at 12:15
  • They told everyone @graham, and the SABRE 4 engine doesn't go sub-zero.

    see https://space.stackexchange.com/q/39921/20636

     –  Jan 12 '21 at 17:57
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Systems that do this exist and more are being introduced. It's just that they hide their appearance and look somewhat different to what would be expected from what you describe.

  • Orbital Sciences Corporation (now owned by Northrop Grumman) have been air launching the Pegasus satellite launcher since 1990 (almost 30 years).

  • Virgin Galactic's 'White Knight' One & Two craft - intended to carry dedicated suborbital craft are well known.

  • And, the 'new kid on the street' that meets your description is the 'Stratolaunch" system . You knew that. The Stratolaunch 'mothership' has the largest wingspan of any aircraft ever built.

This is the "first stage" :-)

enter image description here

Image from here

Surprisingly (perhaps) release height for launch vehicles is 'only' 35,000 feet (10,700 metres).
Launch payload is 225+ tons.

Stratolaunch had originally proposed the development of a range of launch craft but in early 2019 announced that these plans had been abandoned and that they were intending to launch existing Pegasus XL launchers.

Wikipedia Stratolaunch Systems

enter image description here

From here

Note the outline of Virgin Galactic's 'White Knight Two" on the above diagram. WK2 performs a similar role to 'Stratolaunch', but at a far smaller payload mass and with a dedicated payload.

While Stratolaunch originally proposed a range of craft that they intended to develop, in early 2019 they announced that these plans had been cancelled and that their sole payload initially would be the Northrop Grumman (previously Orbital Sciences Corporation) Pegasus air launched satellite launcher.

Pegasus payload

  • The Pegasus is an air-launched rocket developed by Orbital Sciences Corporation (now part of Northrop Grumman Innovation Systems after Northrop Grumman acquired Orbital ATK). Capable of carrying small payloads of up to 443 kilograms (977 lb) into low Earth orbit, Pegasus first flew in 1990 and remains active as of 2018. The vehicle consists of three solid propellant stages and an optional monopropellant fourth stage. Pegasus is released from its carrier aircraft at approximately 40,000 ft (12,000 m), and its first stage has a wing and a tail to provide lift and attitude control while in the atmosphere.

Prior "mothership"

  • Initially, a NASA-owned B-52 Stratofortress NB-008 served as the carrier aircraft. By 1994, Orbital had transitioned to their "Stargazer" L-1011, a converted airliner which was formerly owned by Air Canada. The name "Stargazer" is an homage to the television series Star Trek: The Next Generation: the character Jean-Luc Picard was captain of a ship named Stargazer prior to the events of the series, and his first officer William Riker once served aboard a ship named Pegasus.

L-1011 Stargazer launching Orbital ATK Pegasus XL rocket December 15th 2016. orbiting the CYGNSS spacecraft for NASA.

enter image description here

Video - release, ignition, ...

NASA page

Russell McMahon
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  • For anyone else from the rest of the world: 35000 feet is 10.7km. – Michael Apr 16 '19 at 11:36
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    @Michael I'm from the rest of the world - I'm in New Zealand. I try to shape my answers to suit the most likely audience - I sometimes use dual units or just get it wrong :-). In this case, altitude in feet is extremely time honoured and the majority of aircraft altitude information is in feet. I realise that that doe snot apply in space launch applications. | I'll edit my answer based on your input :-). – Russell McMahon Apr 16 '19 at 11:54
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    @Michael Feet are used for altitude in aviation in nearly every part of the world. Similarly, knots are typically used for airspeed. – reirab Apr 16 '19 at 18:36
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    Just keep in mind that being lifted by the Stratolaunch does not by itself do very much to get a payload into orbit. It will get you about 10km up at about 85m/s, so at best it will only get a payload a couple percent of the way into orbit. It's most useful at overcoming logistical challenges of launch, such as being able to choose your inclination (which can save more than a few % of fuel) and not having to worry about launching over populated areas. I think it would be a stretch to compare it to a first stage. – Avi Cherry Apr 16 '19 at 21:34
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    Also to add: you can pretty much ignore the weather at 10km and you can optimize your first stage nozzle for a lower ambient pressure. More info here: https://space.stackexchange.com/questions/5531/why-arent-all-satellite-carrying-rockets-launched-from-airplanes – Avi Cherry Apr 16 '19 at 21:44
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    It feels like this answer is incomplete because it doesn't mention Virgin Galactic, to name just one comparable concept. – Everyday Astronaut Apr 17 '19 at 10:13
  • @EverydayAstronaut Indeed. Note I said "Systems that do this exist and more are being introduced." I thought about mentioning the XL systems Pegasus and similar. White Knight Two is shown on the bottom image - no doubt included by the image makers due to its comparable task. I'll add a few comments thereon. – Russell McMahon Apr 17 '19 at 11:41
  • What about a vertical takeoff jet stage? – WarpPrime Apr 17 '19 at 12:52
  • @18ballz Vertical take off is possible BUT if you are going to use a jet you may as well play to its strengths. Wings (& a pilot) give you better flyback than Elon achieves, allows lower thrust per weight (some jet aircraft have > 1:1 Thrust:weight but they are usually very high performance fighters of limited payload. The several people currently doing manned personal VTO craft show that it's doable, – Russell McMahon Apr 17 '19 at 13:23
  • @AviCherry Why do you say 85 m/s? Airliners typically fly more like 250 m/s or so real airspeed at those altitudes (IAS is less, but that's not the relevant measure for going to orbit.) It's also important to remember just how much fuel a typical rocket burns through at the beginning of a launch. The Space Shuttle stack burned through half of its entire mass in the first roughly 95 seconds of flight, for example. From a quick calculation, it looks like it used fuel equal to over 10% of the stack mass to reach the velocity that Stratolaunch should provide. – reirab Apr 17 '19 at 19:32
  • I say that because so far, the Stratolaunch's top tested speed is 85m/s, so right off the bat you seem off by a factor of 3. I still think you're confused about how little of an orbital rocket's energy goes into altitude. Here's ascent data: https://education.ti.com/~/media/4113B908B22F4F5085BE8319260A02B0 At 95 seconds, the shuttle will be moving at about 1000m/s at about 100,000 feet. When the shuttle is 10km up, it's already moving at 400m/s. Currently, Stratolaunch can get a rocket to 5% of orbital altitude, and 1.5% of orbital velocity. And going 'up' is the cheapest part. – Avi Cherry Apr 17 '19 at 23:05
  • @AviCherry A rocket's change in altitude or velocity alone doesn't reflect how much energy it spent. At 95 seconds, the Shuttle's moving at 1000m/s at 100,000 feet, but it used an enormous amount of energy to get there; the vast majority courtesy of 27,000 kN of combined thrust from the Solid Rocket Boosters. Roughly thirty seconds later, the Shuttle is at 140,000 feet, the boosters detach, and it is still only a fraction of the way to orbital velocity (at around 7700m/s). It makes up the entire rest of that velocity using the three main engines with a combined thrust of only about 5700 kN. – srikor Apr 19 '19 at 04:21
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    @AviCherry You are correct that the total launch vehicle energy is a small fraction of its final value at 35000 feet. The killer energy wise is the aerodynamic energy losses which increase with the cube of velocity and decrease logarithmically with altitude. (Actually linearly with decreasing air density which decreases logarithmically). Air density halves ~= every 14000 feet. || Qbar = max aerodynamic pressure point varies with design but was ~ 35000 feet for Shuttle and ~= 45,000 feet for Saturn V (and falcon 9). – Russell McMahon Apr 19 '19 at 07:14
  • By being able to launch at LOWER velocity at a point where air density is about 5 x lower than at sea level reduces Q by 5x, pushes Qbar up to a much higher altitude and lower value and allows major reductions in craft ruggedness and/or improvements in craft design, ... . The cost of the propellants are not the limiting factor - it's the ability to carry them that counts. Full tanks at 35,000 feet and if desired the ability to build a bigger or less rugged craft are very worthwhile – Russell McMahon Apr 19 '19 at 07:15
  • The faster your craft can accelerate leads to less time it needs to ascend to orbit and the less energy it loses effectively "hovering against gravity". – Russell McMahon Apr 19 '19 at 07:20
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    @RussellMcMahon I understand that intuitively it seems obvious atmospheric drag is a significant factor in getting to orbit, but for large launch vehicles, it's not (in terms of energy required). I don't pretend to understand the math, but here are exact numbers in delta v: Saturn V: 40m/s Shuttle: 107 Atlas I: 110 Titan IV/Centaur: 156 I've seen estimates for F9 that puts it between 50-75m/s So the most you could save would be 0.5%-1.5%. – Avi Cherry Apr 19 '19 at 20:34
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There are two major barriers: one is that thrust-to-weight ratio of jet engines is pretty poor (2 J58s massing more than 15 times what 9 Rutherfords do), the other is that it's hard to make an engine that performs efficiently over the wide range of speeds and altitudes that a first stage wants to cover.

That said, Boeing at one point toyed with a concept using recoverable jet-powered modules as the first stage of a three-stage-to-orbit reusable launcher.

Russell Borogove
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    I'm not advocating the concept, but you might save some mass by not carrying oxidizer for the jet engines. – Organic Marble Apr 16 '19 at 00:22
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    Absolutely -- that's the most attractive thing about using jet engines. On orbital ascent, though, the useful run time of air-breathers is so short that the added weight of the engine almost eats up the oxidizer savings. – Russell Borogove Apr 16 '19 at 00:25
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    Agree completely. – Organic Marble Apr 16 '19 at 00:28
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    The fact that the air can be used as an oxidizer is just an extra bonus. The more important benefit is that it's reaction mass – that's why air-breathing engines suffer less from the rocket equation. This would hold up even for a high-bypass turbofan on mars, whose turbine is exclusively fed with canned oxidizer but the fan with CO₂ from the atmosphere. – leftaroundabout Apr 18 '19 at 12:47
  • ...and more usefully, it would even hold up for a supersonic turbojet like the Olympus 593, if both oxygen and fuel are injected into the combustion chamber. – leftaroundabout Apr 18 '19 at 12:57
  • Maybe the combined TWR of the J58s is lower than the TWR of 9 Rutherfords. – WarpPrime Apr 18 '19 at 13:17
  • I'm curious now. Let's see, an F-1 engine burned 2,578 kg per second of fuel plus oxidizer. Air is about 1 kg/m^3 at sea level. So, for an intake of 1 m^2 and ignoring the complications of aerodynamics, that corresponds to an intake air speed of 2,578 m/s. An intake of 10 m^2 (diameter ~3.6 m) gets 258 m/s. So just in terms of reaction mass, it must be hard for an air-breather to match what a conventional rocket puts out. – Greg Apr 18 '19 at 14:26
  • @Greg Now try it at Mach 2. – Russell Borogove Apr 18 '19 at 14:50
  • The fuel consumption of the jet engines probably are significantly lower than the rocket engines, but the jets lose their efficiency at faster speeds and altitudes. – WarpPrime Aug 05 '19 at 18:00
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Effectively it has been done, but not in the way you're thinking (or probably intending).

The USAF launched its ASAT missile into space from an F-15 fighter aircraft. While not an orbital rocket, it's close.

And for orbital rockets, there's Pegasus, launched from a converted L1011 Tristar mother aircraft. It's not a concept that ever really took off, despite multiple attempts (and several still being developed), mostly because of the minimal benefits and high complexity combined with very low payload capacity for the rocket (due to size constraints of the mother aircraft, can't fit something the diameter of a Proton or a Falcon 9 under a jetliner after all).

jwenting
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    Besides what was mentioned, there's Virgin Galactic's SpaceShipTwo and LauncherOne. But I don't think an air-breathing stage has the advantage that it might seem to have. The air doesn't go high enough to make the extra complexity and weight worthwhile. Just make the tanks a little bigger. Bezos, when he started Blue Origin, thought that rockets were a horrible way to get into space. He started out studying every launch scheme he could find, and concluded that rockets are a great way to get things into space. – Greg Apr 16 '19 at 19:53
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    @Greg thought of mentioning that one, the X-15, and some of the original concepts for the Space Shuttle which also featured an air breathing mother aircraft launching the orbiter from its back. The main advantage of it is that you're not bound by fixed launch sites, but that's a very small advantage compared to the many disadvantages. – jwenting Apr 17 '19 at 03:50
  • As I'm sure you are aware, you can reach or transit LEO at net velocities far below that required to stay in LEO. Without looking up the figures I'd imagine (and I may be very wrong) that asat vehicles do not come near orbital velocity requirements. – Russell McMahon Apr 19 '19 at 10:37
  • You are correct :-) - the ASM135 asat has a published maximum velocity of > 0.85 orbital velocity. Closer that I'd have imagined.

    https://wiki2.org/en/ASM-135_ASAT

     – Russell McMahon Apr 19 '19 at 10:53
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For straight vertical, it just doesn't make sense. Jet engines have a low TWR, you are adding a lot of complexity, and it won't gain you much at all. The only way it make some sense is from an airplane type delivery, which will give you a more ideal launch location, and some speed and altitude when you drop the rocket. But keep in mind, it requires a huge plane for a relatively small rocket, and launching from the air reduces the kinds of cryogenics you can use.

Overall, it just isn't really worth it, unless you can heavily take advantage of an airplane type mode, as a few launch providers, such as the Pegasus rocket.

PearsonArtPhoto
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  • re " ... Jet engines have a low TWR ..." -> Many do, but it seems likely that that's in large part due to design optimisation for other factors. A select few jet aircraft can climb vertically along with airframe, lift and control surfaces, fuel tanks for sustained flight over much longer periods that needed for VTO to orbit use, avionics, landing gear, ,,, . Ye Olde Lightning, F22, ... . – Russell McMahon Apr 19 '19 at 10:57
  • Most of those that can climb vertically have a relatively low mass, and can't run the engines for long. Granted a rocket also wouldn't need to use the engines for long, but... – PearsonArtPhoto Apr 19 '19 at 12:14
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Though TWR (thrust to weight ratio) of jet engines are low, you can still do it; if not one jet engine, use four jet engines. If not four jet engines, use 30 jet engines. You are going to reuse all jet engines anyway and fly them only just for few minutes. So you aren't going to lose a lot of money. But maximum speed they can provide is around Mach 3, which is less than half the speed of first stage of electron rocket can provide around Mach 7. If you consider kinetic energy though, jet engines can provide just fifth of it: $\frac{\text{(Mach 3)}^2}{\text{(Mach 7)}^2} \approx \frac{1}{5}$ . So not really that useful.

And you got to perform stage separation inside Earth's atmosphere flying at Mach 3 probably less than 20km (ceiling altitude of SR-71) and space is above 100km. Considering atmosphere pressures at 20km, it's just too dangerous and complicated to do.

Only advantage I can see, you need only one vacuum optimized Rutherford engines instead of ten. But Rocket Lab is 3D printing rocket engines and the pumps run on battery-power (not complicated). So it's really cheap. All in all, you are not saving much.

Putting more efforts towards recovering first stage of electron rocket would be a wiser decision.

No Nonsense
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SRD
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