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Why aren't air breathing engines used as small first stages

2

1

$begingroup$

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. This works, so why doesn't NASA or other aerospace companies use this?

engines rocketlab electron kerbal-space-program ramjet

share|improve this question

asked 2 hours ago

18ballz18ballz

156

New contributor

18ballz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
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  Someone else will post a detailed answer, but humans have not yet been able to make a working scramjet. That limits air breathing engines to < Mach 5 which is not a big help. Conventional first stages simply work better.
  $endgroup$
  – Organic Marble
  2 hours ago
  

  
  
  
  


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  Oh, in the KSP simulator, the hybrid jet engine can go up to Mach 4, where it loses thrust. However, it has a good high-altitude performance. Is there any jet engine that can do this?
  $endgroup$
  – 18ballz
  2 hours ago
  

  
  
  
  


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  The highest altitude high-speed aircraft engine I know of is the Pratt and Whitney J58. It could run at around Mach 3 at around 85,000 ft. There are some developments like the Sabre engine that could do better if they actually are built and work.
  $endgroup$
  – Organic Marble
  2 hours ago
  
  
  

  
  
  
  


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  $begingroup$
  That's good! So what thrust does the J58 have?
  $endgroup$
  – 18ballz
  2 hours ago
  

  
  
  
  


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  $begingroup$
  Roughly 30,000 lbf.
  $endgroup$
  – Organic Marble
  2 hours ago
  

  
  
  
  

 | 
show 3 more comments

2

1

$begingroup$

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. This works, so why doesn't NASA or other aerospace companies use this?

engines rocketlab electron kerbal-space-program ramjet

share|improve this question

asked 2 hours ago

18ballz18ballz

156

New contributor

18ballz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Someone else will post a detailed answer, but humans have not yet been able to make a working scramjet. That limits air breathing engines to < Mach 5 which is not a big help. Conventional first stages simply work better.
  $endgroup$
  – Organic Marble
  2 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Oh, in the KSP simulator, the hybrid jet engine can go up to Mach 4, where it loses thrust. However, it has a good high-altitude performance. Is there any jet engine that can do this?
  $endgroup$
  – 18ballz
  2 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  The highest altitude high-speed aircraft engine I know of is the Pratt and Whitney J58. It could run at around Mach 3 at around 85,000 ft. There are some developments like the Sabre engine that could do better if they actually are built and work.
  $endgroup$
  – Organic Marble
  2 hours ago
  
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  That's good! So what thrust does the J58 have?
  $endgroup$
  – 18ballz
  2 hours ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Roughly 30,000 lbf.
  $endgroup$
  – Organic Marble
  2 hours ago
  

  
  
  
  

 | 
show 3 more comments

$begingroup$

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. This works, so why doesn't NASA or other aerospace companies use this?

engines rocketlab electron kerbal-space-program ramjet

share|improve this question

asked 2 hours ago

18ballz18ballz

156

New contributor

18ballz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

$endgroup$

share|improve this question

asked 2 hours ago

18ballz18ballz

156

New contributor

18ballz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

share|improve this question

asked 2 hours ago

18ballz18ballz

156

New contributor

18ballz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 2 hours ago

18ballz18ballz

156

New contributor

18ballz is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.

asked 2 hours ago

18ballz18ballz

156

2 Answers
2

active

oldest

votes

3

$begingroup$

Take a look at the SABRE engine. The goal is to achieve single stage to 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 no longer 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.

share|improve this answer

answered 44 mins ago

Anthony XAnthony X

9,50513681

$endgroup$

add a comment | 

3

$begingroup$

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.

share|improve this answer

edited 1 hour ago

answered 2 hours ago

Russell BorogoveRussell Borogove

89.3k3300384

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I'm not advocating the concept, but you might save some mass by not carrying oxidizer for the jet engines.
  $endgroup$
  – Organic Marble
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  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.
  $endgroup$
  – Russell Borogove
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Agree completely.
  $endgroup$
  – Organic Marble
  1 hour ago
  

  
  
  
  

add a comment | 

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3

$begingroup$

Take a look at the SABRE engine. The goal is to achieve single stage to 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 no longer 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.

share|improve this answer

answered 44 mins ago

Anthony XAnthony X

9,50513681

$endgroup$

add a comment | 

3

$begingroup$

Take a look at the SABRE engine. The goal is to achieve single stage to 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 no longer 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.

share|improve this answer

answered 44 mins ago

Anthony XAnthony X

9,50513681

$endgroup$

add a comment | 

$begingroup$

Take a look at the SABRE engine. The goal is to achieve single stage to 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 no longer 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.

share|improve this answer

answered 44 mins ago

Anthony XAnthony X

9,50513681

$endgroup$

share|improve this answer

answered 44 mins ago

Anthony XAnthony X

9,50513681

answered 44 mins ago

Anthony XAnthony X

9,50513681

answered 44 mins ago

Anthony XAnthony X

9,50513681

3

$begingroup$

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.

share|improve this answer

edited 1 hour ago

answered 2 hours ago

Russell BorogoveRussell Borogove

89.3k3300384

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I'm not advocating the concept, but you might save some mass by not carrying oxidizer for the jet engines.
  $endgroup$
  – Organic Marble
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  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.
  $endgroup$
  – Russell Borogove
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Agree completely.
  $endgroup$
  – Organic Marble
  1 hour ago
  

  
  
  
  

add a comment | 

3

$begingroup$

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.

share|improve this answer

edited 1 hour ago

answered 2 hours ago

Russell BorogoveRussell Borogove

89.3k3300384

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  I'm not advocating the concept, but you might save some mass by not carrying oxidizer for the jet engines.
  $endgroup$
  – Organic Marble
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  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.
  $endgroup$
  – Russell Borogove
  1 hour ago
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Agree completely.
  $endgroup$
  – Organic Marble
  1 hour ago
  

  
  
  
  

add a comment | 

$begingroup$

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.

share|improve this answer

edited 1 hour ago

answered 2 hours ago

Russell BorogoveRussell Borogove

89.3k3300384

$endgroup$

share|improve this answer

edited 1 hour ago

answered 2 hours ago

Russell BorogoveRussell Borogove

89.3k3300384

answered 2 hours ago

Russell BorogoveRussell Borogove

89.3k3300384

answered 2 hours ago

Russell BorogoveRussell Borogove

89.3k3300384