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Which one of these Isp's for the Dawn spacecraft is wrong?

Why we don't use cyclotron for ion thrusters?Does the Dawn spacecraft have the potential for an extended mission?How will the ion thruster powered Dawn spacecraft enter orbit around Ceres?Why does the Dawn spacecraft have three ion engines at different angles?How does one spacecraft best visit multiple asteroids?Dawn Spacecraft: Why are the orbits so high?If the Dawn probe visits a third asteroid after Ceres and Vesta, which asteroid could it visit?Xenon vs Hydrazine, “Should I Stay or Should I go?” Dawn mission decisionsWhich way will the Neumann drive (on the ISS) point, what will be its maximum possible thrust?Ceres gravity from spherical harmonics from Dawn, how to get the coefficients, definitions and potential?Why was Dawn placed into an orbit that would only be stable for “decades”

2

$begingroup$

The Wikipedia article NASA Solar Technology Application Readiness says:

The ions are accelerated through two fine grids with roughly a 1300 V difference between them for 2.3 kW operation, with a thrust of 20-92 mN, a specific impulse of 1950-3100 N·s/kg and a total impulse capability of 2.65 x106 Ns.

and the Wikipedia article subsection Dawn (spacecraft); Propulsion system says:

The Dawn spacecraft was propelled by three xenon ion thrusters derived from NSTAR technology used by the Deep Space 1 spacecraft, using one at a time. They have a specific impulse of 3,100 s and produce a thrust of 90 mN.

Both numerical values are about 3,000.

But Isp in seconds is obtained by dividing Isp in N·s/kg (which has units of velocity) by Earth's standard gravity of 9.80665 m/s^2, so one of those numbers is off by roughly a factor of 10.

• Which one is wrong, or are they both wrong?


• If so, then what's the right number?

ion-thruster dawn-mission

share|improve this question

edited May 18 at 19:14

uhoh

asked May 18 at 19:02

uhohuhoh

43.9k19168549

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  thought tempting, identify-this-error is probably a bad idea for a tag
  $endgroup$
  – uhoh
  May 18 at 19:07
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  It's possible that some future person will be searching for the correct Isp for Dawn. Such a person is more likely to come here if the question title asks for the correct answer, rather than disputing the wrong answer.
  $endgroup$
  – DrSheldon
  May 19 at 1:25
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Maybe that's how they nearly missed their initial target...
  $endgroup$
  – AtmosphericPrisonEscape
  May 19 at 1:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @DrSheldon I understand the concept, but in this case I think it's most appropriate to leave it as-is. This is exactly the question I needed answering, and it is exactly the question that has been addressed in the posted and accepted answer. If a future person queries a search engine, they will now get the right number in either article in Wikipedia. People don't usually come to Stack Exchange to get a specific numerical value like an engine's Isp, and this question will serve as a warning that even if they do move on to a more encyclopedic website like Wikipedia, numbers there can be wrong!
  $endgroup$
  – uhoh
  May 19 at 1:42
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  @uhoh validate-my-hypothesis might be a good one :). I've wanted one for validating a line of thinking for awhile now. There's a lot of open-ended questions, but I've seen a lot of people asking with an attempt at the question first (E.G. this).
  $endgroup$
  – Magic Octopus Urn
  12 hours ago
  
  

  
  
  
  

add a comment | 

2

$begingroup$

The Wikipedia article NASA Solar Technology Application Readiness says:

The ions are accelerated through two fine grids with roughly a 1300 V difference between them for 2.3 kW operation, with a thrust of 20-92 mN, a specific impulse of 1950-3100 N·s/kg and a total impulse capability of 2.65 x106 Ns.

and the Wikipedia article subsection Dawn (spacecraft); Propulsion system says:

The Dawn spacecraft was propelled by three xenon ion thrusters derived from NSTAR technology used by the Deep Space 1 spacecraft, using one at a time. They have a specific impulse of 3,100 s and produce a thrust of 90 mN.

Both numerical values are about 3,000.

But Isp in seconds is obtained by dividing Isp in N·s/kg (which has units of velocity) by Earth's standard gravity of 9.80665 m/s^2, so one of those numbers is off by roughly a factor of 10.

• Which one is wrong, or are they both wrong?


• If so, then what's the right number?

ion-thruster dawn-mission

share|improve this question

edited May 18 at 19:14

uhoh

asked May 18 at 19:02

uhohuhoh

43.9k19168549

$endgroup$

• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  thought tempting, identify-this-error is probably a bad idea for a tag
  $endgroup$
  – uhoh
  May 18 at 19:07
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  It's possible that some future person will be searching for the correct Isp for Dawn. Such a person is more likely to come here if the question title asks for the correct answer, rather than disputing the wrong answer.
  $endgroup$
  – DrSheldon
  May 19 at 1:25
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  Maybe that's how they nearly missed their initial target...
  $endgroup$
  – AtmosphericPrisonEscape
  May 19 at 1:37
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  $begingroup$
  @DrSheldon I understand the concept, but in this case I think it's most appropriate to leave it as-is. This is exactly the question I needed answering, and it is exactly the question that has been addressed in the posted and accepted answer. If a future person queries a search engine, they will now get the right number in either article in Wikipedia. People don't usually come to Stack Exchange to get a specific numerical value like an engine's Isp, and this question will serve as a warning that even if they do move on to a more encyclopedic website like Wikipedia, numbers there can be wrong!
  $endgroup$
  – uhoh
  May 19 at 1:42
  
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  @uhoh validate-my-hypothesis might be a good one :). I've wanted one for validating a line of thinking for awhile now. There's a lot of open-ended questions, but I've seen a lot of people asking with an attempt at the question first (E.G. this).
  $endgroup$
  – Magic Octopus Urn
  12 hours ago
  
  

  
  
  
  

add a comment | 

$begingroup$

The Wikipedia article NASA Solar Technology Application Readiness says:

The ions are accelerated through two fine grids with roughly a 1300 V difference between them for 2.3 kW operation, with a thrust of 20-92 mN, a specific impulse of 1950-3100 N·s/kg and a total impulse capability of 2.65 x106 Ns.

and the Wikipedia article subsection Dawn (spacecraft); Propulsion system says:

The Dawn spacecraft was propelled by three xenon ion thrusters derived from NSTAR technology used by the Deep Space 1 spacecraft, using one at a time. They have a specific impulse of 3,100 s and produce a thrust of 90 mN.

Both numerical values are about 3,000.

But Isp in seconds is obtained by dividing Isp in N·s/kg (which has units of velocity) by Earth's standard gravity of 9.80665 m/s^2, so one of those numbers is off by roughly a factor of 10.

• Which one is wrong, or are they both wrong?


• If so, then what's the right number?

ion-thruster dawn-mission

share|improve this question

edited May 18 at 19:14

uhoh

asked May 18 at 19:02

uhohuhoh

43.9k19168549

$endgroup$

share|improve this question

edited May 18 at 19:14

uhoh

asked May 18 at 19:02

uhohuhoh

43.9k19168549

share|improve this question

edited May 18 at 19:14

uhoh

asked May 18 at 19:02

uhohuhoh

43.9k19168549

asked May 18 at 19:02

uhohuhoh

43.9k19168549

asked May 18 at 19:02

uhohuhoh

43.9k19168549

1 Answer
1

active

oldest

votes

7

$begingroup$

The 3100 s figure is the correct one. Note that a little further along in the same article it says "the engine achieves a specific impulse of one to three thousand seconds."

The N·s/kg unit is equivalent to meters/sec of exhaust velocity, and ~3000 m/s (or ~300 s) is typical for small, pressure fed, chemical bipropellant rockets. If you weren't getting 10 times better specific impulse out of the deal, there would be nothing attractive about complex, power-hungry electric thrusters with thrust levels measured in milliNewtons.

In fact, the citation on the NSTAR wikipedia article goes to a paper titled Performance of the NSTAR ion propulsion system on the Deep Space One mission, which says 1950-3100 s rather than 1950-3100 N·s/kg. The edit history of the Wikipedia article shows a well-meaning attempt to correct an error in unit conversion. The mis-corrected conversion has now been corrected, for the time being.

share|improve this answer

edited May 18 at 19:27

answered May 18 at 19:21

Russell BorogoveRussell Borogove

93k3311396

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Excellent! Wikipedia is now a tiny bit better.
  $endgroup$
  – uhoh
  May 18 at 19:27
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  It would be even better if the article read the way it did before user 2A00:1028:8388:586E:3990:D513:9FA8:2CF defaced the article back in November, 2015. Before that, the article claimed "a specific impulse of 1950-3100 s". The 's' there is rather naked; it would have been better to use 'seconds'. But not N·s/kg.
  $endgroup$
  – David Hammen
  May 18 at 23:45
  
  

  
  
  
  

add a comment | 

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7

$begingroup$

The 3100 s figure is the correct one. Note that a little further along in the same article it says "the engine achieves a specific impulse of one to three thousand seconds."

The N·s/kg unit is equivalent to meters/sec of exhaust velocity, and ~3000 m/s (or ~300 s) is typical for small, pressure fed, chemical bipropellant rockets. If you weren't getting 10 times better specific impulse out of the deal, there would be nothing attractive about complex, power-hungry electric thrusters with thrust levels measured in milliNewtons.

In fact, the citation on the NSTAR wikipedia article goes to a paper titled Performance of the NSTAR ion propulsion system on the Deep Space One mission, which says 1950-3100 s rather than 1950-3100 N·s/kg. The edit history of the Wikipedia article shows a well-meaning attempt to correct an error in unit conversion. The mis-corrected conversion has now been corrected, for the time being.

share|improve this answer

edited May 18 at 19:27

answered May 18 at 19:21

Russell BorogoveRussell Borogove

93k3311396

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Excellent! Wikipedia is now a tiny bit better.
  $endgroup$
  – uhoh
  May 18 at 19:27
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  It would be even better if the article read the way it did before user 2A00:1028:8388:586E:3990:D513:9FA8:2CF defaced the article back in November, 2015. Before that, the article claimed "a specific impulse of 1950-3100 s". The 's' there is rather naked; it would have been better to use 'seconds'. But not N·s/kg.
  $endgroup$
  – David Hammen
  May 18 at 23:45
  
  

  
  
  
  

add a comment | 

7

$begingroup$

The 3100 s figure is the correct one. Note that a little further along in the same article it says "the engine achieves a specific impulse of one to three thousand seconds."

The N·s/kg unit is equivalent to meters/sec of exhaust velocity, and ~3000 m/s (or ~300 s) is typical for small, pressure fed, chemical bipropellant rockets. If you weren't getting 10 times better specific impulse out of the deal, there would be nothing attractive about complex, power-hungry electric thrusters with thrust levels measured in milliNewtons.

In fact, the citation on the NSTAR wikipedia article goes to a paper titled Performance of the NSTAR ion propulsion system on the Deep Space One mission, which says 1950-3100 s rather than 1950-3100 N·s/kg. The edit history of the Wikipedia article shows a well-meaning attempt to correct an error in unit conversion. The mis-corrected conversion has now been corrected, for the time being.

share|improve this answer

edited May 18 at 19:27

answered May 18 at 19:21

Russell BorogoveRussell Borogove

93k3311396

$endgroup$

• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  Excellent! Wikipedia is now a tiny bit better.
  $endgroup$
  – uhoh
  May 18 at 19:27
  

  
  
  
  


• 
  
  
  

  
  1
  

  
  
  
  
  
  

  
  $begingroup$
  It would be even better if the article read the way it did before user 2A00:1028:8388:586E:3990:D513:9FA8:2CF defaced the article back in November, 2015. Before that, the article claimed "a specific impulse of 1950-3100 s". The 's' there is rather naked; it would have been better to use 'seconds'. But not N·s/kg.
  $endgroup$
  – David Hammen
  May 18 at 23:45
  
  

  
  
  
  

add a comment | 

$begingroup$

The 3100 s figure is the correct one. Note that a little further along in the same article it says "the engine achieves a specific impulse of one to three thousand seconds."

The N·s/kg unit is equivalent to meters/sec of exhaust velocity, and ~3000 m/s (or ~300 s) is typical for small, pressure fed, chemical bipropellant rockets. If you weren't getting 10 times better specific impulse out of the deal, there would be nothing attractive about complex, power-hungry electric thrusters with thrust levels measured in milliNewtons.

In fact, the citation on the NSTAR wikipedia article goes to a paper titled Performance of the NSTAR ion propulsion system on the Deep Space One mission, which says 1950-3100 s rather than 1950-3100 N·s/kg. The edit history of the Wikipedia article shows a well-meaning attempt to correct an error in unit conversion. The mis-corrected conversion has now been corrected, for the time being.

share|improve this answer

edited May 18 at 19:27

answered May 18 at 19:21

Russell BorogoveRussell Borogove

93k3311396

$endgroup$

share|improve this answer

edited May 18 at 19:27

answered May 18 at 19:21

Russell BorogoveRussell Borogove

93k3311396

answered May 18 at 19:21

Russell BorogoveRussell Borogove

93k3311396

answered May 18 at 19:21

Russell BorogoveRussell Borogove

93k3311396