How does the impulse-to-weight ratio of a propulsion system indicate an effective design?












5














While going through Sutton's propulsion elements text, I came across an example where he compares the values of impulse-to-weight ratio to that of specific impulse and deems it a "fair design".



The value of the impulse/weight ratio was 187s and the specific impulse was 240s.



I'm having trouble wrapping my head around how comparing the two value lets you determine this.



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    5














    While going through Sutton's propulsion elements text, I came across an example where he compares the values of impulse-to-weight ratio to that of specific impulse and deems it a "fair design".



    The value of the impulse/weight ratio was 187s and the specific impulse was 240s.



    I'm having trouble wrapping my head around how comparing the two value lets you determine this.



    enter image description here










    share|improve this question







    New contributor




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























      5












      5








      5







      While going through Sutton's propulsion elements text, I came across an example where he compares the values of impulse-to-weight ratio to that of specific impulse and deems it a "fair design".



      The value of the impulse/weight ratio was 187s and the specific impulse was 240s.



      I'm having trouble wrapping my head around how comparing the two value lets you determine this.



      enter image description here










      share|improve this question







      New contributor




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











      While going through Sutton's propulsion elements text, I came across an example where he compares the values of impulse-to-weight ratio to that of specific impulse and deems it a "fair design".



      The value of the impulse/weight ratio was 187s and the specific impulse was 240s.



      I'm having trouble wrapping my head around how comparing the two value lets you determine this.



      enter image description here







      launch






      share|improve this question







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      Lil_TEE is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
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          The specific impulse of a propulsion system measures how much impulse you get per unit mass of propellant consumed.



          The impulse/weight ratio (actually impulse/mass, and sometimes called system specific impulse) measures how much impulse you get per total mass of a launcher or stage.



          A theoretical "perfect" rocket, with zero mass for structure, engine, tankage, avionics, and so on, would have the same impulse/weight ratio as the propulsion system specific impulse, but this is obviously impossible; the non-propellant components have mass. So the specific impulse is an unreachable upper limit to the impulse/weight ratio. If you have very little dry mass in your design -- very thin tanks, minimal structural weight, engine whittled by elves from an Unobtainium billet, etc., then the impulse/weight ratio is closer to the specific impulse.



          The example design reaches 78% of the limiting specific impulse.



          Delta IV Heavy has a system specific impulse of about 352 seconds; some rough calculations tell me its propellant-mass-specific impulse averages around 389 seconds, so it gets to about 90% of the limit.



          Falcon 9 FT system specific impulse is about 259 seconds and its propellant-mass-specific impulse averages around 307, so it's about 84%.






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            1 Answer
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            1 Answer
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            active

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            4














            The specific impulse of a propulsion system measures how much impulse you get per unit mass of propellant consumed.



            The impulse/weight ratio (actually impulse/mass, and sometimes called system specific impulse) measures how much impulse you get per total mass of a launcher or stage.



            A theoretical "perfect" rocket, with zero mass for structure, engine, tankage, avionics, and so on, would have the same impulse/weight ratio as the propulsion system specific impulse, but this is obviously impossible; the non-propellant components have mass. So the specific impulse is an unreachable upper limit to the impulse/weight ratio. If you have very little dry mass in your design -- very thin tanks, minimal structural weight, engine whittled by elves from an Unobtainium billet, etc., then the impulse/weight ratio is closer to the specific impulse.



            The example design reaches 78% of the limiting specific impulse.



            Delta IV Heavy has a system specific impulse of about 352 seconds; some rough calculations tell me its propellant-mass-specific impulse averages around 389 seconds, so it gets to about 90% of the limit.



            Falcon 9 FT system specific impulse is about 259 seconds and its propellant-mass-specific impulse averages around 307, so it's about 84%.






            share|improve this answer




























              4














              The specific impulse of a propulsion system measures how much impulse you get per unit mass of propellant consumed.



              The impulse/weight ratio (actually impulse/mass, and sometimes called system specific impulse) measures how much impulse you get per total mass of a launcher or stage.



              A theoretical "perfect" rocket, with zero mass for structure, engine, tankage, avionics, and so on, would have the same impulse/weight ratio as the propulsion system specific impulse, but this is obviously impossible; the non-propellant components have mass. So the specific impulse is an unreachable upper limit to the impulse/weight ratio. If you have very little dry mass in your design -- very thin tanks, minimal structural weight, engine whittled by elves from an Unobtainium billet, etc., then the impulse/weight ratio is closer to the specific impulse.



              The example design reaches 78% of the limiting specific impulse.



              Delta IV Heavy has a system specific impulse of about 352 seconds; some rough calculations tell me its propellant-mass-specific impulse averages around 389 seconds, so it gets to about 90% of the limit.



              Falcon 9 FT system specific impulse is about 259 seconds and its propellant-mass-specific impulse averages around 307, so it's about 84%.






              share|improve this answer


























                4












                4








                4






                The specific impulse of a propulsion system measures how much impulse you get per unit mass of propellant consumed.



                The impulse/weight ratio (actually impulse/mass, and sometimes called system specific impulse) measures how much impulse you get per total mass of a launcher or stage.



                A theoretical "perfect" rocket, with zero mass for structure, engine, tankage, avionics, and so on, would have the same impulse/weight ratio as the propulsion system specific impulse, but this is obviously impossible; the non-propellant components have mass. So the specific impulse is an unreachable upper limit to the impulse/weight ratio. If you have very little dry mass in your design -- very thin tanks, minimal structural weight, engine whittled by elves from an Unobtainium billet, etc., then the impulse/weight ratio is closer to the specific impulse.



                The example design reaches 78% of the limiting specific impulse.



                Delta IV Heavy has a system specific impulse of about 352 seconds; some rough calculations tell me its propellant-mass-specific impulse averages around 389 seconds, so it gets to about 90% of the limit.



                Falcon 9 FT system specific impulse is about 259 seconds and its propellant-mass-specific impulse averages around 307, so it's about 84%.






                share|improve this answer














                The specific impulse of a propulsion system measures how much impulse you get per unit mass of propellant consumed.



                The impulse/weight ratio (actually impulse/mass, and sometimes called system specific impulse) measures how much impulse you get per total mass of a launcher or stage.



                A theoretical "perfect" rocket, with zero mass for structure, engine, tankage, avionics, and so on, would have the same impulse/weight ratio as the propulsion system specific impulse, but this is obviously impossible; the non-propellant components have mass. So the specific impulse is an unreachable upper limit to the impulse/weight ratio. If you have very little dry mass in your design -- very thin tanks, minimal structural weight, engine whittled by elves from an Unobtainium billet, etc., then the impulse/weight ratio is closer to the specific impulse.



                The example design reaches 78% of the limiting specific impulse.



                Delta IV Heavy has a system specific impulse of about 352 seconds; some rough calculations tell me its propellant-mass-specific impulse averages around 389 seconds, so it gets to about 90% of the limit.



                Falcon 9 FT system specific impulse is about 259 seconds and its propellant-mass-specific impulse averages around 307, so it's about 84%.







                share|improve this answer














                share|improve this answer



                share|improve this answer








                edited 3 hours ago

























                answered 3 hours ago









                Russell Borogove

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