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Part of the work I have to do for my degree is to perform a "Hohmann-like" transfer between Earth and Mercury, of which I believe I have accomplished. I have attempted numerous times to confirm that it is "Hohmann-like" with the Professor with no reply, which I find rather vague and my worst nightmare is to get to the Mission Summery presentation and find out it is not in fact "Hohmann-like".

A conformation would be a great relief.

![Transfer to Mercury

Secondly, I done the transfer with both chemical thrusters and electrical. I expected that the Delta V of the electrical thruster would be significantly lower but apparently this was not the case. Shown below:

  Chemical Maneuver Summary
    -----------------
    Impulsive Burn:     TCM
    Spacecraft:         DefaultSC
    Origin:             Earth
    Axes:               VNB
    Delta V Vector:
       Element 1:   -15.391902348528 km/s
       Element 2:   -0.0012308398803 km/s
       Element 3:   -4.4865567022084 km/s

    Mass depletion from MainTank:  
       Delta V:        16.032462395518 km/s
       Isp:            300.00000000000 s
       Mass change:   -4398.9744517915 kg



   Electrical       Maneuver Summary
    -----------------
    Impulsive Burn:     TCM
    Spacecraft:         DefaultSC
    Origin:             Earth
    Axes:               VNB
    Delta V Vector:
       Element 1:   -8.7407066088320 km/s
       Element 2:   -0.0007028275620 km/s
       Element 3:   -6.8141580876057 km/s

    Mass depletion from ElectricTank1:  
       Delta V:        11.082991606896 km/s
       Isp:            4200.0000000000 s
       Mass change:   -815.09870601026 kg

[I have gotten rid of a propagation sequence that I realised did nothing. This explains the difference in the image and the data]

This leaves me under the impression that I have done something wrong. What am I missing?

Thank you.

Mission sequence:

The current mission sequence of both transfers

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  • 1
    $\begingroup$ It must be quite frustrating to have a professor give you a problem and then not give you feedback. Hopefully one of the orbital-mechanical folks here will be able to help out. When you said you've used electrical thruster what jumps out immediately is that your electrical Isp is only 300 seconds, the same as your chemical thruster and that's a big problem. They have 10 times higher Isp or more, so use 1/10th the mass or less, and they have 1/1000 the thrust or less of a chemical rocket. The durations of the burns last for months or years rather than seconds or minutes. $\endgroup$
    – uhoh
    Commented Dec 24, 2021 at 22:15
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    $\begingroup$ I have since changed the ISP to 4200 seconds and the Thrust force to 0.125 N. Apparently it only takes 20 days longer. I'm extremely confused as to why this is. My suspicion is that it is to do with my vary commands as I have no burn command in my chemical transfer. Perhaps it will only work with a single finite burn for the electrical transfer? And yes, the Prof is notoriously difficult to contact, its a well know thing among students. $\endgroup$ Commented Dec 24, 2021 at 22:34
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    $\begingroup$ I made the changes and also added my mission sequence. The presentation is on the 25th of January, so plenty of time. $\endgroup$ Commented Dec 24, 2021 at 22:52
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    $\begingroup$ OK, thank you for the information, I'm new to StackExchange and am unfamiliar with the platform. I assume I just press the "Answer Your Question" box for a partial answer? In regards to the question, in your opinion, what figures should I be looking out for to call it "done". I feel happy to call the chemical version OK, but we were never actually taught anything about electrical propulsion and I'm dubious to settle for "It went where I wanted it", feels like a luck more than understanding. $\endgroup$ Commented Dec 24, 2021 at 23:07
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    $\begingroup$ OK cool. I'll do that now and include everything from both simulations for extra clarity. $\endgroup$ Commented Dec 24, 2021 at 23:13

1 Answer 1

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//Partial Answer as advised//

  • Chemical Thrust Transfer

This section is my answer to the first part of my task. I believe this is correct. My only worry is that is might not qualify as "Hohmann-like", mostly because it seems like a vague specification and I am not sure about the difference between Hohmann and Hohmann-like. Chemical Transfer with Mission Sequence

    Chemical Maneuver Summary
    -----------------
    Impulsive Burn:     TCM
    Spacecraft:         DefaultSC
    Origin:             Earth
    Axes:               VNB
    Delta V Vector:
       Element 1:   -15.391902348528 km/s
       Element 2:   -0.0012308398803 km/s
       Element 3:   -4.4865567022084 km/s

    Mass depletion from MainTank:  
       Delta V:        16.032462395518 km/s
       Isp:            300.00000000000 s
       Mass change:   -4398.9744517915 kg

Electrical Thrust Transfer

This is what I have arrived at for the transfer under electrical thrust. I have set the ISP to 4200 seconds and the constant thrust to 0.125 Newtons with a solar power system that have the shadow bodies of Earth, Moon and Venus.

Electrical Thrust Transfer

 Manoeuvre Summary
    -----------------
    Impulsive Burn:     TCM
    Spacecraft:         DefaultSC
    Origin:             Earth
    Axes:               VNB
    Delta V Vector:
       Element 1:   -8.7407066088320 km/s
       Element 2:   -0.0007028275620 km/s
       Element 3:   -6.8141580876057 km/s

    Mass depletion from ElectricTank1:  
       Delta V:        11.082991606896 km/s
       Isp:            4200.0000000000 s
       Mass change:   -815.09870601026 kg

The further assistance needed:

  • Do the transfers qualify as Hohmann-like?
  • Are the figures of the electrical thrust transfer as expected?
  • Why does GMAT not have a gravity profile for Mercury? Will this pose a problem when I sort out the orbit?
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  • $\begingroup$ Welcome to space! If you have additional questions, do not ask them in your answer. Answer posts are for answers. Either edit your question to include this new information, or ask a new question. $\endgroup$ Commented Dec 25, 2021 at 1:34
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    $\begingroup$ My apologies, I was asked to post a partial answer. $\endgroup$ Commented Dec 25, 2021 at 21:57
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    $\begingroup$ The chemical transfer doesn't look like a Hohmann transfer, the departure and arrival should be on opposite sides of the Sun. $\endgroup$
    – djr
    Commented Dec 28, 2021 at 22:36
  • $\begingroup$ You should also calculate how long it takes to consume 815 kg of propellant with a thrust of 0.125 N and an Isp of 4200 s. $\endgroup$
    – djr
    Commented Dec 29, 2021 at 11:40
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    $\begingroup$ Why don't you just look on Wikipedia and on this site for Hohmann transfers to Mercury ? $\endgroup$
    – Cornelis
    Commented Jan 4, 2022 at 15:00

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