3 ms·

> You would need over 290,000 AEPS engines to match one Merlin 1D engine. Even worse, even an infinite number of AEPS engines would not match a single chemical

> You would need over 290,000 AEPS engines to match one Merlin 1D engine. Even worse, even an infinite number of AEPS engines would not match a single chemical rocket engine of any type, because Hall effect thrusters have a thrust-to-weight ratio of significantly less than 1. So it can't even lift itself off the pad, let alone anything else.

ashtonkem 6y ago
If I’m doing my math right, the 25kg AEPS engine would need to produce 245N to even hover without any payload. So it’s theoretical max is about 0.9% of what’s needed to even lift itself.
- CydeWeys 6y ago
  And of course these figures get much worse when you consider the other components that are a necessary part of the engine working, namely, the xenon fuel tank and massive solar panel array. For a relevant comparison, the thrust-to-weight ratio of the Merlin 1D engine by itself is something like 180:1, but the overall thrust-to-weight ratio of the entire rocket at launch is only 1.4:1. So just in order to get a rocket that can lift off at all and deliver useful payload to orbit you need at least a 3-digit thrust-to-weight ratio on the engine itself. Ion thrusters don't remotely come close.
rbanffy 6y ago
> So it can't even lift itself off the pad, let alone anything else. This is only a problem for take off from large bodies. Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. It'll just take a lot longer to get to that point. Which brings up an interesting option for NTRs: bimodal propulsion. From a single propellant tank you could drive both an NTR and a Hall thruster, depending on your acceleration needs. Hall thrusters aren't heavy (certainly not compared to fission reactors) and you could use your NTRs when you needed high acceleration (such as to reduce transit times or to take off and land from a big rock) and use the low-power extreme-efficiency Halls for most of the trip when you'd be coasting.
- CydeWeys 6y ago
  > This is only a problem for take off from large bodies. Yes, that's what we're talking about here. > Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. Yes, that's what specific impulse measures (which is what most of the larger conversation has been about). As for bimodal propulsion, it makes you wonder if that would be worth doing at all, or if it wouldn't be better to just have separate engines and fuel tanks so each can do what it's best at. I suspect it might end up being less weight and complexity just to use an ion thruster as designed using xenon propellant. Xenon fuel tanks aren't typically all that big anyway.
  - rbanffy 6y ago
    Separate fuel tanks would demand the design to be more mission-specific. Ideally we want reusable spacecraft that can do multiple roles.

ashtonkem 6y ago

If I’m doing my math right, the 25kg AEPS engine would need to produce 245N to even hover without any payload. So it’s theoretical max is about 0.9% of what’s needed to even lift itself.

CydeWeys 6y ago
And of course these figures get much worse when you consider the other components that are a necessary part of the engine working, namely, the xenon fuel tank and massive solar panel array. For a relevant comparison, the thrust-to-weight ratio of the Merlin 1D engine by itself is something like 180:1, but the overall thrust-to-weight ratio of the entire rocket at launch is only 1.4:1. So just in order to get a rocket that can lift off at all and deliver useful payload to orbit you need at least a 3-digit thrust-to-weight ratio on the engine itself. Ion thrusters don't remotely come close.

rbanffy 6y ago

> So it can't even lift itself off the pad, let alone anything else. This is only a problem for take off from large bodies. Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. It'll just take a lot longer to get to that point. Which brings up an interesting option for NTRs: bimodal propulsion. From a single propellant tank you could drive both an NTR and a Hall thruster, depending on your acceleration needs. Hall thrusters aren't heavy (certainly not compared to fission reactors) and you could use your NTRs when you needed high acceleration (such as to reduce transit times or to take off and land from a big rock) and use the low-power extreme-efficiency Halls for most of the trip when you'd be coasting.

CydeWeys 6y ago
> This is only a problem for take off from large bodies. Yes, that's what we're talking about here. > Mass per mass, you'll get a lot more delta-v from a Hall engine than from a Merlin. Yes, that's what specific impulse measures (which is what most of the larger conversation has been about). As for bimodal propulsion, it makes you wonder if that would be worth doing at all, or if it wouldn't be better to just have separate engines and fuel tanks so each can do what it's best at. I suspect it might end up being less weight and complexity just to use an ion thruster as designed using xenon propellant. Xenon fuel tanks aren't typically all that big anyway.
- rbanffy 6y ago
  Separate fuel tanks would demand the design to be more mission-specific. Ideally we want reusable spacecraft that can do multiple roles.