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Post by burningtumbleweed on Aug 28, 2018 6:59:05 GMT
In what scenarios would each type of propellant be at its best? Lighter propellants tend to need heavier tanks, while heavier ones can stuff more mass into a given space, reducing the armor and tank material mass...
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Post by bigbombr on Aug 28, 2018 8:04:09 GMT
High delta-v requirements tend to favor lighter propellants with a higher exhaust velocity (methane, hydrogen and hydrogen deuteride). Heavy armor tends to favor denser propellants (decane, RP-1).
Keep in mind that some propellants have a lower exhaust velocity than you'd expect based on their density. Hydrocarbons tend to make for the best propellants for thermal drives. MPDT's are less picky, as they tend to have excellent exhaust velocity (meaning they don't need a lot of reaction mass, meaning propellant tanks tend to be relatively small).
I personally use pentane for NTR's because it's a decent compromise of density and exhaust velocity, and it has a wide liquid range which would make handling easy IRL. I use MMH/DNTO for combustion rockets because their decent density, wide liquid temperature range and their hypergolicity would make things simpler IRL. For MPDT's I use water, as it's dense, has a pretty good exhaust velocity (in MPDT's, in thermal drives it has a poor exhaust velocity) and would be easy to acquire pretty much anywhere in the solar system.
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Post by AtomHeartDragon on Aug 28, 2018 9:42:38 GMT
There is also the funny aspect that while heavy propellants typically yield higher thrust (making them suited for high manoeuvrability ships), high mass fraction (which is another thing heavy propellants excel at, and something you simply need if you want any sort of sensible delta-v budget with them) is bad for high manoeuvrability ships as it makes handling characteristics change dramatically as propellant depletes. If you make a dense prop ship that can spin on a dime and effortlessly dodge incoming fire when full, you will have made a ship that also instantly murders its crew with acceleration trauma when near empty (this is especially fun when making tankers as they inherently aim for stupidly high mass fraction). You can engineer around it by using different engine groups and progressively shutting them down as you deplete propellant, but since COADE doesn't support throttling, vertical staging nor limiting acceleration on control level, there is no other way around that and no way that would be usable by AI. Of course, drones and missiles don't suffer from such issues (hint), although missiles will be prone to entering wild spins when unencumbered and drones might benefit from sharing propellant or part of it with main fleet for logistics reasons. MPDT's are less picky, as they tend to have excellent exhaust velocity (meaning they don't need a lot of reaction mass, meaning propellant tanks tend to be relatively small). That's because MPDTs always brutally murder ionize molecules of whatever you feed into them while thermal engines need to dump a lot of heat into that. Personally I would aim to share propellant between MPDTs and thermal rockets for the purpose of gear shifting. If you use different propellants with hybrid propulsion your delta-v will fluctuate wildly depending on the order in which you use them and with military, general purpose ships you can't just ensure the order in which you will burn different engines up front. |
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Post by jtyotjotjipaefvj on Aug 28, 2018 10:39:29 GMT
The best way to solve this issue is to have an unarmored carrier running on ridiculously powerful MPDs that spews out drones (or even full capitals) running on hydrocarbon NTRs. That way you can have basically unlimited dv while also having high acceleration and armor when it matters. The only time I'd use HD, the only really light propellant, is when I need a single ship to have both high acceleration and DV close to the 20 km/s mark. Which happens quite rarely. Personally I would aim to share propellant between MPDTs and thermal rockets for the purpose of gear shifting. If you use different propellants with hybrid propulsion your delta-v will fluctuate wildly depending on the order in which you use them and with military, general purpose ships you can't just ensure the order in which you will burn different engines up front. Your dv will fluctuate wildly depending on burn order even if you do share propellant, so limiting yourself to that doesn't make that much sense to me. Especially since the best MPD propellants tend to be terrible with NTRs and totally incombustible. Not to mention that high-powered MPDs can be fast enough for use in combat too. |
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Post by bigbombr on Aug 28, 2018 11:20:06 GMT
Especially since the best MPD propellants tend to be terrible with NTRs and totally incombustible. Not to mention that high-powered MPDs can be fast enough for use in combat too. Methane (and to a lesser extent, other hydrocarbons) works fine in both NTRs and MPDTs. I used to use methane propellant and have dual drives on my capships, but since then I have split between high delta-v and high acceleration designs. The reason for this is that a ship designed for NTRs will have pathetic acceleration while using MPDTs due to the high mass fraction, and a ship designed for MPDTs will have to short a burn time while using NTRs. Furthermore, I try to move away from cryogenics, as they introduce complications. MPDTs can have acceleration high enough for combat use, especially if we could dodge sideways. |
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Post by anotherfirefox on Aug 28, 2018 11:42:01 GMT
Light and heavy..? That's not the logic...
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Post by The Astronomer on Aug 28, 2018 11:48:59 GMT
Propellant density, you mean?
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Post by AtomHeartDragon on Aug 28, 2018 13:57:28 GMT
Personally I would aim to share propellant between MPDTs and thermal rockets for the purpose of gear shifting. If you use different propellants with hybrid propulsion your delta-v will fluctuate wildly depending on the order in which you use them and with military, general purpose ships you can't just ensure the order in which you will burn different engines up front. Your dv will fluctuate wildly depending on burn order even if you do share propellant, so limiting yourself to that doesn't make that much sense to me. Your delta-v cannot depend on the order of propellants used up if you have only one propellant. Sure you will expend your propellant at different rate with NTRs and MPDTs and produce different amounts of delta-v, but there is no preferred order in which you burn both - rather than that the preferred use of NTRs is "never", unless you need to make impulsive trajectory change in deep gravity well, or are otherwise performing rapid manoeuvres (AKA fighting). OTOH, if your engines don't share propellants, you'll generally want to dump high-thrust, low I sp stuff ASAP (preferably along with associated tankage and motors - this is known as staging) and then happily proceed on high I sp stuff. This is the exact order that also happens to work the best for things like liftoff, but also happens to work the absolute worst when you, for example, want to depart your home body's orbit in order to mess up someone's fleet elsewhere. Been there, done that, this was by far the most severe issue I had with Meth-LOx RCS/afterburners on my ships. Sharing propellant you simply don't have this kind of issues. OTOH MPDTs can be fed anything you can ionize unless it's excessively unhealthy for the motor itself. Also, almost everything is combustible when faced with sufficiently BRÜTAL fluorine compound. |
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ghgh
Full Member
Still trying to make kinetics work.
Posts: 136
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Post by ghgh on Aug 31, 2018 14:05:10 GMT
Light and heavy..? That's not the logic... Well.... technically a dense object is going to be heavier then a material that is not dense. So maybe the terminology is less scientific but it gets the same effect. One of the definitions of heavy is having great density.
2.of great density; thick or substantial.
"heavy gray clouds" ·
[more]
synonyms: dense · thick · opaque · soupy · murky · smoggy · impenetrable
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Post by Apotheon on Sept 2, 2018 19:00:09 GMT
Can someone remind me of why methane may be preferrable to hydrogen for nuclear thermal rockets?
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Post by AtomHeartDragon on Sept 2, 2018 19:37:10 GMT
Can someone remind me of why methane may be preferrable to hydrogen for nuclear thermal rockets? It's much denser (smaller tanks, better mass ratios, higher thrust) and is only mildly cryogenic and thus much easier to store.
It doesn't embrittle most materials nor leak through them either.
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Post by Apotheon on Sept 6, 2018 15:42:58 GMT
Can someone remind me of why methane may be preferrable to hydrogen for nuclear thermal rockets? It's much denser (smaller tanks, better mass ratios, higher thrust) and is only mildly cryogenic and thus much easier to store.
It doesn't embrittle most materials nor leak through them either.
Hmmm... I just designed a really small craft and with methane instead of hydrogen deuteride, it was almost half the size, but 20% more mass, BUT 30% cheaper and the propellant cost for the same amount of dV is about 75% cheaper... and this threw me off completely. In other words, mass and cost optimisation are quite irreconcilable, but personally, I'm sticking with mass-optimisation. |
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Post by AtomHeartDragon on Sept 6, 2018 18:45:29 GMT
It's much denser (smaller tanks, better mass ratios, higher thrust) and is only mildly cryogenic and thus much easier to store.
It doesn't embrittle most materials nor leak through them either.
Hmmm... I just designed a really small craft and with methane instead of hydrogen deuteride, it was almost half the size, but 20% more mass, BUT 30% cheaper and the propellant cost for the same amount of dV is about 75% cheaper... and this threw me off completely. In other words, mass and cost optimisation are quite irreconcilable, but personally, I'm sticking with mass-optimisation. You can't really say up-front whether hydrogen deuteride or <insert your hydrocarbon of choice> is superior NTR propellant. It depends on: - How much armour do you want?
- How much are you hurt by ballooning cross-section?
- How much thrust do you need?
- How much are you hurt by high mass fraction when near empty?
- What's your idea of sensible delta-v budget?
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