Quote from: LMT on 01/26/2023 06:51 pmQuote from: TrevorMonty on 01/26/2023 06:19 pmEven lightly loaded NTR is better than Hydrolox, its when you up payload that they really shine.If payload were the driving problem, that might be relevant. But what missions are payload-prohibited, e.g., with methalox Starships and ASCENT engines?Moving forward, ISRU can conjure bulk propellant outside Earth's gravity well, with definite paths to: 1. ISRU hydrolox on the Moon,2. ISRU methalox on Mars,3. ISRU LOX in VLEO, and4. ISRU ASCENT in VLEO, on Mars, and in Mars orbit.-- And ASCENT ion drive clusters can push mass with much higher Isp than NTR, at ~ 1500 s. Maybe creativity and money should be applied to such modern bulk ISRU propulsion scenarios, instead of an idea that's been searching for justification -- and reliable lightweight implementation -- since the 1940s.Do you have link with more info on ASCENT?
Quote from: TrevorMonty on 01/26/2023 06:19 pmEven lightly loaded NTR is better than Hydrolox, its when you up payload that they really shine.If payload were the driving problem, that might be relevant. But what missions are payload-prohibited, e.g., with methalox Starships and ASCENT engines?Moving forward, ISRU can conjure bulk propellant outside Earth's gravity well, with definite paths to: 1. ISRU hydrolox on the Moon,2. ISRU methalox on Mars,3. ISRU LOX in VLEO, and4. ISRU ASCENT in VLEO, on Mars, and in Mars orbit.-- And ASCENT ion drive clusters can push mass with much higher Isp than NTR, at ~ 1500 s. Maybe creativity and money should be applied to such modern bulk ISRU propulsion scenarios, instead of an idea that's been searching for justification -- and reliable lightweight implementation -- since the 1940s.
Even lightly loaded NTR is better than Hydrolox, its when you up payload that they really shine.
Do you have link with more info on ASCENT?
Cryogenic propellant transfer is not a killer for NTR, but an enabler: It means an NTR stage is no longer limited by the excruciatingly low TWR of an NTR trying to claw its way out of a gravity well. It means both a single reactor can operate beyond a single propellant load, and that an NTR stage can dramatically improve its mass fraction using much larger tanks that can be launched empty and filled in orbit. There is no physical mechanism to prevent an NTR stage utilising aerobraking. Not even on Mars, as there are currently 4 vehicles on Mars that carried RTGs, and all entered using Aerobraking, so clearly Planetary Protection is not an impediment.
From what i read they aim for a rather low T/W ratio (2-3) and an ISP of around 900 which is not much better than chemical propulsion ( i remember Kirk Sorensen wrote a good article on the topic ).
Quote from: RON_P on 01/26/2023 05:24 pmFrom what i read they aim for a rather low T/W ratio (2-3) and an ISP of around 900 which is not much better than chemical propulsion ( i remember Kirk Sorensen wrote a good article on the topic ).You mean this article: SSTO is a bad idea, but NTR SSTO is worse ?It's limited to earth to orbit application, so doesn't apply here. Although I don't think he likes NTP in space either based on some comments on this forum.
Quote from: InterestedEngineer on 01/25/2023 04:08 pmQuote from: edzieba on 01/25/2023 07:48 amThere is no physical mechanism to prevent an NTR stage utilising aerobraking. Not even on Mars, as there are currently 4 vehicles on Mars that carried RTGs, and all entered using Aerobraking, so clearly Planetary Protection is not an impediment.Not true, for many reasons:1. A rocket with large tanks is an elongated rocket needs to look something like a Starship. The RTGs you are talking about were classic cone heat shields. So you have to have the same amount of testing as Starship to get aerobraking to work. It hasn't been done beforeOnly if you arbitrarily constrain the vehicle to look like Starship. If your optimisation parameter were minimised shadowshield dimensions, then a conical tank or tank assembly (e.g. stacked increasing diameter spheres) would be the optimum. In extremis, you can end up with a classic sphere-cone shape with the engine at the apex, so classical TPS designs can be applied. Inflatable TPS is also an option, in addition to active cooling (with all that LH2 you are carrying). The rest of the points hinge on the invalid assumption that an NTR rocket must look and behave like Starship.
Quote from: edzieba on 01/25/2023 07:48 amThere is no physical mechanism to prevent an NTR stage utilising aerobraking. Not even on Mars, as there are currently 4 vehicles on Mars that carried RTGs, and all entered using Aerobraking, so clearly Planetary Protection is not an impediment.Not true, for many reasons:1. A rocket with large tanks is an elongated rocket needs to look something like a Starship. The RTGs you are talking about were classic cone heat shields. So you have to have the same amount of testing as Starship to get aerobraking to work. It hasn't been done before
There is no physical mechanism to prevent an NTR stage utilising aerobraking. Not even on Mars, as there are currently 4 vehicles on Mars that carried RTGs, and all entered using Aerobraking, so clearly Planetary Protection is not an impediment.
Quote from: RON_P on 01/26/2023 05:24 pmFrom what i read they aim for a rather low T/W ratio (2-3) and an ISP of around 900 which is not much better than chemical propulsion ( i remember Kirk Sorensen wrote a good article on the topic ).T/W ratio isn't that important for OTV its the overall dry mass of vehicle that counts compared to its wet mass and payload. Here is one comparsion example.455ISP Hydrolox Vehicle.No payload Wet 100t, dry 10t, DV 1027420t payload ie Wet 120t, dry 30t, DV 6185900 Nuclear vehicleWet 120t, dry 30t DV 1223520t payload ie Wet 140, dry 50t DV 908759t payload ie wet 179, dry 89t DV 6167In last example NTR delivered almost 3x payload for same 90t of fuel. Even lightly loaded NTR is better than Hydrolox, its when you up payload that they really shine.
In last example NTR delivered almost 3x payload for same 90t of fuel. Even lightly loaded NTR is better than Hydrolox, its when you up payload that they really shine.
This is because of a larger dry mass and no aerobraking on Mars.
Quote from: InterestedEngineer on 01/27/2023 05:15 amThis is because of a larger dry mass and no aerobraking on Mars.Nor will any non-fantasy engine when such arbitrary constraints are added.
Quote from: edzieba on 01/27/2023 08:57 amQuote from: InterestedEngineer on 01/27/2023 05:15 amThis is because of a larger dry mass and no aerobraking on Mars.Nor will any non-fantasy engine when such arbitrary constraints are added.It’s not arbitrary. The NTR-based architectures I’ve seen (such as by Aerojet-Rocketdyne, seen on FISO in 2020) avoid aerobraking/aerocapture. And they look like this:
Quote from: Robotbeat on 01/27/2023 11:46 amQuote from: edzieba on 01/27/2023 08:57 amQuote from: InterestedEngineer on 01/27/2023 05:15 amThis is because of a larger dry mass and no aerobraking on Mars.Nor will any non-fantasy engine when such arbitrary constraints are added.It’s not arbitrary. The NTR-based architectures I’ve seen (such as by Aerojet-Rocketdyne, seen on FISO in 2020) avoid aerobraking/aerocapture. And they look like this:Attempting to claim a restriction is not arbitrary by citing an architecture designed solely to justify use of SLS alone for heavy lift is not exactly making a great point.
As far as aerobraking being an arbitrary restriction, you design a large LH2 tank and radioactive engine that has to be as far away from the cargo that can aerobrake. I doubt it is possible, and if it is possible, it's impossibly hard and expensive to test and iterate. But you are welcome to try. Do keep in mind basic aerodynamic rules such as center of mass lining up with center of drag.
An NTR based on NERVA specs can't get to Mars with any less fuel mass requirement than a fully refueled Starship from LEO.This is because of a larger dry mass and no aerobraking on Mars.NTR doesn't make sense in the era of low cost refuels to LEO.To halve the transit time to Mars an NTR would require an ISP of 1750. That'd be the lightbulb gas fuel design. A fantasy right now.https://docs.google.com/spreadsheets/d/1ycrQlKql9fft9oLDoW1uP-rotK-Y5yVtH4T3S2ZKgk4
Quote from: InterestedEngineer on 01/27/2023 01:22 pmAs far as aerobraking being an arbitrary restriction, you design a large LH2 tank and radioactive engine that has to be as far away from the cargo that can aerobrake. I doubt it is possible, and if it is possible, it's impossibly hard and expensive to test and iterate. But you are welcome to try. Do keep in mind basic aerodynamic rules such as center of mass lining up with center of drag.Failure of imagination is not a physical constraint. Architectures from inflatable decelerators to side-braking (gee, better not just dismiss that one out-of-hand!) to engine-first with an articulated heatshield, etc, are hardly new or novel designs. Incidentally, centre of mass and centre of drag do not need to line up, and generally don't even for existing capsules, as lifting entries are the norm to allow for steering.
People get mad about all sorts of things these days. I think we can predict that many will not like this idea.
