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The total deltaV of the ship has to be higher than mission deltaV, because an excess of propellant is needed for course correction and to compensate the amout of propellant trapped in the tanks, or the propellant used to cool down the rocket (if it is an NTR).
Is there a rule of thumb to calculate the excess of propellant needed for safety?
I. e. to perform a total mission deltaV of 10 km/s, how much higher must be the spacecraft deltaV?
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I don't think there's any guideline in particular. It depends how reliable your engines and navigation are (if your engine is rated for 330 s and it produces 320 s sometimes you need to carry more propellant) and what the failure options are (if you fall short, does human crew die? Or is your satellite just in a somewhat lower orbit? Can you use reaction control fuel as a backup?).
For interplanetary trips, all craft carry "course correction" fuel to change the direction and speed of travel after the main burn. Orbital craft do a "circularization burn" after orbital insertion.
I wish I had a better answer for you but basically it seems like we're pretty good at hitting the orbit on the nose without extra fuel.
-Josh
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Also extra for boil off as well.
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Quaoar,
I'm not sure that there are any specific rules of thumb, but there are performance standards and margins:
1. know exactly where you are (positioning system accuracy and precision)
2. know exactly where you intend to be after firing the engines (navigation system accuracy and precision)
3. know the exact weight of your vehicle (always varies by mission)
4. know the exact thrust your engines produce (should obviously be tested, but must also be measured during flight)
5. know the exact duration of all phases of the engine firing sequence (also requires testing and measurement)
Once you know all that, then you can calculate performance margins for the engines and onboard propellant quantity. If that's too complicated, then know where you are, how much dV it will take to get where you're going based upon orbital mechanics, and know the exact weight of your vehicle at ignition.
Useful tidbits of knowledge from real-world experience with on-orbit refueling and propulsion aboard ISS (stuff that could easily injure or kill people if due diligence is not followed):
On-Orbit Propulsion System Performance of ISS Visiting Vehicles
FAA is responsible for setting standards for human space flight safety (there are very few set numbers, but lots of specific design practices):
Recommended Practices for Human Space Flight Occupant Safety
Since you want to use nuclear engines, read through the radiation safety sections documents:
Guidelines and Capabilities for Designing Human Missions
For those curious about how missed-thrust affects propellant margin for solar electric or nuclear electric propulsion:
Automated Missed-Thrust Propellant Margin Analysis for Low-Thrust Trajectories
From the doc:
Spacecraft employing low-thrust propulsion must be able to recover from an event in which the spacecraft stops thrusting due to an unforeseen problem. Extra propellant is carried to allow the spacecraft to reach its target in the form of propellant margin. An automated method is presented for estimating the propellant margin a spacecraft should carry. The method is applied to three cases: a transfer to Mars, a gravity-assist trajectory to the asteroid Psyche, and a solar sail transfer to the near-Earth asteroid 1991 VG. Examining different power levels for the Mars transfer, it is shown that a 5% margin is sufficient to recover from a 20 days missed thrust for a 10 kW spacecraft, whereas a 15% margin is required for a 20 kW spacecraft. The gravity-assist case exhibits greater sensitivity to missed thrust, but this problem is alleviated by adding a coast arc before the gravity assist in the nominal case. For the solar sail mission, a spacecraft outage of more than 10 days late in the mission may delay arrival by more than 200 days. The effect of missed thrust on a trajectory must be assessed on a mission-by-mission basis, and it cannot be adequately managed with a general guideline.
For those without access:
Edit (more food for thought for low-thrust and impulsive transfers):
DESIGNING ROBUST LOW-THRUST INTERPLANETARY TRAJECTORIES SUBJECT TO ONE TEMPORARY ENGINE FAILURE
Automated Sensitivity Analysis of Interplanetary Trajectories for Optimal Mission Design
This one actually has the code in the doc:
Automated trajectory design for impulsive and low thrust interplanetary mission analysis
Last edited by kbd512 (2018-11-20 23:10:04)
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Thanks to all and happy Thanksgiving
Last edited by Quaoar (2018-11-21 05:17:00)
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Happy Thanksgiving, to all!!!
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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Yes, Happy Thanksgiving to all NewMars friends and much joy to the Holiday seasons still coming.
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