Fuel/DV safety margins for real spacecraft

[Tyko]

Just wondering how much of a DV margin is typically allotted in real spacecraft. The only time I've ever read about a margin was the Apollo 11 lander which had about 60 seconds of extra fuel for the landing rocket - which they used most of to manually shift their landing site off a boulder-strewn plain.

 

[PB666]

Think about correction burns that are made, for instance, that is the question to ask, how much fuel is reserved for correction burns and how much is typically spent.

BTW, now a days, space craft that land on the moon would directly target their landing site to the last meter. This is what separates pioneers from colonist or repeate missions.

I would ask the folks at the JSC or GFC how much of a flights fuel is reserved for correction burns and are these made with RCS or fuel remnants.

My assumption is that for cryogenic rockets the fuel is burnt and correction and station keeping is up to the RCS system.

 

[Racescort666]

It's largely dependent on the mission though. For example, the fuel margin for a deep space probe is likely double or more than what they need to accomplish the primary mission. However, they usually plan on having an optionally extended mission. Fuel margins for sending resupplies to the ISS or other single use craft with short durations is probably much smaller.

[Pand5461]

I'd say, it must be a fair bit, probably a few hundreds m/s.

Communication satellites: more dV = more lifetime, about 15 m/s per year is needed to maintain position at GEO. With modern satellites being active for 10 years and more, that gives at least 150 m/s.

Orbital probes at other planets: all the same.

Manned spacecraft: Soyuz has about 350 m/s after separation from the launcher. Space Shuttles had about the same, IIRC. Apollo could be fueled to 2 km/s but LEO was not the orbit that ship was designed for. Buran could have about 1500 m/s if the program was not cancelled.

[KSK]

The Shuttle OMS (Orbital Maneuvering System) had enough onboard propellant for about 300m/s worth of maneuvers. Around a third of that was required for the de-orbit burn. 

[Tyko]

cool...thanks for the replies. What about for launch vehicles and fuel margins during ascent? I tend to plan about 5% DV Margins for launch/circularization. I'm just wondering how close to "real" i'm getting

Edited October 25, 2017 by Tyko

[linuxgurugamer]

I wish I could design my ships with that close of a margin. I usually have between 1 and 2000 Delta V when I come back.

[KSK]

9 hours ago, Tyko said:

cool...thanks for the replies. What about for launch vehicles and fuel margins during ascent? I tend to plan about 5% DV Margins for launch/circularization. I'm just wondering how close to "real" i'm getting

I don't know what the actual numbers are but I would guess the margins are pretty slim. Launch vehicles can fly a 'dogleg' ascent trajectory to help them hit the right orbital plane, for example if they're launching from a latitude that wouldn't let them hit that plane directly, or if some additional margin in the launch window is needed. However doglegs do require extra propellant.

Given that most launch windows tend to be quite tight anyway despite there being a mechanism for widening them, I'm thinking that launch vehicles don't fly with that much of a propellant margin. 

5% sounds pretty close to real, allowing for the fact that you're hand-flying your launch vehicle!