34 votes
Accepted

What is and what isn't ullage in rocket science?

But I still don't completely understand what is or isn't ullage in rocket science context. Ullage technically is the space in a tank of liquid which is gas-filled instead of liquid-filled. For a ...
Russell Borogove's user avatar
18 votes
Accepted

Why does the Falcon 9 use turbopumps as well as helium pressurisation system?

Pumps are great at creating positive pressure, hundreds of bars. But with tank at ~1 bar, atmospheric pressure, they can only create a grand total of 1 bar of suction - can't go more vacuum than ...
SF.'s user avatar
  • 55k
17 votes
Accepted

How is hot staging of Starship expected to provide such a large (10%) increase in total mass to LEO?

The main benefit is that the second stage never stops accelerating, retaining close to maximum acceleration at all times. One of most significant losses in rocket launches are gravitational losses; ...
SF.'s user avatar
  • 55k
11 votes
Accepted

Why do Soviet and Soviet-derived rockets hot-stage instead of using ullage motors?

Paraphrasing parts of an answer to a different question, hot-staging has a few advantages: It's less complex than staging using ullage motors since fewer parts are involved (whole rocket motors and ...
DarkDust's user avatar
  • 12.5k
10 votes

Why did the S-IV-B refer to "jettison"ing its ullage?

There are two separate systems for ullage on the S-IVB (500 series only). The first one is used for the first start of the S-IVB during launch. Some information can be found in the AS-503 Saturn V ...
indy91's user avatar
  • 1,991
7 votes

Ullage control with single engine stage?

If the upper stage is fired promptly after first-stage shutdown, it's possible that no ullage is needed at all. The thrust from the first stage engines settles the upper stage propellants. Any ...
Russell Borogove's user avatar
6 votes

Why did the ullage motors for the S-IVB TLI burn shut down prior to S-IVB ignition?

This document states (sorry for image, I couldn't copy the text) that after the ullage engines cut off, the LH2 Continuous Propulsive Vent System kicks in and continues to settle the propellants. (...
Organic Marble's user avatar
6 votes
Accepted

What do the terms milli-g and microgravity mean? How are they used?

tl;dr: milli-G and micro-G refer to literal, specific-ish levels of acceleration. "Microgravity" is a general term for freefall which is not used synonymously with "micro-G." ...
Erin Anne's user avatar
  • 11.2k
6 votes

Why does the Falcon 9 use turbopumps as well as helium pressurisation system?

In a turbopump feed system it is still necessary to pressurize the tanks slightly (10 to 50 lb/in2) in order to prevent pump cavitation. Rocket Propulsion Elements, Sutton, 4th edition, p. 223 ...
Organic Marble's user avatar
6 votes

Starship deorbit process

NOTE: after doing the below sanity check I have softened my position a lot. I don't think it's completely obvious from what's publicly available how the deorbit will be conducted. A lot of the prior ...
Erin Anne's user avatar
  • 11.2k
5 votes

Can propellant tanks in a pressure-feed rocket be pressurized by burning the propellant inside those same tanks? Recipe for Ka-BOOM?

You have a big problem with hydrolox or metholox--the combustion products condense out at far above the temperature of the liquid in the tank. Thus you get no pressure. Even with Aerozine/nitrogen ...
Loren Pechtel's user avatar
4 votes

How (the heck) will SpaceX's Starship's future pressure-fed thrusters work at "any gee’s, any attitude"?

The propellants are gaseous methane and oxygen propellants, they provide their own pressure just as the nitrogen used in the cold gas thrusters does. No diaphragms, bellows, or bladders involved. A ...
Christopher James Huff's user avatar
4 votes
Accepted

Are these sliding ullage bulkheads in the OTRAG rocket? How do they seal?

Details here: https://www.opsjournal.org/DocumentLibrary/Uploads/OTRAG_u2_final_LK%20.pdf Each 3 m tank segment had a tank bottom which was perforated so the tanks could be filled continuously. The ...
amI's user avatar
  • 720
3 votes

Why did Apollo missions open the propulsive Hydrogen vents during the coast phase?

The Press Reference section on the third stage states The continuous vent system is used to provide a thrust force required to position propellants at the aft end of each tank during coast. So there ...
Organic Marble's user avatar
3 votes

Ullage control with single engine stage?

Ullage for the Apollo command/service module and the lunar module was performed by those reaction control system engines which pointed in the same direction (+X) as the main engine. These engines are ...
DrSheldon's user avatar
  • 47.9k
3 votes
Accepted

How might an internal combustion engine generate substantial external thrust?

The patent indicates they're aiming for exhaust pressures in the region of 10-20 psi (0.7-1.3 bar), which gives a thruster chamber pressure of 5-10 psi (0.35-0.7 bar), which generates 0.5-2 lbf of ...
Hobbes's user avatar
  • 128k
2 votes

Why did the ullage motors for the S-IVB TLI burn shut down prior to S-IVB ignition?

The S-IVB had ACS thrusters, one of which was used for ullage but not part of the "ullage thrusters" usually listed.
BeginOblivion's user avatar
2 votes

How will Starship get ullage during launch?

I think we know the answer to this question now. According to Musk's June 25 Twitter Spaces chat... SpaceX plan to hot stage for the next orbital launch attempt. So the upper stage engines will start ...
psioniq's user avatar
  • 21
1 vote

Starship maintaining tank pressure during reentry and landing

When Starship re-enters most of the energy of re-entry will be absorbed or deflected by the tiles. Some heat will be absorbed into the tank, however as stated any resultant pressure build up can be ...
Slarty's user avatar
  • 9,303
1 vote

Why does the Vulcan-Centaur upper stage do ullage burns after engine shutdown?

TL;DR: During Low-G coasting, pulsing its RCS thrusters at a reduced duty cycle reduces consumption for a system that also has a number of other jobs (vehicle control, roll). Settling acceleration has ...
blobbymcblobby's user avatar
1 vote

Starship deorbit process

Starship is currently still in the development phase, so what is seen on current Starship test vehicles and test flights does not fully predict the final operational design. For example after the ...
Steve Pemberton's user avatar
1 vote

Can propellant tanks in a pressure-feed rocket be pressurized by burning the propellant inside those same tanks? Recipe for Ka-BOOM?

In addition to the issue in Loren's answer of the combustion products not necessarily being good pressurants some of the combustion products are also likely to be contaminants to the propellants. For ...
Alan Birtles's user avatar
  • 2,066
1 vote

Ullage control on stage shutdown?

Yes, it is only done on engine startup. Remember, ullage is done so that liquid propellant is fed to the engines, instead of gas bubbles. When a tank is allowed to sit in microgravity for a ...
DrSheldon's user avatar
  • 47.9k

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