Major US Launches this Week: • SUN FL Falcon9 / Starlink • TUE CA Falcon9 / Starlink • WED FL Falcon9 / Starlink • FRI FL Falcon9 / Starlink (500th launch/landing of an orbital-class rocket) • SAT CA Falcon9 / Starlink ______ Updated: 6 SEP / 1430 (all times US Eastern) Last week Engineering Fun:
Entry Burn.
Why not just do a landing burn only?
Because aft-end aerodynamic heating is an engineering nightmare.
Heating and Drag ~ (air density)x(speed squared) for a given orientation (attitude).
In the upper atmosphere, air density ≈ 0, so heating and drag are small.
Upon entering the middle/lower atmosphere, density becomes more and more significant.
Also, speed has been increasing due to gravity free-fall after reaching ballistic apex.
The entry burn quickly slows the booster’s falling speed before aero heating becomes damaging.
The booster then slows aerodynamically as air density increases and drag > weight.
It’s fun to replay the vid around entry burn and watch the speed numbers.
Sometimes, you can see the speed increase for a few seconds after entry-burn shutdown,
until air density increases enough for drag to start slowing the fall.
Yeah, pretty sure that’s how it works, without a slide rule and tables of data.
Agree, Doug. That’s exactly how it works.
And then, there’s the hoverslam…
Going into my way back machine, wasn’t a re-entry burn also used to mitigate damage to ablative shielding? Granted, the shielding was going to take damage, hence the term “ablative” but I seem to recall the idea was rocket exhaust used as a counter to the heating which occurs.
Retro rockets are used to reduce orbital speed (25,000+ kph) to suborbital speed, so that the new perigee dips into the atmosphere where either it burns up or a heat shield protects it while it slows via drag to parachute speed (see: crew capsules and film-return capsules from back in my day).
Falcon9’s Stg-1 is wayyy slower than orbital speed.
It’s doing about 8000 kph when the re-entry burn starts as it enters the thick atmosphere.
There are no heat shield tiles, so speed is reduced to avoid damage.
Starship enters from orbital speed, so it uses thermal tiles
(they’re non-ablative so they can be re-used).
Normal single-use ablative tiles sluff off material to carry heat energy away.