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One thing which surprised me a bit was how fast it left the launch pad. It took just a second or a little bit more. With Starship it appeared as if it was stru

One thing which surprised me a bit was how fast it left the launch pad. It took just a second or a little bit more. With Starship it appeared as if it was struggling to lift off, staying for a while burning fuel until it left the launch pad and even then it felt a bit slow at accelerating. I'll see tomorrow if it was just an illusion.

ericbarrett 2y ago
This has to do with the launch vehicle's thrust to weight ratio (TWR) on the ground with a full tank. It's usually between 1.1 and 1.4. The Russian Proton has a famously high ground TWR and is known for "leaping" off the pad. Maybe Starliner is the same in this initial crewed LEO config.
Ductapemaster 2y ago
I’m not a rocket scientist but I do know the two rockets use different propellants. The Atlas V uses solid propellant, and the Starship uses liquid. I know liquid engines have the ability to throttle them, and I would guess solid does not (or a more limited capacity) so that could be at play here. Starship could ignite and then throttle up, which would leave it on the pad longer. I’m sure someone more knowledgeable than I can clarify!
- fastball 2y ago
  Only the SRBs use solid propellant (APCP). The first stage is kerolox and the upper stage is hydrolox. Although you are a bit off the mark, you did actually hit on the likely reason for the faster liftoff, which is that the SRBs have a much higher thrust-to-weight ratio than the SpaceX Superheavy booster.
rdruxn 2y ago
Starship (including the Super Heavy) weighs about 10x as much as the Starliner (including the Atlas V)
gorkish 2y ago
For one, Starship is 392 ft vs 172ft of Atlas V + Starliner. Even if they accelerate off the pad at the same rate, Starship is going to look like it's moving a lot slower.
tempnow987 2y ago
The better comparison is probably Falcon 9 and Crew Dragon in terms of mass / mission etc. Starship is in a totally different class For Falcon 9 - it will look slower on takeoff as well. One reason not mentioned yet is that after engine start, Falcon is held down until all vehicle systems are verified as functioning normally before release for liftoff. So they can do pressure / thrust etc checks on the ground before releasing. With solid rocket motors especially - once you light those you are pretty committed.
- ericbarrett 2y ago
  Mass does not strictly matter, it is the ground TWR of the launch config. Heavier rockets do tend to have a lower TWR but it is not a universal rule. Starliner is also held for confirmation of all systems being functional even with the SRBs. Better to let them burn out on the pad and clean up the mess than to risk release with, say, asymmetrical thrust. That said I'm sure there are variances between the two rockets' procedures which could make one appear faster.
  - tempnow987 2y ago
    Interesting that they hold with SRB's lit. I think SLS does 90% thrust on main engines - then lights SRB's but SRB ignition is takeoff. A problem at that point is launch escape abort (capsule to safety) if for example one SRB lit but the other didn't for some reason. Not sure it can otherwise manage the asymmetric thrust.
DylanSp 2y ago
As mentioned in other comments, the TWR at liftoff is what makes the difference. If you look at footage of Shuttle and Saturn V launches, you can really see the difference between the Saturn V's TWR of ~1.2 and Shuttle's TWR of ~1.5.

ericbarrett 2y ago

This has to do with the launch vehicle's thrust to weight ratio (TWR) on the ground with a full tank. It's usually between 1.1 and 1.4. The Russian Proton has a famously high ground TWR and is known for "leaping" off the pad. Maybe Starliner is the same in this initial crewed LEO config.

Ductapemaster 2y ago

I’m not a rocket scientist but I do know the two rockets use different propellants. The Atlas V uses solid propellant, and the Starship uses liquid. I know liquid engines have the ability to throttle them, and I would guess solid does not (or a more limited capacity) so that could be at play here. Starship could ignite and then throttle up, which would leave it on the pad longer. I’m sure someone more knowledgeable than I can clarify!

fastball 2y ago
Only the SRBs use solid propellant (APCP). The first stage is kerolox and the upper stage is hydrolox. Although you are a bit off the mark, you did actually hit on the likely reason for the faster liftoff, which is that the SRBs have a much higher thrust-to-weight ratio than the SpaceX Superheavy booster.

rdruxn 2y ago

Starship (including the Super Heavy) weighs about 10x as much as the Starliner (including the Atlas V)

gorkish 2y ago

For one, Starship is 392 ft vs 172ft of Atlas V + Starliner. Even if they accelerate off the pad at the same rate, Starship is going to look like it's moving a lot slower.

tempnow987 2y ago

The better comparison is probably Falcon 9 and Crew Dragon in terms of mass / mission etc. Starship is in a totally different class For Falcon 9 - it will look slower on takeoff as well. One reason not mentioned yet is that after engine start, Falcon is held down until all vehicle systems are verified as functioning normally before release for liftoff. So they can do pressure / thrust etc checks on the ground before releasing. With solid rocket motors especially - once you light those you are pretty committed.

ericbarrett 2y ago
Mass does not strictly matter, it is the ground TWR of the launch config. Heavier rockets do tend to have a lower TWR but it is not a universal rule. Starliner is also held for confirmation of all systems being functional even with the SRBs. Better to let them burn out on the pad and clean up the mess than to risk release with, say, asymmetrical thrust. That said I'm sure there are variances between the two rockets' procedures which could make one appear faster.
- tempnow987 2y ago
  Interesting that they hold with SRB's lit. I think SLS does 90% thrust on main engines - then lights SRB's but SRB ignition is takeoff. A problem at that point is launch escape abort (capsule to safety) if for example one SRB lit but the other didn't for some reason. Not sure it can otherwise manage the asymmetric thrust.

DylanSp 2y ago

As mentioned in other comments, the TWR at liftoff is what makes the difference. If you look at footage of Shuttle and Saturn V launches, you can really see the difference between the Saturn V's TWR of ~1.2 and Shuttle's TWR of ~1.5.